Skip to Content
Rule

National Ambient Air Quality Standards for Particulate Matter

Action

Final Rule.

Summary

Based on its review of the air quality criteria and the national ambient air quality standards (NAAQS) for particulate matter (PM), the EPA is making revisions to the suite of standards for PM to provide requisite protection of public health and welfare and to make corresponding revisions to the data handling conventions for PM and to the ambient air monitoring, reporting, and network design requirements. The EPA also is making revisions to the prevention of significant deterioration (PSD) permitting program with respect to the NAAQS revisions.

With regard to primary (health-based) standards for fine particles (generally referring to particles less than or equal to 2.5 micrometers (μm) in diameter, PM 2.5), the EPA is revising the annual PM 2.5 standard by lowering the level to 12.0 micrograms per cubic meter (μg/m3) so as to provide increased protection against health effects associated with long- and short-term exposures (including premature mortality, increased hospital admissions and emergency department visits, and development of chronic respiratory disease), and to retain the 24-hour PM 2.5 standard at a level of 35 μg/m3. The EPA is revising the Air Quality Index (AQI) for PM 2.5 to be consistent with the revised primary PM 2.5 standards. With regard to the primary standard for particles generally less than or equal to 10 µm in diameter (PM 10), the EPA is retaining the current 24-hour PM 10 standard to continue to provide protection against effects associated with short-term exposure to thoracic coarse particles (i.e., PM 10-2.5). With regard to the secondary (welfare-based) PM standards, the EPA is generally retaining the current suite of secondary standards (i.e., 24-hour and annual PM 2.5 standards and a 24-hour PM 10 standard). Non-visibility welfare effects are addressed by this suite of secondary standards, and PM-related visibility impairment is addressed by the secondary 24-hour PM 2.5 standard.

Unified Agenda

 

Table of Contents Back to Top

Tables Back to Top

DATES: Back to Top

The final rule is effective on March 18, 2013.

ADDRESSES: Back to Top

Section X.B requests comments on an information collection request regarding changes to the monitoring requirements. Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2007-0492, to the EPA by one of the following methods:

  • www.regulations.gov: Follow the on-line instructions for submitting comments.
  • Email: a-and-r-Docket@epa.gov.
  • Fax: 202-566-9744.
  • Mail: Docket No. EPA-HQ-OAR-2007-0492, Environmental Protection Agency, Mail code 6102T, 1200 Pennsylvania Ave. NW., Washington, DC 20460. Please include a total of two copies.
  • Hand Delivery: Docket No. EPA-HQ-OAR-2007-0492, Environmental Protection Agency, EPA West, Room 3334, 1301 Constitution Ave. NW., Washington, DC. Such deliveries are only accepted during the Docket's normal hours of operation, and special arrangements should be made for deliveries of boxed information.

Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-2007-0492. The EPA's policy is that all comments received will be included in the public docket without change and may be made available online at www.regulations.gov, including any personal information provided, unless the comment includes information claimed to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Do not submit information that you consider to be CBI or otherwise protected through www.regulations.gov or email. The www.regulations.gov Web site is an “anonymous access” system, which means the EPA will not know your identity or contact information unless you provide it in the body of your comment. If you send an email comment directly to the EPA without going through www.regulations.gov your email address will be automatically captured and included as part of the comment that is placed in the public docket and made available on the Internet. If you submit an electronic comment, the EPA recommends that you include your name and other contact information in the body of your comment and with any disk or CD-ROM you submit. If the EPA cannot read your comment due to technical difficulties and cannot contact you for clarification, the EPA may not be able to consider your comment. Electronic files should avoid the use of special characters, any form of encryption, and be free of any defects or viruses. For additional information about EPA's public docket visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm. Comments on this information collection request should also be sent to the Office of Management and Budget (OMB). See section X.B below for additional information regarding submitting comments to OMB.

Docket: The EPA has established a docket for this action under Docket No. EPA-HQ-OAR-2007-0492. All documents in the docket are listed on the www.regulations.gov Web site. This includes documents in the rulemaking docket (Docket ID No. EPA-HQ-OAR-2007-0492) and a separate docket, established for 2009 Integrated Science Assessment (Docket No. EPA-HQ-ORD-2007-0517), that has have been incorporated by reference into the rulemaking docket. All documents in these dockets are listed on the www.regulations.gov Web site. Although listed in the index, some information is not publicly available, e.g., CBI or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, is not placed on the Internet and may be viewed, with prior arrangement, at the EPA Docket Center. Publicly available docket materials are available either electronically in www.regulations.gov or in hard copy at the Air and Radiation Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution Ave. NW., Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays. The telephone number for the Public Reading Room is (202) 566-1744 and the telephone number for the Air and Radiation Docket and Information Center is (202) 566-1742. For additional information about EPA's public docket visit the EPA Docket Center homepage at: http://www.epa.gov/epahome/dockets.htm.

FOR FURTHER INFORMATION CONTACT: Back to Top

Ms. Beth M. Hassett-Sipple, Health and Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail code C504-06, Research Triangle Park, NC 27711; telephone: (919) 541-4605; fax: (919) 541-0237; email: hassett-sipple.beth@epa.gov.

SUPPLEMENTARY INFORMATION: Back to Top

General Information Back to Top

Availability of Related Information

A number of the documents that are relevant to this rulemaking are available through the EPA's Office of Air Quality Planning and Standards (OAQPS) Technology Transfer Network (TTN) Web site at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_index.html. These documents include the Plan for Review of the National Ambient Air Quality Standards for Particulate Matter (U.S. EPA, 2008a), available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html, the Integrated Science Assessment for Particulate Matter (U.S. EPA, 2009a), available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_isa.html, the Quantitative Health Risk Assessment for Particulate Matter (U.S. EPA, 2010a), available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html, the Particulate Matter Urban-Focused Visibility Assessment (U.S. EPA 2010b), available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html, and the Policy Assessment for the Review of the Particulate Matter National Ambient Air Quality Standards (U.S. EPA, 2011a), available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. These and other related documents are also available for inspection and copying in the EPA docket identified above.

Table of Contents Back to Top

The following topics are discussed in this preamble:

I. Executive Summary

A. Purpose of This Regulatory Action

B. Summary of Major Provisions

C. Costs and Benefits

II. Background

A. Legislative Requirements

B. Review of the Air Quality Criteria and Standards for PM

1. Previous PM NAAQS Reviews

2. Litigation Related to the 2006 PM Standards

3. Current PM NAAQS Review

C. Related Control Programs To Implement PM Standards

D. Summary of Proposed Revisions to the PM NAAQS

E. Organization and Approach to Final PM NAAQS Decisions

III. Rationale for Final Decisions on the Primary PM 2.5 Standards

A. Background

1. General Approach Used in Previous Reviews

2. Remand of Primary Annual PM 2.5 Standard

3. General Approach Used in the Policy Assessment for the Current Review

B. Overview of Health Effects Evidence

C. Overview of Quantitative Characterization of Health Risks

D. Conclusions on the Adequacy of the Current Primary PM 2.5 Standards

1. Introduction

a. Evidence- and Risk-based Considerations in the Policy Assessment

b. CASAC Advice

c. Administrator's Proposed Conclusions Concerning the Adequacy of the Current Primary PM 2.5 Standards

2. Comments on the Need for Revision

3. Administrator's Final Conclusions Concerning the Adequacy of the Current Primary PM 2.5 Standards

E. Conclusions on the Elements of the Primary Fine Particle Standards

1. Indicator

2. Averaging Time

3. Form

a. Annual Standard

b. 24-Hour Standard

4. Level

a. General Approach for Considering Standard Levels

b. Proposed Decisions on Level

i. Consideration of Alternative Standard Levels in the Policy Assessment

ii. CASAC Advice

iii. Administrator's Proposed Decisions on the Primary PM 2.5 Standard Levels

c. Comments on Standard Levels

i. Annual Standard Level

ii. 24-Hour Standard Level

d. Administrator's Final Conclusions on the Primary PM 2.5 Standard Levels

F. Administrator's Final Decisions on the Primary PM 2.5 Standards

IV. Rationale for Final Decision on Primary PM 10 Standard

A. Background

1. Previous Reviews of the PM NAAQS

a. Reviews Completed in 1987 and 1997

b. Review Completed in 2006

2. Litigation Related to the 2006 Primary PM 10 Standards

3. General Approach Used in the Current Review

B. Health Effects Related to Exposure to Thoracic Coarse Particles

C. Consideration of the Current and Potential Alternative Standards in the Policy Assessment

1. Consideration of the Current Standard in the Policy Assessment

2. Consideration of Potential Alternative Standards in the Policy Assessment

D. CASAC Advice

E. Administrator's Proposed Conclusions Concerning the Adequacy of the Current Primary PM 10 Standard

F. Public Comments on the Administrator's Proposed Decision To Retain the Primary PM 10 Standard

G. Administrator's Final Decision on the Primary PM 10 Standard

V. Communication of Public Health Information

VI. Rationale for Final Decisions on the Secondary PM Standards

A. Background

1. Approaches Used in Previous Reviews

2. Remand of 2006 Secondary PM 2.5 Standards

3. General Approach Used in the Policy Assessment for the Current Review

B. Proposed Decisions on Secondary PM Standards

1. PM-related Visibility Impairment

a. Nature of PM-related Visibility Impairment

i. Relationship Between Ambient PM and Visibility

ii. Temporal Variations of Light Extinction

iii. Periods During the Day of Interest for Assessment of Visibility

iv. Exposure Durations of Interest

v. Periods of Fog and Rain

b. Public Perception of Visibility Impairment

c. Summary of Proposed Conclusions

i. Adequacy

ii. Indicator

iii. Averaging Time

iv. Form

v. Level

vi. Administrator's Proposed Conclusions

vii. Related Technical Analysis

2. Other (Non-Visibility) PM-related Welfare Effects

a. Evidence of Other Welfare Effects Related to PM

b. CASAC Advice

c. Summary of Proposed Decisions Regarding Other Welfare Effects

C. Comments on Proposed Rule

1. Comments on Proposed Secondary PM Standard for Visibility Protection

a. Overview of Comments

b. Indicator

i. Comments on Calculated vs. Directly Measured Light Extinction

ii. Comments on Specific Aspects of Calculated Light Extinction Indicator

c. Averaging Time

d. Form

e. Level

i. Comments on Visibility Preference Studies

ii. Specific Comments on Level

f. Need for a Distinct Secondary Standard

g. Legal Issues

h. Relationship With Regional Haze Program

2. Comments on the Proposed Decision Regarding Non-Visibility Welfare Effects

D. Conclusions on Secondary PM Standards

1. Conclusions Regarding Secondary PM Standards To Address Non-Visibility Welfare Effects

2. Conclusions Regarding Secondary PM Standards for Visibility Protection

E. Administrator's Final Decisions on Secondary PM Standards

VII. Interpretation of the NAAQS for PM

A. Amendments to Appendix N: Interpretation of the NAAQS for PM 2.5

1. General

2. Monitoring Considerations

3. Requirements for Data Use and Reporting for Comparison With the NAAQS for PM 2.5

4. Comparisons with the PM 2.5 NAAQS

B. Exceptional Events

C. Updates for Data Handling Procedures for Reporting the Air Quality Index

VIII. Amendments to Ambient Monitoring and Reporting Requirements

A. Issues Related to 40 CFR Part 53 (Reference and Equivalent Methods)

1. PM 2.5 and PM 10-2.5 Federal Equivalent Methods

2. Use of Chemical Speciation Network (CSN) Methods to Support the Proposed New Secondary PM 2.5 Visibility Index NAAQS

B. Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)

1. Terminology Changes

2. Special Considerations for Comparability of PM 2.5 Ambient Air Monitoring Data to the NAAQS

a. Revoking Use of Population-Oriented as a Condition for Comparability of PM 2.5 Monitoring Sites to the NAAQS

b. Applicability of Micro- and Middle-scale Monitoring Sites to the Annual PM 2.5 NAAQS

3. Changes to Monitoring for the National Ambient Air Monitoring System

a. Background

b. Primary PM 2.5 NAAQS

i. Addition of a Near-road Component to the PM 2.5 Monitoring Network

ii. Use of PM 2.5 Continuous FEMs at SLAMS

c. Revoking PM 10-2.5 Speciation Requirements at NCore Sites

d. Measurements for the Proposed New PM 2.5 Visibility Index NAAQS

4. Revisions to the Quality Assurance Requirements for SLAMS, SPMs, and PSD

a. Quality Assurance Weight of Evidence

b. Quality Assurance Requirements for the Chemical Speciation Network

c. Waivers for Maximum Allowable Separation of Collocated PM 2.5 Samplers and Monitors

5. Revisions To Probe and Monitoring Path Siting Criteria

a. Near-road Component to the PM 2.5 Monitoring Network

b. CSN Network

c. Reinsertion of Table E-1 to Appendix E

6. Additional Ambient Air Monitoring Topics

a. Annual Monitoring Network Plans and Periodic Assessment

b. Operating Schedules

c. Data Reporting and Certification for CSN and IMPROVE Data

d. Requirements for Archiving Filters

IX. Clean Air Act Implementation Requirements for the PM NAAQS

A. Designation of Areas

1. Overview of Clean Air Act Designations Requirements

2. Proposed Designations Schedules

3. Comments and Responses

4. Final Intended Designations Schedules

B. Section 110(a)(2) Infrastructure SIP Requirements

C. Implementing the Revised Primary Annual PM 2.5 NAAQS in Nonattainment Areas

D. Prevention of Significant Deterioration and Nonattainment New Source Review Programs for the Revised Primary Annual PM 2.5 NAAQS

1. Prevention of Significant Deterioration

a. Transition Provision (Grandfathering)

i. Proposal

ii. Comments and Responses

iii. Final Action

b. Modeling Tools and Guidance Applicable to the Revised Primary Annual PM 2.5 NAAQS

c. PSD Screening Tools: Significant Emissions Rates, Significant Impact Levels, and Significant Monitoring Concentration

d. PSD Increments

e. Other PSD Transition Issues

2. Nonattainment New Source Review

E. Transportation Conformity Program

F. General Conformity Program

X. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

B. Paperwork Reduction Act

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

G. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, Distribution, or Use

I. National Technology Transfer and Advancement Act

J. Executive Order 12898: Federal Actions To Address Environmental Justice in Minority Populations and Low-Income Populations

K. Congressional Review Act

References

I. Executive Summary Back to Top

A. Purpose of This Regulatory Action

Sections 108 and 109 of the Clean Air Act (CAA) govern the establishment, review, and revision, as appropriate, of the national ambient air quality standards (NAAQS) to protect public health and welfare. The CAA requires periodic review of the air quality criteria—the science upon which the standards are based—and the standards themselves. This rulemaking is being done pursuant to these statutory requirements. The schedule for completing this review is established by a court order.

In 2006, the EPA completed its last review of the PM NAAQS. In that review, the EPA took three principal actions: (1) With regard to fine particles (generally referring to particles less than or equal to 2.5 micrometers (μm) in diameter, PM 2.5), at that time, the EPA revised the level of the primary 24-hour PM 2.5 standard from 65 to 35 μg/m [3] and retained the level of the primary annual PM 2.5 standard; (2) With regard to the primary standards for particles less than or equal to 10 µm in diameter (PM 10), the EPA retained the primary 24-hour PM 10 standard to continue to provide protection against effects associated with short-term exposure to thoracic coarse particles (i.e., PM 10-2.5) and revoked the primary annual PM 10 standard; and (3) the EPA also revised the secondary standards to be identical in all respects to the primary standards.

In subsequent litigation, the U.S. Court of Appeals for the District of Columbia Circuit remanded the primary annual PM 2.5 standard to the EPA because the Agency had failed to explain adequately why the standard provided the requisite protection from both short- and long-term exposures to fine particles, including protection for at-risk populations such as children. The court remanded the secondary PM 2.5 standards to the EPA because the Agency failed to explain adequately why setting the secondary standards identical to the primary standards provided the required protection for public welfare, including protection from PM-related visibility impairment.

The EPA initiated this review in June 2007. Between 2007 and 2011, the EPA prepared draft and final Integrated Science Assessments, Risk and Exposure Assessments, and Policy Assessments. Multiple drafts of all of these documents were subject to review by the public and were peer reviewed by the EPA's Clean Air Scientific Advisory Committee (CASAC). The EPA proposed revisions to the primary and secondary PM NAAQS on June 29, 2012 (77 FR 38890). This final rulemaking is the final step in the review process.

In this rulemaking, the EPA is revising the suite of standards for PM to provide requisite protection of public health and welfare. The EPA is revising the PSD permitting regulations to address the changes in the PM NAAQS. In addition, the EPA is updating the AQI for PM 2.5 and making changes in the data handling conventions for PM and ambient air monitoring, reporting, and network design requirements to correspond with the changes to the PM NAAQS.

B. Summary of Major Provisions

With regard to the primary standards for fine particles, the EPA is revising the annual PM 2.5 standard by lowering the level from 15.0 to 12.0 μg/m [3] so as to provide increased protection against health effects associated with long-and short-term exposures. The EPA is retaining the level (35 μg/m [3] ) and the form (98th percentile) of the 24-hour PM 2.5 standard to continue to provide supplemental protection against health effects associated with short-term exposures. This action provides increased protection for children, older adults, persons with pre-existing heart and lung disease, and other at-risk populations against an array of PM 2.5-related adverse health effects that include premature mortality, increased hospital admissions and emergency department visits, and development of chronic respiratory disease. The EPA also is eliminating spatial averaging provisions as part of the form of the annual standard to avoid potential disproportionate impacts on at-risk populations.

The final decisions for the primary annual and 24-hour PM 2.5 standards are within the ranges that CASAC advised the Agency to consider. These decisions are based on an integrative assessment of an extensive body of new scientific evidence, which substantially strengthens what was known about PM 2.5-related health effects in the last review, including extended analyses of key epidemiological studies, and evidence of health effects observed at lower ambient PM 2.5 concentrations, including effects in areas that likely met the current standards. The revised suite of PM 2.5 standards also reflects consideration of a quantitative risk assessment that estimates public health risks likely to remain upon just meeting the current and various alternative standards. Based on this information, the Administrator concludes that the current primary PM 2.5 standards are not requisite to protect public health with an adequate margin of safety, as required by the CAA, and that these revisions are warranted to provide the appropriate degree of increased public health protection.

With regard to the primary standard for thoracic coarse particles (PM 10-2.5), the EPA is retaining the current 24-hour PM 10 standard, with a level of 150 μg/m [3] and a one-expected exceedance form, to continue to provide protection against effects associated with short-term exposure to PM 10-2.5 including premature mortality and increased hospital admissions and emergency department visits. In reaching this decision, the Administrator concludes that the available health evidence and air quality information for PM 10-2.5, taken together with the considerable uncertainties and limitations associated with that information, suggests that a standard is needed to protect against short-term exposure to all types of PM 10-2.5 and that the degree of public health protection provided against short-term exposures to PM 10-2.5 does not need to be increased beyond that provided by the current PM 10 standard.

With regard to the secondary PM standards, the Administrator is retaining the current suite of secondary PM standards, except for a change to the form of the annual PM 2.5 standard. Specifically, the EPA is retaining the current secondary 24-hour PM 2.5 and PM 10 standards, and is revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard. This suite of secondary standards addresses PM-related non-visibility welfare effects including ecological effects, effects on materials, and climate impacts. With respect to PM-related visibility impairment, the Administrator has identified a target degree of protection, defined in terms of a PM 2.5 visibility index (based on speciated PM 2.5 mass concentrations and relative humidity data to calculate PM 2.5 light extinction), a 24-hour averaging time, and a 90th percentile form, averaged over 3 years, and a level of 30 deciviews (dv), which she judges to be requisite to protect public welfare with regard to visual air quality (VAQ). The EPA's analysis of monitoring data provides the basis for concluding that the current secondary 24-hour PM 2.5 standard would provide sufficient protection, and in some areas greater protection, relative to this target protection level. Adding a distinct secondary standard to address visibility would not affect this protection. Since sufficient protection from visibility impairment will be provided for all areas of the country without adoption of a distinct secondary standard, and adoption of a distinct secondary standard will not change the degree of over-protection of VAQ provided for some areas of the country by the secondary 24-hour PM 2.5 standard, the Administrator judges that adoption of a distinct secondary standard, in addition to the current suite of secondary standards, is not needed to provide requisite protection for both visibility and non-visibility related welfare effects.

The revisions to the PM NAAQS trigger a process under which states (and tribes, if they choose) will make recommendations to the Administrator regarding designations, identifying areas of the country that either meet or do not meet the revised NAAQS. States will also review, modify and supplement their existing state implementation plans (SIPs), as needed. With regard to these implementation-related activities, the EPA intends to promulgate a separate implementation rule on a schedule that provides timely clarity to the states, tribes, and other parties responsible for NAAQS implementation. The NAAQS revisions also affect the applicable air permitting requirement, but cause no significant change to the transportation conformity and general conformity processes. The EPA is revising its PSD regulations to provide limited grandfathering from the requirements that result from the revised PM NAAQS.

On other topics, the EPA is changing the AQI for PM 2.5 to be consistent with the revised primary PM 2.5 NAAQS. The EPA also is revising the data handling procedures for PM 2.5 consistent with the revised PM 2.5 NAAQS including the computations necessary for determining when the standards are met and the measurement data that are appropriate for comparison to the standards. With regard to monitoring-related activities, the EPA is updating several aspects of the monitoring regulations and specifically requiring that a small number of PM 2.5 monitors be relocated to be collocated with measurements of other pollutants (e.g., nitrogen dioxide, carbon monoxide) in the near-road environment.

C. Costs and Benefits

In setting the NAAQS, the EPA may not consider the costs of implementing the standards. This was confirmed by the United States Supreme Court in Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001), as noted in section II.A of this rule. As has traditionally been done in NAAQS rulemaking, the EPA has conducted a Regulatory Impact Analysis (RIA) to provide the public with information on the potential costs and benefits of attaining several alternative PM 2.5 standards. In NAAQS rulemaking, the RIA is done for informational purposes only, and the final decisions on the NAAQS in this rulemaking are not in any way based on consideration of the information or analyses in the RIA. The RIA fulfills the requirements of Executive Orders 13563 and 12866. The summary of the RIA, which is discussed in more detail below in section X.A, estimates benefits ranging from $4,000 million to $9,100 million at a 3 percent discount rate and $3,600 million to $8,200 million at a 7 percent discount rate in 2020 and costs ranging from $53 million to $350 million per year at a 7 percent discount rate.

II. Background Back to Top

A. Legislative Requirements

Two sections of the CAA govern the establishment, review and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to identify and list certain air pollutants and then to issue air quality criteria for those pollutants. The Administrator is to list those air pollutants that in her “judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare;” “the presence of which in the ambient air results from numerous or diverse mobile or stationary sources;” and “for which * * * [the Administrator] plans to issue air quality criteria * * *” Air quality criteria are intended to “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of [a] pollutant in the ambient air * * *” 42 U.S.C. 7408(b). Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate “primary” and “secondary” NAAQS for pollutants for which air quality criteria are issued. Section 109(b)(1) defines a primary standard as one “the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health.” [1] A secondary standard, as defined in section 109(b)(2), must “specify a level of air quality the attainment and maintenance of which, in the judgment of the Administrator, based on such criteria, is requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of [the] pollutant in the ambient air.” [2]

The requirement that primary standards provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information available at the time of standard setting. It was also intended to provide a reasonable degree of protection against hazards that research has not yet identified. See Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980); American Petroleum Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981); American Farm Bureau Federation v. EPA, 559 F. 3d 512, 533 (D.C. Cir. 2009); Association of Battery Recyclers v. EPA, 604 F. 3d 613, 617-18 (D.C. Cir. 2010). Both kinds of uncertainties are components of the risk associated with pollution at levels below those at which human health effects can be said to occur with reasonable scientific certainty. Thus, in selecting primary standards that provide an adequate margin of safety, the Administrator is seeking not only to prevent pollution levels that have been demonstrated to be harmful but also to prevent lower pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree. The CAA does not require the Administrator to establish a primary NAAQS at a zero-risk level or at background concentration levels, see Lead Industries v. EPA, 647 F.2d at 1156 n.51, but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety.

In addressing the requirement for an adequate margin of safety, the EPA considers such factors as the nature and severity of the health effects involved, the size of at-risk population(s), and the kind and degree of the uncertainties that must be addressed. The selection of any particular approach to providing an adequate margin of safety is a policy choice left specifically to the Administrator's judgment. See Lead Industries Association v. EPA, 647 F.2d at 1161-62; Whitman v. American Trucking Associations, 531 U.S. 457, 495 (2001).

In setting standards that are “requisite” to protect public health and welfare, as provided in section 109(b), the EPA's task is to establish standards that are neither more nor less stringent than necessary for these purposes. In so doing, the EPA may not consider the costs of implementing the standards. See generally, Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant considerations in the promulgation of national ambient air quality standards.”American Petroleum Institute v. Costle, 665 F. 2d at 1185.

Section 109(d)(1) requires that “not later than December 31, 1980, and at 5-year intervals thereafter, the Administrator shall complete a thorough review of the criteria published under section 108 and the national ambient air quality standards * * * and shall make such revisions in such criteria and standards and promulgate such new standards as may be appropriate * * *” Section 109(d)(2) requires that an independent scientific review committee “shall complete a review of the criteria * * * and the national primary and secondary ambient air quality standards * * * and shall recommend to the Administrator any new * * * standards and revisions of existing criteria and standards as may be appropriate. * * *” Since the early 1980's, this independent review function has been performed by the CASAC. [3]

B. Review of the Air Quality Criteria and Standards for PM

1. Previous PM NAAQS Reviews

The EPA initially established NAAQS for PM under section 109 of the CAA in 1971. Since then, the Agency has made a number of changes to these standards to reflect continually expanding scientific information, particularly with respect to the selection of indicator [4] and level. Table 1 provides a summary of the PM NAAQS that have been promulgated to date. These decisions are briefly discussed below.

Table 1—Summary of National Ambient Air Quality Standards Promulgated for PM 1971-2006 a Back to Top
Final rule Indicator Averaging time Level Form
aWhen not specified, primary and secondary standards are identical.
bThe level of the 24-hour standard is defined as an integer (zero decimal places) as determined by rounding. For example, a 3-year average 98th percentile concentration of 35.49 μg/m3would round to 35 μg/m3and thus meet the 24-hour standard and a 3-year average of 35.50 μg/m3would round to 36 and, hence, violate the 24-hour standard (40 CFR part 50, appendix N).
cThe level of the annual standard is defined to one decimal place (i.e., 15.0 μg/m3) as determined by rounding. For example, a 3-year average annual mean of 15.04 μg/m3would round to 15.0 μg/m3and, thus, meet the annual standard and a 3-year average of 15.05 μg/m3would round to 15.1 μg/m3and, hence, violate the annual standard (40 CFR part 50, appendix N).
dThe level of the standard was to be compared to measurements made at sites that represent “community-wide air quality” recording the highest level, or, if specific requirements were satisfied, to average measurements from multiple community-wide air quality monitoring sites (“spatial averaging”).
eThe EPA tightened the constraints on the spatial averaging criteria by further limiting the conditions under which some areas may average measurements from multiple community-oriented monitors to determine compliance (See 71 FR 61165 to 61167, October 17, 2006).
1971—36 FR 8186 April 30, 1971 TSP 24-hour 260 μg/m3(primary) 150 μg/m3 (secondary) Not to be exceeded more than once per year.
Annual 75 μg/m3 (primary) Annual average.
1987—52 FR 24634 July 1, 1987 PM 10 24-hour 150 μg/m3 Not to be exceeded more than once per year on average over a 3-year period.
Annual 50 μg/m3 Annual arithmetic mean, averaged over 3 years.
1997—62 FR 38652 July 18, 1997 PM 2.5 24-hour 65 μg/m3 98th percentile, averaged over 3 years.b
Annual 15.0 μg/m3 Annual arithmetic mean, averaged over 3 years. c d
PM 10 24-hour 150 μg/m3 Initially promulgated 99th percentile, averaged over 3 years; when 1997 standards for PM 10 were vacated, the form of 1987 standards remained in place (not to be exceeded more than once per year on average over a 3-year period).
Annual 50 μg/m3 Annual arithmetic mean, averaged over 3 years.
2006—71 FR 61144 October 17, 2006 PM 2.5 24-hour Annual 35 μg/m3 15.0 μg/m3 98th percentile, averaged over 3 years.b Annual arithmetic mean, averaged over 3 years. c e
PM 10 24-hour 150 μg/m3 Not to be exceeded more than once per year on average over a 3-year period.

In 1971, the EPA established NAAQS for PM based on the original air quality criteria document (DHEW, 1969; 36 FR 8186, April 30, 1971). The reference method specified for determining attainment of the original standards was the high-volume sampler, which collects PM up to a nominal size of 25 to 45 μm (referred to as total suspended particles or TSP). The primary standards (measured by the indicator TSP) were 260 μg/m [3] , 24-hour average, not to be exceeded more than once per year, and 75 μg/m [3] , annual geometric mean. The secondary standard was 150 μg/m [3] , 24-hour average, not to be exceeded more than once per year.

In October 1979, the EPA announced the first periodic review of the criteria and NAAQS for PM, and significant revisions to the original standards were promulgated in 1987 (52 FR 24634, July 1, 1987). In that decision, the EPA changed the indicator for PM from TSP to PM 10, the latter including particles with an aerodynamic diameter less than or equal to a nominal 10 µm, which delineates thoracic particles (i.e., that subset of inhalable particles small enough to penetrate beyond the larynx to the thoracic region of the respiratory tract). The EPA also revised the primary standards by (1) replacing the 24-hour TSP standard with a 24-hour PM 10 standard of 150 μg/m [3] with no more than one expected exceedance per year and (2) replacing the annual TSP standard with a PM 10 standard of 50 μg/m [3] , annual arithmetic mean. The secondary standard was revised by replacing it with 24-hour and annual PM 10 standards identical in all respects to the primary standards. The revisions also included a new reference method for the measurement of PM 10 in the ambient air and rules for determining attainment of the new standards. On judicial review, the revised standards were upheld in all respects. Natural Resources Defense Council v. EPA, 902 F. 2d 962 (D.C. Cir. 1990).

In April 1994, the EPA announced its plans for the second periodic review of the criteria and NAAQS for PM, and promulgated significant revisions to the NAAQS in 1997 (62 FR 38652, July 18, 1997). Most significantly, the EPA determined that although the PM NAAQS should continue to focus on thoracic particles (PM 10), the fine and coarse fractions of PM 10 should be considered separately. New standards were added, using PM 2.5 as the indicator for fine particles. The PM 10 standards were retained for the purpose of regulating the coarse fraction of PM 10 (referred to as thoracic coarse particles or PM 10-2.5). [5] The EPA established two new PM 2.5 standards: an annual standard of 15.0 μg/m [3] , based on the 3-year average of annual arithmetic mean PM 2.5 concentrations from single or multiple monitors sited to represent community-wide air quality [6] and a 24-hour standard of 65 μg/m [3] , based on the 3-year average of the 98th percentile of 24-hour PM 2.5 concentrations at each population-oriented monitor [7] within an area. Also, the EPA established a new reference method for the measurement of PM 2.5 in the ambient air and rules for determining attainment of the new standards. To continue to address thoracic coarse particles, the annual PM 10 standard was retained, while the form, but not the level, of the 24-hour PM 10 standard was revised to be based on the 99th percentile of 24-hour PM 10 concentrations at each monitor in an area. The EPA revised the secondary standards by making them identical in all respects to the primary standards.

Following promulgation of the revised PM NAAQS in 1997, petitions for review were filed by a large number of parties, addressing a broad range of issues. In May 1998, a three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit issued an initial decision that upheld the EPA's decision to establish fine particle standards, holding that “the growing empirical evidence demonstrating a relationship between fine particle pollution and adverse health effects amply justifies establishment of new fine particle standards.”American Trucking Associations v. EPA, 175 F. 3d 1027, 1055-56 (D.C. Cir. 1999), rehearing granted in part and denied in part, 195 F. 3d 4 (D.C. Cir. 1999), affirmed in part and reversed in part, Whitman v. American Trucking Associations, 531 U.S. 457 (2001). The panel also found “ample support” for the EPA's decision to regulate coarse particle pollution, but vacated the 1997 P.M. 10 standards, concluding, in part, that PM 10 is a “poorly matched indicator for coarse particulate pollution” because it includes fine particles. Id. at 1053-55. Pursuant to the court's decision, the EPA removed the vacated 1997 P.M. 10 standards from the CFR (69 FR 45592, July 30, 2004) and deleted the regulatory provision (at 40 CFR 50.6(d)) that controlled the transition from the pre-existing 1987 P.M. 10 standards to the 1997 P.M. 10 standards. The pre-existing 1987 P.M. 10 standards remained in place (65 FR 80776, December 22, 2000). The court also upheld the EPA's determination not to establish more stringent secondary standards for fine particles to address effects on visibility (175 F. 3d at 1027).

More generally, the panel held (over a strong dissent) that the EPA's approach to establishing the level of the standards in 1997, both for the PM and for the ozone NAAQS promulgated on the same day, effected “an unconstitutional delegation of legislative authority.”Id. at 1034-40. Although the panel stated that “the factors EPA uses in determining the degree of public health concern associated with different levels of ozone and PM are reasonable,” it remanded the rule to the EPA, stating that when the EPA considers these factors for potential non-threshold pollutants “what EPA lacks is any determinate criterion for drawing lines” to determine where the standards should be set. Consistent with the EPA's long-standing interpretation and D.C. Circuit precedent, the panel also reaffirmed its prior holdings that in setting NAAQS, the EPA is “not permitted to consider the cost of implementing those standards.”Id. at 1040-41.

On EPA's petition for rehearing, the panel adhered to its position on these points. American Trucking Associations v. EPA, 195 F. 3d 4 (D.C. Cir. 1999). The full Court of Appeals denied the EPA's request for rehearing en banc, with five judges dissenting. Id. at 13. Both sides filed cross appeals on these issues to the United States Supreme Court, which granted certiorari. In February 2001, the Supreme Court issued a unanimous decision upholding the EPA's position on both the constitutional and cost issues. Whitman v. American Trucking Associations, 531 U.S. 457, 464, 475-76. On the constitutional issue, the Court held that the statutory requirement that NAAQS be “requisite” to protect public health with an adequate margin of safety sufficiently cabined the EPA's discretion, affirming the EPA's approach of setting standards that are neither more nor less stringent than necessary. The Supreme Court remanded the case to the Court of Appeals for resolution of any remaining issues that had not been addressed in that court's earlier rulings. Id. at 475-76. In March 2002, the Court of Appeals rejected all remaining challenges to the standards, holding under the statutory standard of review that the EPA's PM 2.5 standards were reasonably supported by the administrative record and were not “arbitrary and capricious.”American Trucking Association v. EPA, 283 F. 3d 355, 369-72 (D.C. Cir. 2002).

In October 1997, the EPA published its plans for the next periodic review of the air quality criteria and NAAQS for PM (62 FR 55201, October 23, 1997). After CASAC and public review of several drafts, the EPA's National Center for Environmental Assessment (NCEA) finalized the Air Quality Criteria Document for Particulate Matter (henceforth, AQCD or the “Criteria Document”) in October 2004 (U.S. EPA, 2004) and OAQPS finalized an assessment document, Particulate Matter Health Risk Assessment for Selected Urban Areas (Abt Associates, 2005), and the Review of the National Ambient Air Quality Standards for Particulate Matter: Policy Assessment of Scientific and Technical Information, in December 2005 (henceforth, “Staff Paper,” U.S. EPA, 2005). In conjunction with its review of the Staff Paper, CASAC provided advice to the Administrator on revisions to the PM NAAQS (Henderson, 2005a). In particular, most CASAC PM Panel members favored revising the level of the primary 24-hour PM 2.5 standard within the range of 35 to 30 μg/m [3] with a 98th percentile form, in concert with revising the level of the primary annual PM 2.5 standard within the range of 14 to 13 μg/m [3] (Henderson, 2005a, p.7). For thoracic coarse particles, the Panel had reservations in recommending a primary 24-hour PM 10-2.5 standard, and agreed that there was a need for more research on the health effects of thoracic coarse particles (Henderson, 2005b). With regard to secondary standards, most Panel members strongly supported establishing a new, distinct secondary PM 2.5 standard to protect urban visibility (Henderson, 2005a, p. 9).

On January 17, 2006, the EPA proposed to revise the primary and secondary NAAQS for PM (71 FR 2620) and solicited comment on a broad range of options. Proposed revisions included: (1) Revising the level of the primary 24-hour PM 2.5 standard to 35 μg/m [3] ; (2) revising the form, but not the level, of the primary annual PM 2.5 standard by tightening the constraints on the use of spatial averaging; (3) replacing the primary 24-hour PM 10 standard with a 24-hour standard defined in terms of a new indicator, PM 10-2.5, which was qualified so as to include any ambient mix of PM 10-2.5 dominated by particles generated by high-density traffic on paved roads, industrial sources, and construction sources, and to exclude any ambient mix of particles dominated by rural windblown dust and soils and agricultural and mining sources (71 FR 2667 to 2668), set at a level of 70 μg/m [3] based on the 3-year average of the 98th percentile of 24-hour PM 10-2.5 concentrations; (4) revoking the primary annual PM 10 standard; and (5) revising the secondary standards by making them identical in all respects to the proposed suite of primary standards for fine and coarse particles. [8] Subsequent to the proposal, CASAC provided additional advice to the EPA in a letter to the Administrator requesting reconsideration of CASAC's recommendations for both the primary and secondary PM 2.5 standards as well as the standards for thoracic coarse particles (Henderson, 2006a).

On October 17, 2006, the EPA published revisions to the PM NAAQS to provide increased protection of public health and welfare (71 FR 61144). With regard to the primary and secondary standards for fine particles, the EPA revised the level of the primary 24-hour PM 2.5 standard to 35 μg/m [3] , retained the level of the primary annual PM 2.5 standard at 15.0 μg/m [3] , and revised the form of the primary annual PM 2.5 standard by adding further constraints on the optional use of spatial averaging. The EPA revised the secondary standards for fine particles by making them identical in all respects to the primary standards. With regard to the primary and secondary standards for thoracic coarse particles, the EPA retained the level and form of the 24-hour PM 10 standard (such that the standard remained at a level of 150 μg/m [3] with a one-expected exceedance form and retained the PM 10 indicator) and revoked the annual PM 10 standard. The EPA also established a new Federal Reference Method (FRM) for the measurement of PM 10-2.5 in the ambient air (71 FR 61212 to 13). Although the standards for thoracic coarse particles were not defined in terms of a PM 10-2.5 indicator, the EPA adopted a new FRM for PM 10-2.5 to facilitate consistent research on PM 10-2.5 air quality and health effects and to promote commercial development of Federal Equivalent Methods (FEMs) to support future reviews of the PM NAAQS (71 FR 61212/2).

Following issuance of the final rule, CASAC articulated its concern that the “EPA's final rule on the NAAQS for PM does not reflect several important aspects of the CASAC's advice” (Henderson et al., 2006b, p. 1). With regard to the primary PM 2.5 annual standard, CASAC expressed serious concerns regarding the decision to retain the level of the standard at 15 μg/m [3] . Specifically, CASAC stated, “It is the CASAC's consensus scientific opinion that the decision to retain without change the annual PM 2.5 standard does not provide an `adequate margin of safety * * * requisite to protect the public health' (as required by the Clean Air Act), leaving parts of the population of this country at significant risk of adverse health effects from exposure to fine PM” (Henderson et al., 2006b, p. 2). Furthermore, CASAC pointed out that its recommendations “were consistent with the mainstream scientific advice that EPA received from virtually every major medical association and public health organization that provided their input to the Agency” (Henderson et al., 2006b, p. 2). [9] With regard to EPA's final decision to retain the 24-hour PM 10 standard for thoracic coarse particles, CASAC had mixed views with regard to the decision to retain the 24-hour standard and the continued use of PM 10 as the indicator of coarse particles, while also recognizing the need to have a standard in place to protect against effects associated with short-term exposures to thoracic coarse particles (Henderson et al., 2006b, p. 2). With regard to the EPA's final decision to revise the secondary PM 2.5 standards to be identical in all respects to the revised primary PM 2.5 standards, CASAC expressed concerns that its advice to establish a distinct secondary standard for fine particles to address visibility impairment was not followed and emphasized “that continuing to rely on the primary standard to protect against all PM-related adverse environmental and welfare effects assures neglect, and will allow substantial continued degradation, of visual air quality over large areas of the country” (Henderson et al, 2006b, p. 2).

2. Litigation Related to the 2006 PM Standards

Several parties filed petitions for review following promulgation of the revised PM NAAQS in 2006. These petitions addressed the following issues: (1) Selecting the level of the primary annual PM 2.5 standard; (2) retaining PM 10 as the indicator of a standard for thoracic coarse particles, retaining the level and form of the 24-hour PM 10 standard, and revoking the PM 10 annual standard; and (3) setting the secondary PM 2.5 standards identical to the primary standards. On February 24, 2009, the U.S. Court of Appeals for the District of Columbia Circuit issued its opinion in the case American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). The court remanded the primary annual PM 2.5 NAAQS to the EPA because the EPA failed to adequately explain why the standard provided the requisite protection from both short- and long-term exposures to fine particles, including protection for at-risk populations such as children. American Farm Bureau Federation v. EPA, 559 F. 3d 512, 520-27 (D.C. Cir. 2009). With regard to the standards for PM 10, the court upheld the EPA's decisions to retain the 24-hour PM 10 standard to provide protection from thoracic coarse particle exposures and to revoke the annual PM 10 standard. American Farm Bureau Federation v. EPA, 559 F. 2d at 533-38. With regard to the secondary PM 2.5 standards, the court remanded the standards to the EPA because the Agency's decision was “unreasonable and contrary to the requirements of section 109(b)(2)” of the CAA. The court further concluded that the EPA failed to adequately explain why setting the secondary PM standards identical to the primary standards provided the required protection for public welfare, including protection from visibility impairment. American Farm Bureau Federation v. EPA, 559 F. 2d at 528-32.

The decisions of the court with regard to these three issues are discussed further in sections III.A.2, IV.A.2, and VI.A.2 below. The EPA is responding to the court's remands as part of the current review of the PM NAAQS.

3. Current PM NAAQS Review

The EPA initiated the current review of the air quality criteria for PM in June 2007 with a general call for information (72 FR 35462, June 28, 2007). In July 2007, the EPA held two “kick-off” workshops on the primary and secondary PM NAAQS, respectively (72 FR 34003 to 34004, June 20, 2007). [10] These workshops provided an opportunity for a public discussion of the key policy-relevant issues around which the EPA would structure this PM NAAQS review and the most meaningful new science that would be available to inform our understanding of these issues.

Based in part on the workshop discussions, the EPA developed a draft Integrated Review Plan outlining the schedule, process, and key policy-relevant questions that would guide the evaluation of the air quality criteria for PM and the review of the primary and secondary PM NAAQS (U.S. EPA, 2007a). On November 30, 2007, the EPA held a consultation with CASAC on the draft Integrated Review Plan (72 FR 63177, November 8, 2007), which included the opportunity for public comment. The final Integrated Review Plan (U.S. EPA, 2008a) incorporated comments from CASAC (Henderson, 2008) and the public on the draft plan as well as input from senior Agency managers. 11 12

A major element in the process for reviewing the NAAQS is the development of an Integrated Science Assessment. This document provides a concise evaluation and integration of the policy-relevant science, including key science judgments upon which the risk and exposure assessments build. As part of the process of preparing the PM Integrated Science Assessment, NCEA hosted a peer review workshop in June 2008 on preliminary drafts of key Integrated Science Assessment chapters (73 FR 30391, May 27, 2008). CASAC and the public reviewed the first external review draft Integrated Science Assessment (U.S. EPA, 2008b; 73 FR 77686, December 19, 2008) at a meeting held on April 1 to 2, 2009 (74 FR 2688, February 19, 2009). Based on CASAC (Samet, 2009e) and public comments, NCEA prepared a second draft Integrated Science Assessment (U.S. EPA, 2009b; 74 FR 38185, July 31, 2009), which was reviewed by CASAC and the public at a meeting held on October 5 and 6, 2009 (74 FR 46586, September 10, 2009). Based on CASAC (Samet, 2009f) and public comments, NCEA prepared the final Integrated Science Assessment titled Integrated Science Assessment for Particulate Matter, December 2009 (U.S. EPA, 2009a; 74 FR 66353, December 15, 2009).

Building upon the information presented in the PM Integrated Science Assessment, the EPA prepared Risk and Exposure Assessments that provide a concise presentation of the methods, key results, observations, and related uncertainties. In developing the Risk and Exposure Assessments for this PM NAAQS review, OAQPS released two planning documents: Particulate Matter National Ambient Air Quality Standards: Scope and Methods Plan for Health Risk and Exposure Assessment and Particulate Matter National Ambient Air Quality Standards: Scope and Methods Plan for Urban Visibility Impact Assessment (henceforth, Scope and Methods Plans, U.S. EPA, 2009c,d; 74 FR 11580, March 18, 2009). These planning documents outlined the scope and approaches that staff planned to use in conducting quantitative assessments as well as key issues that would be addressed as part of the assessments. In designing and conducting the initial health risk and visibility impact assessments, the Agency considered CASAC comments (Samet 2009a,b) on the Scope and Methods Plans made during an April 2009 consultation (74 FR 7688, February 19, 2009) as well as public comments. CASAC and the public reviewed two draft assessment documents, Risk Assessment to Support the Review of the PM 2.5 Primary National Ambient Air Quality Standards: External Review Draft, September 2009 (U.S. EPA, 2009e) and Particulate Matter Urban-Focused Visibility Assessment—External Review Draft, September 2009 (U.S. EPA, 2009f) at a meeting held on October 5 and 6, 2009 (74 FR 46586, September 10, 2009). Based on CASAC (Samet 2009c,d) and public comments, OAQPS staff revised these draft documents and released second draft assessment documents (U.S. EPA, 2010d,e) in January and February 2010 (75 FR 4067, January 26, 2010) for CASAC and public review at a meeting held on March 10 and 11, 2010 (75 FR 8062, February 23, 2010). Based on CASAC (Samet, 2010a,b) and public comments on the second draft assessment documents, the EPA revised these documents and released final assessment documents titled Quantitative Health Risk Assessment for Particulate Matter, June 2010 (henceforth, “Risk Assessment,” U.S. EPA, 2010a) and Particulate Matter Urban-Focused Visibility Assessment—Final Document, July 2010 (henceforth, “Visibility Assessment,” U.S. EPA, 2010b) (75 FR 39252, July 8, 2010).

Based on the scientific and technical information available in this review as assessed in the Integrated Science Assessment and Risk and Exposure Assessments, the EPA staff prepared a Policy Assessment. The Policy Assessment is intended to help “bridge the gap” between the relevant scientific information and assessments and the judgments required of the Administrator in reaching decisions on the NAAQS (Jackson, 2009, attachment, p. 2). American Farm Bureau Federation v. EPA, 559 F. 3d at 521. The Policy Assessment is not a decision document; rather it presents the EPA staff conclusions related to the broadest range of policy options that could be supported by the currently available information. A preliminary draft Policy Assessment (U.S. EPA, 2009g) was released in September 2009 for informational purposes and to facilitate discussion with CASAC at the October 5 and 6, 2009 meeting on the overall structure, areas of focus, and level of detail to be included in the Policy Assessment. The EPA considered CASAC's comments on this preliminary draft in developing a first draft Policy Assessment (U.S. EPA, 2010c; 75 FR 4067, January 26, 2010) that built upon the information presented and assessed in the final Integrated Science Assessment and second draft Risk and Exposure Assessments. The EPA presented an overview of the first draft Policy Assessment at a CASAC meeting on March 10, 2010 (75 FR 8062, February 23, 2010) and it was discussed during public CASAC teleconferences on April 8 and 9, 2010 (75 FR 8062, February 23, 2010) and May 7, 2010 (75 FR 19971, April 16, 2010).

The EPA developed a second draft Policy Assessment (U.S. EPA, 2010f; 75 FR 39253, July 8, 2010) based on CASAC (Samet, 2010c) and public comments on the first draft Policy Assessment. CASAC reviewed the second draft document at a meeting on July 26 and 27, 2010 (75 FR 32763, June 9, 2010). The EPA staff considered CASAC (Samet, 2010d) and public comments on the second draft Policy Assessment in preparing a final Policy Assessment titled Policy Assessment for the Review of the Particulate Matter National Ambient Air Quality Standards, April, 2011 (U.S. EPA, 2011a; 76, FR 22665, April 22, 2011). This document includes final staff conclusions on the adequacy of the current PM standards and alternative standards for consideration.

The schedule for the rulemaking in this review is subject to a court order in a lawsuit filed in February 2012 by a group of plaintiffs who alleged that the EPA had failed to perform its mandatory duty, under section 109(d)(1), to complete a review of the PM NAAQS within the period provided by statute. American Lung Association and National Parks Conservation Association v. EPA, D.D.C. No. 12-cv-00243 (consol. with No. 12-cv-00531) Court orders in that case provide that the EPA sign a notice of proposed rulemaking concerning its review of the PM NAAQS no later than June 14, 2012 and a notice of final rulemaking no later than December 14, 2012.

On June 14, 2012, the EPA issued its proposed decision to revise the NAAQS for PM (77 FR 38890, June 29, 2012) (henceforth “proposal”). In the proposal, the EPA identified revisions to the standards, based on the air quality criteria for PM, and to related data handling conventions and ambient air monitoring, reporting, and network design requirements. The EPA proposed revisions to the PSD permitting program with respect to the proposed NAAQS revisions. The Agency also proposed changes to the AQI for PM 2.5, consistent with the proposed primary PM 2.5 standards. The proposal solicited public comments on alternative primary and secondary standards and related matters. The proposal is summarized in section II.D below.

The EPA held two public hearings to receive public comment on the proposed revisions to the PM NAAQS (77 FR 39205, July 2, 2012). One hearing took place in Philadelphia, PA on July 17, 2012 and a second hearing took place in Sacramento, CA on July 19, 2012. At these public hearings, the EPA heard testimony from 168 individuals representing themselves or specific interested organizations.

The EPA received more than 230,000 comments from members of the public and various interest groups on the proposed revisions to the PM NAAQS by the close of the public comment period on August 31, 2012. Major issues raised in the public comments are discussed throughout the preamble of this final action. A more detailed summary of all significant comments, along with the EPA's responses (henceforth “Response to Comments”) can be found in the docket for this rulemaking (Docket No. EPA-HQ-OAR-2007-0492) (U.S. EPA, 2012a).

In the proposal, the EPA recognized that there were a number of new scientific studies on the health effects of PM that had been published since the mid-2009 cutoff date for inclusion in the Integrated Science Assessment. [13] As in the last PM NAAQS review, the EPA committed to conduct a provisional review and assessment of any significant “new” studies published since the close of the Integrated Science Assessment, including studies submitted to the EPA during the public comment period. The purpose of the provisional science assessment was to ensure that the Administrator was fully aware of the “new” science that has developed since 2009 before making final decisions on whether to retain or revise the current PM NAAQS. The EPA screened and surveyed the recent health literature, including studies submitted during the public comment period, and conducted a provisional assessment (U.S. EPA, 2012b) that places the results of those studies of potentially greatest policy relevance in the context of the findings of the Integrated Science Assessment (U.S. EPA, 2009a). This provisional assessment, including a summary of the key conclusions, can be found in the rulemaking docket (EPA-HQ-OAR-2007-0492).

The provisional assessment found that the “new” studies expand the scientific information considered in the Integrated Science Assessment and provide important insights on the relationship between PM exposure and health effects. The provisional assessment also found that the “new” studies generally strengthen the evidence that long- and short-term exposures to fine particles are associated with a wide range of health effects. Some of the “new” epidemiological studies report effects in areas with lower PM 2.5-concentrations than those in earlier studies considered in the Integrated Science Assessment. “New” toxicological and epidemiological studies continue to link various health effects with a range of fine particle sources and components. With regard to thoracic coarse particles, the provisional assessment recognized that a limited number of “new” studies provide evidence of an association with short-term PM 10-2.5 exposures and increased asthma-related emergency department visits in children, but continue to provide no evidence of an association between long-term PM 10-2.5 exposure and mortality. Further, the provisional assessment found that the results reported in “new” studies do not materially change any of the broad scientific conclusions regarding the health effects of PM exposure made in the Integrated Science Assessment.

The EPA believes it was important to conduct a provisional assessment in this proceeding, so that the Administrator would be aware of the science that developed too recently for inclusion in the Integrated Science Assessment. However, it is also important to note that the EPA's review of that science to date has been limited to screening, surveying, and preparing a provisional assessment of these studies. Having performed this limited provisional assessment, the EPA must decide whether to consider the “new” studies in this review and to take such steps as may be necessary to include them in the basis for the final decision, or to reserve such action for the next review of the PM NAAQS.

As in prior NAAQS reviews, the EPA is basing its decision in this review on studies and related information included in the Integrated Science Assessment, Risk and Exposure Assessment, and Policy Assessment, which have undergone CASAC and public review. The studies assessed in the Integrated Science Assessment, and the integration of the scientific evidence presented in that document, have undergone extensive critical review by the EPA, CASAC, and the public during the development of the Integrated Science Assessment. The rigor of that review makes these studies, and their integrative assessment, the most reliable source of scientific information on which to base decisions on the NAAQS. NAAQS decisions can have profound impacts on public health and welfare, and NAAQS decisions should be based on studies that have been rigorously assessed in an integrative manner not only by the EPA but also by the statutorily-mandated independent advisory committee, CASAC, and have been subject as well to the public review that accompanies this process. As described above, the provisional assessment did not and could not provide that kind of in-depth critical review.

This decision is consistent with the EPA's practice in prior NAAQS reviews. Since the 1970 amendments, the EPA has taken the view that NAAQS decisions are to be based on scientific studies and related information that have been assessed as a part of the pertinent air quality criteria. See e.g., 36 FR 8186 (April 30, 1971) (the EPA based original NAAQS for six pollutants on scientific studies discussed in air quality criteria documents and limited consideration of comments to those concerning validity of scientific basis); 38 FR 25678, 25679-25680 (September 14, 1973) (the EPA revised air quality criteria for sulfur oxides to provide basis for reevaluation of secondary NAAQS). This longstanding interpretation was strengthened by new legislative requirements enacted in 1977, which added section 109(d)(2) of the CAA concerning CASAC review of air quality criteria. The EPA has consistently followed this approach. 52 FR 24634, 24637 (July 1, 1987) (after review by CASAC, the EPA issued a post-proposal addendum to the PM Air Quality Criteria Document, to address certain new scientific studies not included in the 1982 Air Quality Criteria Document); 61 FR 25566, 25568 (May 22, 1996) (after review by CASAC, the EPA issued a post-proposal supplement to the 1982 Air Quality Criteria Document to address certain new health studies not included in the 1982 Air Quality Criteria Document or 1986 Addendum). The EPA reaffirmed this approach in its decision not to revise the ozone NAAQS in 1993, as well as in its final decision on the PM NAAQS in the 1997 and 2006 reviews. 58 FR 13008, 13013 to 13014 (March 9, 1993) (ozone review); 62 FR 38652, 38662 (July 18, 1997) and 71 FR 61141, 61148 to 61149 (October 17, 2006) (PM reviews) (The EPA conducted a provisional assessment but based the final PM decisions on studies and related information included in the air quality criteria that had been reviewed by CASAC).

As discussed in the EPA's 1993 decision not to revise the NAAQS for ozone, `new' studies may sometimes be of such significance that it is appropriate to delay a decision on revision of NAAQS and to supplement the pertinent air quality criteria so the “new” studies can be taken into account (58 FR, 13013 to 13014, March 9, 1993). In this proceeding, the provisional assessment of recent studies concludes that, taken in context, the “new” information and findings do not materially change any of the broad scientific conclusions regarding the health effects of PM exposure made in the Integrated Science Assessment (U.S. EPA, 2012b). For this reason, reopening the air quality criteria review would not be warranted even if there were time to do so under the court order governing the schedule for this rulemaking. Accordingly, the EPA is basing the final decisions in this review on the studies and related information included in the PM air quality criteria that have undergone CASAC and public review. The EPA will consider the “new” published studies for purposes of decision making in the next periodic review of the PM NAAQS, which will provide the opportunity to fully assess them through a more rigorous review process involving the EPA, CASAC, and the public.

C. Related Control Programs To Implement PM Standards

States are primarily responsible for ensuring attainment and maintenance of NAAQS once the EPA has established them. Under section 110 of the CAA and related provisions, states are to submit, for the EPA's approval, SIPs that provide for the attainment and maintenance of such standards through control programs directed to sources of the pollutants involved. The states, in conjunction with the EPA, also administer the PSD permitting program (CAA sections 160 to 169). In addition, federal programs provide for nationwide reductions in emissions of PM and other air pollutants through the federal motor vehicle and motor vehicle fuel control program under title II of the Act (CAA sections 202 to 250) which involves controls for emissions from mobile sources and controls for the fuels used by these sources, and new source performance standards (NSPS) for stationary sources under section 111 of the CAA.

Currently, there are 35 areas in the U.S. that are designated as nonattainment for the current annual PM 2.5 standard and 32 areas in the U.S. that are designated as nonattainment for the current 24-hour PM 2.5 standards. With the revisions to the PM NAAQS that are being finalized in this rule, the EPA will work with the states to conduct a new area designation process. Those states with new nonattainment areas will be required to develop SIPs to attain the standards. In developing their attainment plans, states will have to take into account projected emission reductions from federal and state rules that have already been adopted at the time of plan submittal. A number of significant emission reduction programs that will lead to reductions of PM and its precursors are in place today or are expected to be in place by the time any new SIPs will be due. Examples of such rules include regulations for onroad and nonroad engines and fuels, the utility and industrial boilers toxics rules, and various other programs already adopted by states to reduce emissions from key emissions sources. States will then evaluate the level of additional emission reductions needed for each nonattainment area to attain the standards “as expeditiously as practicable” and adopt new state regulations, as appropriate. Section IX includes additional discussion of designation and implementation issues associated with the revised PM NAAQS.

D. Summary of Proposed Revisions to the PM NAAQS

For reasons discussed in the proposal, the Administrator proposed to revise the current primary and secondary PM standards. With regard to the primary PM 2.5 standards, the Administrator proposed to revise the level of the annual PM 2.5 standard from 15.0 μg/m [3] to a level within a range of 12.0 to 13.0 μg/m [3] and to retain the level of the 24-hour PM 2.5 standard at 35 µg/m [3] . The Administrator also proposed to eliminate spatial averaging provisions as part of the form of the annual standard to avoid potential disproportionate impacts on at-risk populations. The EPA proposed to revise the AQI for PM 2.5, consistent with the proposed primary PM 2.5 standards.

With regard to the primary coarse particle standard, the EPA proposed to retain the current 24-hour PM 10 standard to continue to provide protection against effects associated with short-term exposure to thoracic coarse particles (i.e., PM 10-2.5).

With regard to the secondary PM standards, the EPA proposed to revise the suite of secondary PM standards by adding a distinct standard for PM 2.5 to address PM-related visibility impairment. The separate secondary standard was proposed to be defined in terms of a PM 2.5 visibility index, which would use speciated PM 2.5 mass concentrations and relative humidity data to calculate PM 2.5 light extinction, translated to the deciview (dv) scale, similar to the Regional Haze Program; a 24-hour averaging time; a 90th percentile form averaged over 3 years; and a level set at one of two options—either 30 or 28 dv. The EPA also proposed to retain the current secondary standards generally to address non-visibility welfare effects.

The EPA also proposed to revise the data handling procedures consistent with the revised primary and secondary standards for PM 2.5 including the computations necessary for determining when these standards are met and the measurement data that are appropriate for comparison to the standards. With regard to monitoring-related activities, the EPA proposed to update several aspects of the monitoring regulations and specifically to require that a small number of PM 2.5 monitors be relocated to be collocated with measurements of other pollutants (e.g., nitrogen dioxide, carbon monoxide) in the near-road environment.

E. Organization and Approach to Final PM NAAQS Decisions

This action presents the Administrator's final decisions on the review of the current primary and secondary PM 2.5 and PM 10 standards. Consistent with the decisions made by the EPA in the last review and with the conclusions in the Integrated Science Assessment and Policy Assessment, fine and thoracic coarse particles continue to be considered as separate subclasses of PM pollution. Primary standards for fine particles and for thoracic coarse particles are addressed in sections III and IV, respectively. Changes to the AQI for PM 2.5, consistent with the revised primary PM 2.5 standards, are addressed in section V. Secondary standards for fine and coarse particles are addressed in section VI. Related data handling conventions and exceptional events are addressed in section VII. Updates to the monitoring regulations are addressed in section VIII. Implementation activities, including PSD-related actions, are addressed in section IX. Section X addresses applicable statutory and executive order reviews.

Today's final decisions addressing standards for fine and coarse particles are based on a thorough review in the Integrated Science Assessment of scientific information on known and potential human health and welfare effects associated with exposure to these subclasses of PM at levels typically found in the ambient air. These final decisions also take into account: (1) Staff assessments in the Policy Assessment of the most policy-relevant information in the Integrated Science Assessment as well as a quantitative health risk assessment and urban-focused visibility assessment based on that information; (2) CASAC advice and recommendations, as reflected in its letters to the Administrator, its discussions of drafts of the Integrated Science Assessment, Risk and Exposure Assessments, and Policy Assessment at public meetings, and separate written comments prepared by individual members of the CASAC PM Review Panel; (3) public comments received during the development of these documents, both in connection with CASAC meetings and separately; and (4) extensive public comments received on the proposed rulemaking.

III. Rationale for Final Decisions on the Primary PM 2.5 Standards Back to Top

This section presents the Administrator's final decision regarding the need to revise the current primary PM 2.5 standards and, more specifically, regarding revisions to the level and form of the existing primary annual PM 2.5 standard in conjunction with retaining the existing primary 24-hour PM 2.5 standard. As discussed more fully below, the rationale for the final decision is based on a thorough review, in the Integrated Science Assessment, of the latest scientific information, published through mid-2009, on human health effects associated with long- and short-term exposures to fine particles in the ambient air. The final decisions also take into account: (1) Staff assessments of the most policy-relevant information presented and assessed in the Integrated Science Assessment and staff analyses of air quality and human risks presented in the Risk Assessment and the Policy Assessment, upon which staff conclusions regarding appropriate considerations in this review are based; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the Integrated Science Assessment, Risk Assessment, and Policy Assessment at public meetings, in separate written comments, and in CASAC's letters to the Administrator; (3) the multiple rounds of public comments received during the development of these documents, both in connection with CASAC meetings and separately; and (4) extensive public comments received on the proposal.

In developing this final rule, the Administrator recognizes that the CAA requires her to reach a public health policy judgment as to what standards would be requisite—neither more nor less stringent than necessary—to protect public health with an adequate margin of safety, based on scientific evidence and technical assessments that have inherent uncertainties and limitations. This judgment requires making reasoned decisions as to what weight to place on various types of evidence and assessments, and on the related uncertainties and limitations. Thus, in selecting the final standards, the Administrator is seeking not only to prevent fine particle concentrations that have been demonstrated to be harmful but also to prevent lower fine particle concentrations that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree.

As discussed below, as well as in more detail in the proposal, a substantial amount of new research has been conducted since the close of the science assessment in the last review of the PM 2.5 NAAQS (U.S. EPA, 2004), with important new information coming from epidemiological studies, in particular. This body of evidence includes hundreds of new epidemiological studies conducted in many countries around the world. In its assessment of the evidence judged to be most relevant to making decisions on elements of the primary PM 2.5 standards, the EPA has placed greater weight on U.S. and Canadian studies using PM 2.5 measurements, since studies conducted in other countries may reflect different demographic and air pollution characteristics. [14]

The newly available research studies as well as the earlier body of scientific evidence presented and assessed in the Integrated Science Assessment have undergone intensive scrutiny through multiple layers of peer review and opportunities for public review and comment. In developing this final rule, the EPA has drawn upon an integrative synthesis of the entire body of evidence concerning exposure to ambient fine particles and a broad range of health endpoints (U.S. EPA, 2009a, Chapters 2, 4, 5, 6, 7, and 8) focusing on those health endpoints for which the Integrated Science Assessment concludes that there is a causal or likely causal relationship with long- or short-term PM 2.5 exposures. The EPA has also considered health endpoints for which the Integrated Science Assessment concludes there is evidence suggestive of a causal relationship with long-term PM 2.5 exposures.

The EPA has also drawn upon a quantitative risk assessment based upon the scientific evidence described and assessed in the Integrated Science Assessment. These analyses, discussed in the Risk Assessment (U.S. EPA, 2010a) and Policy Assessment (U.S. EPA, 2011a, chapter 2), have also undergone intensive scrutiny through multiple layers of peer review and multiple opportunities for public review and comment.

Although important uncertainties remain in the qualitative and quantitative characterizations of health effects attributable to ambient fine particles, progress has been made in addressing these uncertainties in this review. The EPA's review of this information has been extensive and deliberate. This intensive evaluation of the scientific evidence and quantitative assessments has provided a comprehensive and adequate basis for regulatory decision making at this time.

This section describes the integrative synthesis of the evidence and technical information contained in the Integrated Science Assessment, the Risk Assessment, and the Policy Assessment with regard to the current and alternative standards. The EPA notes that the final decision for retaining or revising the current primary PM 2.5 standards is a public health policy judgment made by the Administrator. The Administrator's final decision draws upon scientific information and analyses related to health effects and risks; judgments about uncertainties that are inherent in the scientific evidence and analyses; CASAC advice; and comments received in response to the proposal.

In presenting the rationale for the final decisions on the primary PM 2.5 standards, this section begins with a summary of the approaches used in setting the initial primary PM 2.5 NAAQS in 1997 and in reviewing and revising those standards in 2006 (section III.A.1). The DC Circuit Court of Appeals remand of the primary annual PM 2.5 standard in 2009 is discussed in section III.A.2. Taking into consideration this history, section III.A.3 describes the EPA's general approach used in the current review for considering the need to retain or revise the current suite of fine particle standards, taking into account public comment on the proposed approach.

The scientific evidence and quantitative risk assessment were presented in sections III.B and III.C of the proposal, respectively (77 FR 38906 to 38917, June 29, 2012) and are outlined in sections III.B and III.C below. Subsequent sections of this preamble provide a more complete discussion of the Administrator's rationale, in light of key issues raised in public comments, for concluding that it is appropriate to revise the suite of current primary PM 2.5 standards (section III.D), as well as a more complete discussion of the Administrator's rationale for retaining or revising the specific elements of the primary PM 2.5 standards, namely the indicator (section III.E.1); averaging time (section III.E.2); form (section III.E.3); and level (section III.E.4). A summary of the final decisions to revise the suite of primary PM 2.5 standards is presented in section III.F.

A. Background

There are currently two primary PM 2.5 standards providing public health protection from effects associated with fine particle exposures. The annual standard is set at a level of 15.0 μg/m [3] , based on the 3-year average of annual arithmetic mean PM 2.5 concentrations from single or multiple monitors sited to represent community-wide air quality. The 24-hour standard is set at a level of 35 μg/m [3] , based on the 3-year average of the 98th percentile of 24-hour PM 2.5 concentrations at each population-oriented monitor within an area.

The past and current approaches for reviewing the primary PM 2.5 standards described below are all based most fundamentally on using information from epidemiological studies to inform the selection of PM 2.5 standards that, in the Administrator's judgment, protect public health with an adequate margin of safety. Such information can be in the form of air quality distributions over which health effect associations have been observed in scientific studies or in the form of concentration-response functions that support quantitative risk assessment. However, evidence- and risk-based approaches using information from epidemiological studies to inform decisions on PM 2.5 standards are complicated by the recognition that no population threshold, below which it can be concluded with confidence that PM 2.5-related effects do not occur, can be discerned from the available evidence. [15] As a result, any general approach to reaching decisions on what standards are appropriate necessarily requires judgments about how to translate the information available from the epidemiological studies into a basis for appropriate standards. This includes consideration of how to weigh the uncertainties in the reported associations across the distributions of PM 2.5 concentrations in the studies and the uncertainties in quantitative estimates of risk, in the context of the entire body of evidence before the Agency. Such approaches are consistent with setting standards that are neither more nor less stringent than necessary, recognizing that a zero-risk standard is not required by the CAA.

1. General Approach Used in Previous Reviews

The general approach used to translate scientific information into standards in the previous PM NAAQS reviews focused on consideration of alternative standard levels that were somewhat below the long-term mean PM 2.5 concentrations reported in key epidemiological studies (U.S. EPA, 2011a, section 2.1.1). This approach recognized that the strongest evidence of PM 2.5-related associations occurs where the bulk of the data exists, which is over a range of concentrations around the long-term (i.e., annual) mean.

In setting primary PM 2.5 annual and 24-hour standards for the first time in 1997, the Agency relied primarily on an evidence-based approach that focused on epidemiological evidence, especially from short-term exposure studies of fine particles judged to be the strongest evidence at that time (U.S. EPA, 2011a, section 2.1.1.1). The EPA selected a level for the annual standard that was at or below the long-term mean PM 2.5 concentrations in studies providing evidence of associations with short-term PM 2.5 exposures, placing greatest weight on those short-term exposure studies that reported clearly statistically significant associations with mortality and morbidity effects. Further consideration of long-term mean PM 2.5 concentrations associated with mortality and respiratory effects in children did not provide a basis for establishing a lower annual standard level. The EPA did not place much weight on quantitative risk estimates from the very limited risk assessment conducted, but did conclude that the risk assessment results confirmed the general conclusions drawn from the epidemiological evidence that a serious public health problem was associated with ambient PM levels allowed under the then current PM 10 standards (62 FR 38665/1, July 18, 1997).

The EPA considered the epidemiological evidence and data on air quality relationships to set an annual PM 2.5 standard that was intended to be the “generally controlling” standard; i.e., the primary means of lowering both long- and short-term ambient concentrations of PM 2.5. [16] In conjunction with the annual standard, the EPA also established a 24-hour PM 2.5 standard to provide supplemental protection against days with high peak concentrations, localized “hotspots,” and risks arising from seasonal emissions that might not be well controlled by an annual standard (62 FR 38669/3).

In 2006, the EPA used a different evidence-based approach to assess the appropriateness of the levels of the 24-hour and annual PM 2.5 standards (U.S. EPA, 2011a, section 2.1.1.2). Based on an expanded body of epidemiological evidence that was stronger and more robust than that available in the 1997 review, including additional studies of both short- and long-term exposures, the EPA decided that using evidence of effects associated with periods of exposure that were most closely matched to the averaging time of each standard was the most appropriate public health policy approach for evaluating the scientific evidence to inform selecting the level of each standard. Thus, the EPA relied upon evidence from the short-term exposure studies as the principal basis for revising the level of the 24-hour PM 2.5 standard from 65 to 35 µg/m [3] to protect against effects associated with short-term exposures. The EPA relied upon evidence from long-term exposure studies as the principal basis for retaining the level of the annual PM 2.5 standard at 15 µg/m [3] to protect against effects associated with long-term exposures. This approach essentially took the view that short-term studies were not appropriate to inform decisions relating to the level of the annual standard, and long-term studies were not appropriate to inform decisions relating to the level of the 24-hour standard. With respect to quantitative risk-based considerations, the EPA determined that the estimates of risks likely to remain upon attainment of the 1997 suite of PM 2.5 standards were indicative of risks that could be reasonably judged important from a public health perspective and, thus, supported revision of the standards. However, the EPA judged that the quantitative risk assessment had important limitations and did not provide an appropriate basis for selecting the levels of the revised standards in 2006 (71 FR 61174/1-2, October 17, 2006).

2. Remand of Primary Annual PM 2.5 Standard

As noted above in section II.B.2, several parties filed petitions for review in the U.S. Court of Appeals for the District of Columbia Circuit following promulgation of the revised PM NAAQS in 2006. These petitions challenged several aspects of the final rule including the level of the primary PM 2.5 annual standard. The primary 24-hour PM 2.5 standard was not challenged by any of the litigants and, thus, was not considered in the court's review and decision.

On judicial review, the D.C. Circuit remanded the primary annual PM 2.5 NAAQS to the EPA on grounds that the Agency failed to adequately explain why the annual standard provided the requisite protection from both short- and long-term exposures to fine particles including protection for at-risk populations. American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). With respect to human health protection from short-term PM 2.5 exposures, the court considered the different approaches used by the EPA in the 1997 and 2006 p.m. NAAQS decisions, as summarized in section III.A.1 above. The court found that the EPA failed to adequately explain why a primary 24-hour PM 2.5 standard by itself would provide the protection needed from short-term exposures and remanded the primary annual PM 2.5 standard to the EPA “for further consideration of whether it is set at a level requisite to protect the public health while providing an adequate margin of safety from the risk of short-term exposures to PM 2.5.”American Farm Bureau Federation v. EPA, 559 F. 3d at 520-24.

With respect to protection from long-term exposure to fine particles, the court found that the EPA failed to adequately explain how the primary annual PM 2.5 standard provided an adequate margin of safety for children and other at-risk populations. The court found that the EPA did not provide a reasonable explanation of why certain morbidity studies, including a study of children in Southern California showing lung damage associated with long-term PM 2.5 exposure (Gauderman et al., 2000) and a multi-city study (24-Cities Study) evaluating decreased lung function in children associated with long-term PM 2.5 exposures (Raizenne et al., 1996), did not warrant a more stringent annual PM 2.5 standard. Id. at 522-23. Specifically, the court found that:

EPA was unreasonably confident that, even though it relied solely upon long-term mortality studies, the revised standard would provide an adequate margin of safety with respect to morbidity among children. Notably absent from the final rule, moreover, is any indication of how the standard will adequately reduce risk to the elderly or to those with certain heart or lung diseases despite (a) the EPA's determination in its proposed rule that those subpopulations are at greater risk from exposure to fine particles and (b) the evidence in the record supporting that determination. Id. at 525.

In addition, the court held that the EPA had not adequately explained its decision to base the level of the annual standard essentially exclusively on the results of long-term studies and the 24-hour standard level essentially exclusively on the results of short-term studies. See 559 F. 3d at 522 (“[e]ven if the long-term studies available today are useful for setting an annual standard * * * it is not clear why the EPA no longer believes it useful to look as well to short-term studies in order to design the suite of standards that will most effectively reduce the risks associated with short-term exposure”); see also Id. at 523-24 (holding that the EPA had not adequately explained why a standard based on levels in short-term exposure studies alone provided appropriate protection from health effects associated with short-term PM 2.5 exposures given the stated need to lower the entire air quality distribution, and not just peak concentrations, in order to control against short-term effects).

In remanding the primary annual PM 2.5 standard for reconsideration, the court did not vacate the standard, Id. at 530, so the standard remains in effect and is therefore the standard considered by the EPA in this review.

3. General Approach Used in the Policy Assessment for the Current Review

This review is based on an assessment of a much expanded body of scientific evidence, more extensive air quality data and analyses, and a more comprehensive quantitative risk assessment relative to the information available in past reviews, as presented and assessed in the Integrated Science Assessment and Risk Assessment and discussed in the Policy Assessment. As a result, the EPA's general approach to reaching conclusions about the adequacy of the current suite of PM 2.5 standards and potential alternative standards that are appropriate to consider was broader and more integrative than in past reviews. Our general approach also reflected consideration of the issues raised by the court in its remand of the primary annual PM 2.5 standard as discussed in section III.A.2 above, since decisions made in this review, and the rationales for those decisions, will comprise the Agency's response to the remand.

The EPA's general approach took into account both evidence-based and risk-based considerations and the uncertainties related to both types of information, as well as advice from CASAC (Samet, 2010c,d) and public comments on the first and second draft Policy Assessments (U.S. EPA, 2010c,f). In so doing, the EPA staff developed a final Policy Assessment (U.S. EPA, 2011a) which provided as broad an array of policy options as was supported by the available information, recognizing that the selection of a specific approach to reaching final decisions on the primary PM 2.5 standards will reflect the judgments of the Administrator as to what weight to place on the various approaches and types of information available in the current review.

The Policy Assessment concluded it was most appropriate to consider the protection against PM 2.5-related mortality and morbidity effects, associated with both long- and short-term exposures, afforded by the annual and 24-hour standards taken together, as was done in the 1997 review, rather than to consider each standard separately, as was done in the 2006 review (U.S. EPA, 2011a, section 2.1.3). [17] As the EPA recognized in 1997, there are various ways to combine two standards to achieve an appropriate degree of public health protection. The extent to which these two standards are interrelated in any given area depends in large part on the relative levels of the standards, the peak-to-mean ratios that characterize air quality patterns in an area, and whether changes in air quality designed to meet a given suite of standards are likely to be of a more regional or more localized nature.

In considering the combined effect of annual and 24-hour standards, the Policy Assessment recognized that changes in PM 2.5 air quality designed to meet an annual standard would likely result not only in lower annual average PM 2.5 concentrations but also in fewer and lower peak 24-hour PM 2.5 concentrations. The Policy Assessment also recognized that changes designed to meet a 24-hour standard would result not only in fewer and lower peak 24-hour PM 2.5 concentrations but also in lower annual average PM 2.5 concentrations. Thus, either standard could be viewed as providing protection from effects associated with both short- and long-term exposures, with the other standard serving to address situations where the daily peak and annual average concentrations are not consistently correlated.

In considering the currently available evidence, the Policy Assessment recognized that the short-term exposure studies were primarily drawn from epidemiological studies that associated variations in area-wide health effects with monitor(s) that measured the variation in daily PM 2.5 concentrations over the course of several years. The strength of the associations in these data was demonstrably in the numerous “typical” days within the air quality distribution, not in the peak days. See also 71 FR 61168, October 17, 2006 and American Farm Bureau Federation v. EPA, 559 F. 3d at 523, 524 (making the same point). The quantitative risk assessments conducted for this and previous reviews demonstrated the same point; that is, much, if not most of the aggregate risk associated with short-term exposures results from the large number of days during which the 24-hour average concentrations are in the low-to mid-range, below the peak 24-hour concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, section 3.1.2.2). In addition, there was no evidence suggesting that risks associated with long-term exposures were likely to be disproportionately driven by peak 24-hour concentrations. [18]

For these reasons, the Policy Assessment concluded that strategies that focused primarily on reducing peak days were less likely to achieve reductions in the PM 2.5 concentrations that were most strongly associated with the observed health effects. Furthermore, the Policy Assessment concluded that a policy approach that focused on reducing peak exposures would most likely result in more uneven public health protection across the U.S. by either providing inadequate protection in some areas or overprotecting in other areas (U.S. EPA, 2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). This is because, as discussed above, reductions based on control of peak days are less likely to control the bulk of the air quality distribution.

The Policy Assessment concluded that a policy goal of setting a “generally controlling” annual standard that will lower a wide range of ambient 24-hour PM 2.5 concentrations, as opposed to focusing on control of peak 24-hour PM 2.5 concentrations, was the most effective and efficient way to reduce total population risk and so provide appropriate protection. This approach, in contrast to one focusing on a generally controlling 24-hour standard, would likely reduce aggregate risks associated with both long- and short-term exposures with more consistency and would likely avoid setting national standards that could result in relatively uneven protection across the country, due to setting standards that are either more or less stringent than necessary in different geographical areas (U.S. EPA, 2011a, p. 2-9).

The Policy Assessment also concluded that an annual standard intended to serve as the primary means for providing protection from effects associated with both long- and short-term PM 2.5 exposures cannot be expected to offer sufficient protection against the effects of all short-term PM 2.5 exposures. As a result, in conjunction with a generally controlling annual standard, the Policy Assessment concluded it was appropriate to consider setting a 24-hour standard to provide supplemental protection, particularly for areas with high peak-to-mean ratios possibly associated with strong local or seasonal sources, or PM 2.5-related effects that may be associated with shorter-than-daily exposure periods (U.S. EPA, 2011a, p. 2-10).

The Policy Assessment's consideration of the protection afforded by the current and alternative suites of standards focused on PM 2.5-related health effects associated with long-term exposures for which the magnitude of quantitative estimates of risks to public health generated in the risk assessment was appreciably larger in terms of overall incidence and percent of total mortality or morbidity effects than for short-term PM 2.5-related effects. Nonetheless, the EPA also considered health effects and estimated risks associated with short-term exposures. In both cases, the Policy Assessment placed greatest weight on health effects that had been judged in the Integrated Science Assessment to have a causal or likely causal relationship with PM 2.5 exposures, while also considering health effects judged to be suggestive of a causal relationship or evidence that focused on specific at-risk populations. The Policy Assessment placed relatively greater weight on statistically significant associations that yielded relatively more precise effect estimates and that were judged to be robust to confounding by other air pollutants. In the case of short-term exposure studies, the Policy Assessment placed greatest weight on evidence from large multi-city studies, while also considering associations in single-city studies.

In translating information from epidemiological studies into the basis for reaching staff conclusions on the adequacy of the current suite of standards, the Policy Assessment considered a number of factors. As an initial matter, the Policy Assessment considered the extent to which the currently available evidence and related uncertainties strengthens or calls into question conclusions from the last review regarding associations between fine particle exposures and health effects. The Policy Assessment also considered evidence of health effects in at-risk populations and the potential impacts on such populations. Further, the Policy Assessment explored the extent to which PM 2.5-related health effects had been observed in areas where air quality distributions extend to lower concentrations than previously reported or in areas that would likely have met the current suite of standards.

In translating information from epidemiological studies into the basis for reaching staff conclusions on standard levels for consideration (U.S. EPA, 2011a, sections 2.1.3 and 2.3.4), the Policy Assessment first recognized the absence of discernible thresholds in the concentration-response functions from long- and short-term PM 2.5 exposure studies (U.S. EPA, 2011a, section 2.4.3). [19] In the absence of any discernible thresholds, the Agency's general approach for identifying appropriate standard levels for consideration involved characterizing the range of PM 2.5 concentrations over which we have the most confidence in the associations reported in epidemiological studies. In so doing, the Policy Assessment recognized that there is no single factor or criterion that comprises the “correct” approach, but rather there are various approaches that are reasonable to consider for characterizing the confidence in the associations and the limitations and uncertainties in the evidence. Identifying the implications of various approaches for reaching conclusions on the range of alternative standard levels that is appropriate to consider can help inform the final decisions to either retain or revise the standards. Today's final decisions also take into account public health policy judgments as to the degree of health protection that is to be achieved.

In reaching staff conclusions on the range of annual standard levels that was appropriate to consider, the Policy Assessment focused on identifying an annual standard that provided requisite protection from effects associated with both long- and short-term exposures. In so doing, the Policy Assessment explored different approaches for characterizing the range of PM 2.5 concentrations over which our confidence in the nature of the associations for both long- and short-term exposures is greatest, as well as the extent to which our confidence is reduced at lower PM 2.5 concentrations.

First, the Policy Assessment recognized that the approach that most directly addressed this issue considered studies that analyzed confidence intervals around concentration-response relationships and in particular, analyses that averaged across multiple concentration-response models rather than considering a single concentration-response model. [20] The Policy Assessment explored the extent to which such analyses had been published for studies of health effects associated with long- or short-term PM 2.5 exposures. Such analyses could potentially be used to characterize a concentration below which uncertainty in a concentration-response relationship substantially increases or is judged to be indicative of an unacceptable degree of uncertainty about the existence of a continuing concentration-response relationship. The Policy Assessment concluded that identifying this area of uncertainty in the concentration-response relationship could be used to inform identification of alternative standard levels that are appropriate to consider.

Further, the Policy Assessment explored other approaches that considered different statistical metrics from epidemiological studies. The Policy Assessment first took into account the general approach used in previous PM reviews which focused on consideration of alternative standard levels that were somewhat below the long-term mean PM 2.5 concentrations reported in epidemiological studies using air quality distributions based on composite monitor concentrations. [21] This approach recognized that the strongest evidence of PM 2.5-related associations occurs at concentrations around the long-term (i.e., annual) mean. In using this approach, the Policy Assessment placed greatest weight on those long- and short-term exposure studies that reported statistically significant associations with mortality and morbidity effects.

In extending this approach, the Policy Assessment also considered information beyond a single statistical metric of PM 2.5 concentrations (i.e., the mean) to the extent such information was available. Pursuant to an express comment from CASAC (Samet 2010d, p. 2), the Policy Assessment utilized distributional statistics (i.e., statistical characterization of an entire distribution of data) to identify the broader range of PM 2.5 concentrations that had the most influence on the calculation of relative risk estimates in both long- and short-term exposure epidemiological studies. Thus, the Policy Assessment considered the part of the distribution of PM 2.5 concentrations in which the data analyzed in the study (i.e., air quality and population-level data, as discussed below) were most concentrated, specifically, the range of PM 2.5 concentrations around the long-term mean over which our confidence in the magnitude and significance of associations observed in the epidemiological studies was greatest. The Policy Assessment then focused on the lower part of the distribution to characterize where the data became appreciably more sparse and, thus, where our understanding of the magnitude and significance of the associations correspondingly became more uncertain. The Policy Assessment recognized there was no single percentile value within a given distribution that was most appropriate or “correct” to use to characterize where our confidence in the associations becomes appreciably lower. The Policy Assessment concluded that the range from the 25th to 10th percentiles is a reasonable range to consider as a region where we had appreciably less confidence in the associations observed in epidemiological studies. [22]

In considering distributional statistics from epidemiological studies, the final Policy Assessment focused on two types of population-level metrics that CASAC advised were most useful to consider in identifying the PM 2.5 concentrations most influential in generating the health effect estimates reported in the epidemiological studies. [23] Consistent with CASAC advice, the most relevant information was the distribution of health events (e.g., deaths, hospitalizations) occurring within a study population in relation to the distribution of PM 2.5 concentrations. However, in recognizing that access to health event data can be restricted, the Policy Assessment also considered the number of study participants within each study area as an appropriate surrogate for health event data.

The Policy Assessment recognized that an approach considering analyses of confidence intervals around concentration-response functions was intrinsically related to an approach that considered different distributional statistics. Both of these approaches could be employed to understand the broader distribution of PM 2.5 concentrations which correspond to the health events reported in epidemiological studies. In applying these approaches, the Policy Assessment, consistent with CASAC advice (Samet, 2010d, p. 3), considered PM 2.5 concentrations from long- and short-term PM 2.5 exposure studies using composite monitor distributions.

In reaching staff conclusions on alternative standard levels that were appropriate to consider, the Policy Assessment also included a broader consideration of the uncertainties and limitations of the current scientific evidence. Most notably, these uncertainties are related to the heterogeneity observed in the epidemiological studies in the eastern versus western parts of the U.S., the relative toxicity of PM 2.5 components, and the potential role of co-pollutants (U.S. EPA, 2011a, pp. 2-25 to 2-26). The limitations and uncertainties associated with the currently available scientific evidence, including the availability of fewer studies toward the lower range of alternative annual standard levels being considered in this proposal, are summarized in section III.B below and further discussed in section III.B.2 of the proposal.

The Policy Assessment recognized that the level of protection afforded by the NAAQS relies both on the level and the form of the standard. The Policy Assessment concluded that a policy approach that used data based on composite monitor distributions to identify alternative standard levels, and then compared those levels to concentrations at maximum monitors to determine whether an area meets a given standard, inherently has the potential to build in some margin of safety (U.S. EPA, 2011a, p. 2-14). [24] This conclusion was consistent with CASAC's comments on the second draft Policy Assessment, in which CASAC expressed its preference for focusing on an approach using composite monitor distributions “because of its stability, and for the additional margin of safety it provides” when “compared to the maximum monitor perspective” (Samet, et al., 2010d, pp. 2 to 3).

In reaching staff conclusions on alternative 24-hour standard levels that are appropriate to consider for setting a 24-hour standard intended to supplement the protection afforded by a generally controlling annual standard, the Policy Assessment considered currently available short-term PM 2.5 exposure studies. The evidence from these studies informed our understanding of the protection afforded by the suite of standards against effects associated with short-term exposures. In considering the short-term exposure studies, the Policy Assessment evaluated both the distributions of 24-hour PM 2.5 concentrations, with a focus on the 98th percentile concentrations (to the extent such data were available) to match the form of the current 24-hour PM 2.5 standard, as well as the long-term mean PM 2.5 concentrations reported in these studies. In addition to considering the epidemiological evidence, the Policy Assessment also considered air quality information based on county-level 24-hour and annual design values [25] to understand the policy implications of the alternative standard levels supported by the underlying science. In particular, the Policy Assessment considered the extent to which different combinations of alternative annual and 24-hour standards would support the policy goal of focusing on a generally controlling annual standard in conjunction with a 24-hour standard that would provide supplemental protection. In so doing, the Policy Assessment discussed the roles that each standard might be expected to play in the protection afforded by alternative suites of standards.

Beyond these evidence-based considerations, the Policy Assessment also considered the quantitative risk estimates and the key observations presented in the Risk Assessment. This assessment included an evaluation of 15 urban case study areas and estimated risk associated with a number of health endpoints associated with long- and short-term PM 2.5 exposures (U.S. EPA, 2010a). As part of the risk-based considerations, the Policy Assessment considered estimates of the magnitude of PM 2.5-related risks associated with recent air quality levels and air quality simulated to just meet the current and alternative suites of standards using alternative simulation approaches. The Policy Assessment also characterized the risk reductions, relative to the risks remaining upon just meeting the current standards, associated with just meeting alternative suites of standards. In so doing, the Policy Assessment recognized the uncertainties inherent in such risk estimates, and took such uncertainties into account by considering the sensitivity of the “core” risk estimates to alternative assumptions and methods likely to have substantial impact on the estimates. In addition, the Policy Assessment considered additional analyses characterizing the representativeness of the urban study areas within a broader national context to understand the roles that the annual and 24-hour standards may play in affording protection against effects related to both long- and short-term PM 2.5 exposures.

Based on the approach discussed above, the Policy Assessment reached conclusions related to the primary PM 2.5 standards that reflected an understanding of both evidence-based and risk-based considerations to inform two overarching questions related to: (1) The adequacy of the current suite of PM 2.5 standards and (2) revisions to the standards that were appropriate to consider in this review to protect against health effects associated with both long- and short-term exposures to fine particles. When evaluating the health protection afforded by the current or any alternative suites of standards considered, the Policy Assessment took into account the four basic elements of the NAAQS: The indicator, averaging time, form, and level.

The general approach for reviewing the primary PM 2.5 standards described above provided a comprehensive basis that helped to inform the Administrator's judgments in reaching her proposed and final decisions to revise the current suite of primary fine particle NAAQS and in responding to the remand of the 2006 primary annual PM 2.5 standard.

B. Overview of Health Effects Evidence

This section outlines the key information presented in section III.B of the proposal (77 FR 38906 to 38911, June 29, 2012) and discussed more fully in the Integrated Science Assessment (Chapters 2, 4, 5, 6, 7, and 8) and the Policy Assessment (Chapter 2) related to health effects associated with fine particle exposures. Section III.B. of the proposal discusses available information on the health effects associated with exposures to PM 2.5, including the nature of such health effects (section III.B.1) and associated limitations and uncertainties (section III.B.2), at-risk populations (section III.B.3), and potential PM 2.5-related impacts on public health (section III.B.4). As was true in the last two reviews, evidence from epidemiological, controlled human exposure and animal toxicological studies played a key role in the Integrated Science Assessment's evaluation of the scientific evidence.

The 2006 PM NAAQS review concluded that there was “strong epidemiological evidence” for linking long-term PM 2.5 exposures with cardiovascular-related and lung cancer mortality and respiratory-related morbidity and for linking short-term PM 2.5 exposures with cardiovascular-related and respiratory-related mortality and morbidity (U.S. EPA, 2004, p. 9-46; U.S. EPA, 2005, p. 5-4). Overall, the evidence from epidemiological, toxicological, and controlled human exposure studies supported “likely causal associations” between PM 2.5 and both mortality and morbidity from cardiovascular and respiratory diseases, based on “an assessment of strength, robustness, and consistency in results” (U.S. EPA, 2004, p. 9-48). [26]

In this review, based on the expanded body of evidence, the EPA finds that:

(1) In looking across the extensive new scientific evidence available in this review, our overall understanding of health effects associated with fine particle exposures has been greatly expanded. The currently available evidence is largely consistent with evidence available in the last review and substantially strengthens what is known about the effects associated with fine particle exposures.

(2) A number of large multi-city epidemiological studies have been conducted throughout the U.S., including extended analyses of long-term exposure studies that were important to inform decision-making in the last review. The body of currently available scientific evidence has also been expanded greatly by the publication of a number of new multi-city, time-series studies that have used uniform methodologies to investigate the effects of short-term PM 2.5 exposures on public health. This body of evidence provides a more expansive data base and considers multiple locations representing varying regions and seasons that provide evidence of the influence of different air pollution mixes on PM 2.5-associated health effects. These studies provide more precise estimates of the magnitude of effects associated with short-term PM 2.5 exposure than most smaller-scale single-city studies that were more commonly available in the last review. These studies have reported consistent increases in morbidity and/or premature mortality related to ambient PM 2.5 concentrations, with the strongest evidence reported for cardiovascular-related effects.

(3) In addition, the findings of new toxicological and controlled human exposure studies greatly expand and provide stronger support for a number of potential biological mechanisms or pathways for cardiovascular and respiratory effects associated with long- and short-term PM exposures. These studies provide coherence and biological plausibility for the effects observed in epidemiological studies.

(4) Using a more formal framework for reaching causal determinations than used in prior reviews, [27] the EPA concludes that a causal relationship exists between both long- and short-term exposures to PM 2.5 and premature mortality and cardiovascular effects and a likely causal relationship exists between long- and short-term PM 2.5 exposures and respiratory effects. Further, there is evidence suggestive of a causal relationship between long-term PM 2.5 exposures and other health effects, including developmental and reproductive effects (e.g., low birth weight, infant mortality) and carcinogenic, mutagenic, and genotoxic effects (e.g., lung cancer mortality). [28]

(5) The newly available evidence significantly strengthens the link between long- and short-term exposure to PM 2.5 and premature mortality, while providing indications that the magnitude of the PM 2.5-mortality association with long-term exposures may be larger than previously estimated. The strongest evidence comes from recent studies investigating long-term exposure to PM 2.5 and cardiovascular-related mortality. The evidence supporting a causal relationship between long-term PM 2.5 exposure and mortality also includes consideration of new studies that demonstrated an improvement in community health following reductions in ambient fine particles.

(6) Several new studies have examined the association between cardiovascular effects and long-term PM 2.5 exposures in multi-city studies conducted in the U.S. and Europe. While studies were not available in the last review with regard to long-term exposure and cardiovascular-related morbidity, recent studies have provided new evidence linking long-term exposure to PM 2.5 with an array of cardiovascular effects such as heart attacks, congestive heart failure, stroke, and mortality. This evidence is coherent with studies of short-term exposure to PM 2.5 that have observed associations with a continuum of effects ranging from subtle changes in indicators of cardiovascular health to serious clinical events, such as increased hospitalizations and emergency department visits due to cardiovascular disease and cardiovascular mortality.

(7) Extended analyses of studies available in the last review as well as new epidemiological studies conducted in the U.S. and abroad provide stronger evidence of respiratory-related morbidity effects associated with long-term PM 2.5 exposure. The strongest evidence for respiratory-related effects is from studies that evaluated decrements in lung function growth, increased respiratory symptoms, and asthma development. The strongest evidence from short-term PM 2.5 exposure studies has been observed for increased respiratory-related emergency department visits and hospital admissions for chronic obstructive pulmonary disease (COPD) and respiratory infections.

(8) The body of scientific evidence is somewhat expanded from the 2006 review but is still limited with respect to associations between long-term PM 2.5 exposures and developmental and reproductive effects as well as cancer, mutagenic, and genotoxic effects. The strongest evidence for an association between PM 2.5 and developmental and reproductive effects comes from epidemiological studies of low birth weight and infant mortality, especially due to respiratory causes during the post-neonatal period (i.e., 1 month-12 months of age). With regard to cancer effects, “[m]ultiple epidemiologic studies have shown a consistent positive association between PM 2.5 and lung cancer mortality, but studies have generally not reported associations between PM 2.5 and lung cancer incidence” (U.S. EPA 2009a p. 2-13).

(9) Efforts to evaluate the relationships between PM composition and health effects continue to evolve. While many constituents of PM 2.5 can be linked with differing health effects, the evidence is not yet sufficient to allow differentiation of those constituents or sources that may be more closely related to specific health outcomes nor to exclude any individual component or group of components associated with any source categories from the fine particle mixture of concern.

(10) Specific groups within the general population are at increased risk for experiencing adverse health effects related to PM exposures. The currently available evidence expands our understanding of previously identified at-risk populations (i.e., children, older adults, and individuals with pre-existing heart and lung disease) and supports the identification of additional at-risk populations (e.g., persons with lower socioeconomic status, genetic differences). Evidence for PM-related effects in these at-risk populations has expanded and is stronger than previously observed. There is emerging, though still limited, evidence for additional potentially at-risk populations, such as those with diabetes, people who are obese, pregnant women, and the developing fetus.

(11) The population potentially affected by PM 2.5 is large. In addition, large subgroups of the U.S. population have been identified as at-risk populations. While individual effect estimates from epidemiological studies may be small in size, the public health impact of the mortality and morbidity associations can be quite large given the extent of exposure. Taken together, this suggests that exposure to ambient PM 2.5 concentrations can have substantial public health impacts.

(12) While the currently available scientific evidence is stronger and more consistent than in previous reviews, providing a strong basis for decision making in this review, the EPA recognizes that important uncertainties and limitations in the health effects evidence remain. Epidemiological studies evaluating health effects associated with long- and short-term PM 2.5 exposures have reported heterogeneity in responses between cities and geographic regions within the U.S. This heterogeneity may be attributed, in part, to differences in the fine particle composition or related to exposure measurement error, which can introduce bias and increased uncertainty in associated health effect estimates. Variability in the associations observed across PM 2.5 epidemiological studies may be due in part to exposure error related to measurement-related issues, the use of central fixed-site monitors to represent population exposure to PM 2.5, models used in lieu of or to supplement ambient measurements, and our limited understanding of factors that may influence exposures (e.g., topography, the built environment, weather, source characteristics, ventilation usage, personal activity patterns, photochemistry). In addition, where PM 2.5 and other pollutants (e.g., ozone, nitrogen dioxide, and carbon monoxide) are correlated, it can be difficult to distinguish the effects of the various pollutants in the ambient mixture (i.e., co-pollutant confounding). [29]

While uncertainties and limitations still remain in the available health effects evidence, the Administrator judges the currently available scientific data base to be stronger and more consistent than in previous reviews providing a strong basis for decision making in this review.

C. Overview of Quantitative Characterization of Health Risks

In addition to a comprehensive evaluation of the health effects evidence available in this review, the EPA conducted an expanded quantitative risk assessment for selected health endpoints to provide additional information and insights to inform decisions on the primary PM 2.5 NAAQS. [30] As discussed in section III.C of the proposal, the approach used to develop quantitative risk estimates associated with PM 2.5 exposures was built on the approach used and lessons learned in the last review and focused on improving the characterization of the overall confidence in the risk estimates, including related uncertainties, by incorporating a number of enhancements, in terms of both the methods and data used in the analyses.

The goals of this quantitative risk assessment were largely the same as those articulated in the risk assessment conducted for the last review. These goals included: (1) To provide estimates of the potential magnitude of premature mortality and/or selected morbidity effects in the population associated with recent ambient levels of PM 2.5 and with simulating just meeting the current and alternative suites of PM 2.5 standards in 15 selected urban study areas, [31] including, where data were available, consideration of impacts on at-risk populations; (2) to develop a better understanding of the influence of various inputs and assumptions on the risk estimates to more clearly differentiate among alternative suites of standards; and (3) to gain insights into the distribution of risks and patterns of risk reductions and the variability and uncertainties in those risk estimates. In addition, the quantitative risk assessment included nationwide estimates of the potential magnitude of premature mortality associated with long-term exposure to recent ambient PM 2.5 concentrations to more broadly characterize this risk on a national scale and to support the interpretation of the more detailed risk estimates generated for selected urban study areas.

The expanded and updated risk assessment conducted in this review included estimates of risk for: (1) All-cause, ischemic heart disease-related, cardiopulmonary-related, and lung cancer-related mortality associated with long-term PM 2.5 exposure; (2) non-accidental, cardiovascular-related, and respiratory-related mortality associated with short-term PM 2.5 exposure; and (3) cardiovascular-related and respiratory-related hospital admissions and asthma-related emergency department visits associated with short-term PM 2.5 exposure. [32]

The Risk Assessment included a core set of risk estimates supplemented by an alternative set of risk results generated using single-factor and multi-factor sensitivity analyses. The core set of risk estimates was developed using the combination of modeling elements and input data sets identified in the Risk Assessment as having higher confidence relative to inputs used in the sensitivity analyses. The results of the sensitivity analyses provided information to evaluate and rank the potential impacts of key sources of uncertainty on the core risk estimates. In addition, the sensitivity analyses represented a set of reasonable alternatives to the core set of risk estimates that fell within an overall set of plausible risk estimates surrounding the core estimates.

The EPA recognized that there were many sources of variability and uncertainty inherent in the inputs to its quantitative risk assessment. [33] The design of the risk assessment included a number of elements to address these issues in order to increase the overall confidence in the risk estimates generated for the 15 urban study areas, including using guidance from the World Health Organization (WHO, 2008) as a framework for characterizing uncertainty in the analyses. [34]

With respect to the sources of variability, the Risk Assessment considered those that contributed to differences in risk across urban study areas, but did not directly affect the degree of risk reduction associated with the simulation of just meeting current or alternative standard levels (e.g., differences in baseline incidence rates, demographics and population behavior). The Risk Assessment also focused on factors that not only introduced variability into risk estimates across study areas, but also played an important role in determining the magnitude of risk reductions upon simulation of just meeting current or alternative standard levels (e.g., peak-to-mean ratios of ambient PM 2.5 concentrations within individual urban study areas and the nature of the rollback approach used to simulate just meeting the current or alternative standards). Key sources of potential variability that were likely to affect population risks included the following: (1) Intra-urban variability in ambient PM 2.5 concentrations, including PM 2.5 composition; (2) variability in the patterns of reductions in PM 2.5 concentrations associated with different rollback approaches when simulating just meeting the current or alternative standards; (3) co-pollutant exposures; (4) factors related to demographic and socioeconomic status; (5) behavioral differences across urban study areas (e.g., time spent outdoors); (6) baseline incidence rates; and (7) longer-term temporal variability in ambient PM 2.5 concentrations reflecting meteorological trends as well as future changes in the mix of PM 2.5 sources, including changes in air quality related to future regulatory actions.

With regard to uncertainties, single and multi-factor sensitivity analyses were combined with a qualitative analysis to assess the impact of potential sources of uncertainty on the core risk estimates. Key sources of uncertainty included: (1) Characterizing intra-urban population exposure in the context of epidemiological studies linking PM 2.5 to specific health effects; (2) statistical fit of the concentration-response functions for short-term exposure-related health endpoints; (3) shape of the concentration-response functions; (4) specifying the appropriate lag structure for short-term exposure studies; (5) transferability of concentration-response functions from study locations to urban study area locations for long-term exposure-related health endpoints; (6) use of single-city versus multi-city studies in the derivation of concentration-response functions; (7) impact of historical air quality on estimates of health risk associated with long-term PM 2.5 exposures; and (8) potential variation in effect estimates reflecting compositional differences in PM 2.5.

Beyond characterizing uncertainty and variability, a number of design elements were included in the risk assessment to increase the overall confidence in the risk estimates generated for the 15 urban study areas (U.S. EPA, 2011a, pp. 2-38 to 2-41). These elements included: (1) Use of a deliberative process for specifying components of the risk model that reflects consideration of the latest research on PM 2.5 exposure and risk (U.S. EPA, 2010a, section 5.1.1); (2) integration of key sources of variability into the design as well as the interpretation of risk estimates (U.S. EPA, 2010a, section 5.1.2); (3) assessment of the degree to which the urban study areas are representative of areas in the U.S. experiencing higher PM 2.5-related risk (U.S. EPA, 2010a, section 5.1.3); and (4) identification and assessment of important sources of uncertainty and the impact of these uncertainties on the core risk estimates (U.S. EPA, 2010a, section 5.1.4). Further, additional analyses examined potential bias and overall confidence in the risk estimates. Greater confidence is associated with risk estimates based on simulated annual mean PM 2.5 concentrations that are within the region of the air quality distribution used in deriving the concentration-response functions where the bulk of the data reside (e.g., within one standard deviation around the long-term mean PM 2.5 concentration) (U.S. EPA, 2011a, p. 2-38).

Key observations and insights from the PM 2.5 risk assessment, together with important caveats and limitations, were discussed in section III.C.3 of the proposal. In general, in considering the set of quantitative risk estimates and related uncertainties and limitations related to long- and short-term PM 2.5 exposure together with consideration of the health endpoints which could not be quantified, the Policy Assessment concluded this information provided strong evidence that risks estimated to remain upon simulating just meeting the current suite of PM 2.5 standards are important from a public health perspective, both in terms of severity and magnitude of effects. Patterns of increasing estimated risk reductions were generally observed as either the annual or 24-hour standard level, or both, were reduced over the ranges considered in the Risk Assessment.

The magnitude of both long- and short-term exposure-related risk estimated to remain upon just meeting the current suite of standards as well as alternative standard levels was strongly associated with the simulated change in annual mean PM 2.5 concentrations. Although long- and short-term exposure-related mortality rates have similar patterns in terms of the subset of urban study areas experiencing risk reductions for the current suite of standard levels, the magnitude of risk remaining is higher for long-term exposure-related mortality and substantially lower for short-term exposure-related mortality. Short-term exposure-related morbidity risk estimates were greater for cardiovascular-related than respiratory-related events and emergency department visits for asthma-related events were significant: Furthermore, most of the aggregate risk associated with short-term exposures was not primarily driven by the small number of days with PM 2.5 concentrations in the upper tail of the air quality distribution, but rather by the large number of days with PM 2.5 concentrations at and around the mean of the distribution, that is, the 24-hour average concentrations that are in the low- to mid-range, well below the peak 24-hour concentrations (U.S. EPA, 2011a, p. 2-3).

With regard to characterizing estimates of PM 2.5-related risk associated with simulation of alternative standards, the Policy Assessment recognized that greater overall confidence was associated with estimates of risk reduction than for estimates of absolute risk remaining (U.S. EPA, 2011a, p. 2-94). Furthermore, the Policy Assessment recognized that estimates of absolute risk remaining for each of the alternative standard levels considered, particularly in the context of long-term exposure-related mortality, may be underestimated. [35] In addition, the Policy Assessment observed that in considering the overall confidence associated with the quantitative analyses, the Risk Assessment recognized that: (1) Substantial variability existed in the magnitude of risk remaining across urban study areas and (2) in general, higher confidence was associated with risk estimates based on PM 2.5 concentrations near the mean PM 2.5 concentrations in the underlying epidemiological studies providing the concentration-response functions (e.g., within one standard deviation of the mean PM 2.5 concentration reported). Furthermore, although the Risk Assessment estimated that the alternative 24-hour standard levels considered (when controlling) would result in additional estimated risk reductions beyond those estimated for alternative annual standard levels alone, these additional estimated reductions were highly variable. Conversely, the Risk Assessment recognized that alternative annual standard levels, when controlling, resulted in more consistent risk reductions across urban study areas, thereby potentially providing a more consistent degree of public health protection (U.S. EPA, 2010a, p. 5-17).

D. Conclusions on the Adequacy of the Current Primary PM 2.5 Standards

1. Introduction

The initial issue to be addressed in the current review of the primary PM 2.5 standards is whether, in view of the advances in scientific knowledge and other information reflected in the Integrated Science Assessment, the Risk Assessment, and the Policy Assessment, the existing standards should be retained or revised. In considering the adequacy of the current suite of PM 2.5 standards, the Administrator has considered the large body of evidence presented and assessed in the Integrated Science Assessment (U.S. EPA, 2009a), the quantitative assessment of risks, staff conclusions and associated rationales presented in the Policy Assessment, views expressed by CASAC, and public comments. The Administrator has taken into account both evidence- and risk-based considerations [36] in developing final conclusions on the adequacy of the current primary PM 2.5 standards.

a. Evidence- and Risk-based Considerations in the Policy Assessment

In considering the available epidemiological evidence in this review, the Policy Assessment took a broader approach than was used in the last review. This approach reflected the more extensive and stronger body of evidence available since the last review on health effects related to both long- and short-term exposure to PM 2.5. As discussed in section III.A.3 above, this broader approach focused on setting the annual standard as the “generally controlling” standard for lowering both short- and long-term PM 2.5 concentrations and so providing requisite protection to public health. In conjunction with such an annual standard, this approach focused on setting the 24-hour standard to provide supplemental protection against days with high peak PM 2.5 concentrations.

In addressing the question whether the evidence now available in this review supports consideration of standards that are more protective than the current PM 2.5 standards, the Policy Assessment considered whether: (1) Statistically significant health effects associations with long- or short-term exposures to fine particles occur in areas that would likely have met the current PM 2.5 standards [see American Trucking Associations, 283 F. 3d at 369, 376 (revision of level of PM NAAQS justified when health effects are observed in areas meeting the existing standard)], and (2) associations with long-term exposures to fine particles extend down to lower air quality concentrations than had previously been observed. With regard to associations observed in long-term PM 2.5 exposure studies, the Policy Assessment recognized that extended follow-up analyses of the ACS and Harvard Six Cities studies provided consistent and stronger evidence of an association with mortality at lower air quality distributions than had previously been observed (U.S. EPA, 2011a, pp. 2-31 to 2-32). The original and reanalysis of the ACS study reported positive and statistically significant effects associated with a long-term mean PM 2.5 concentration of 18.2 μg/m [3] across 50 metropolitan areas for 1979 to 1983 (Pope et al., 1995; Krewski et al., 2000). [37] In extended analyses, positive and statistically significant effects of approximately similar magnitude were associated with declining PM 2.5 concentrations, from an aggregate long-term mean in 58 metropolitan areas of 21.2 µg/m [3] in the original monitoring period (1979 to 1983) to 14.0 µg/m [3] for 116 metropolitan areas in the most recent years evaluated (1999-2000), with an overall average across the two study periods in 51 metropolitan areas of 17.7 µg/m [3] (Pope et al., 2002; Krewski et al., 2009). With regard to the Harvard Six Cities Study, the original and reanalysis reported positive and statistically significant effects associated with a long-term mean PM 2.5 concentration of 18.0 μg/m [3] for 1980 to 1985 (Dockery et al., 1993; Krewski et al., 2000). In an extended follow-up of this study, the aggregate long-term mean concentration across all years evaluated was 16.4 μg/m [3] for 1980 to 1988 [38] (Laden et al., 2006). In an additional analysis of the extended follow-up of the Harvard Six Cities study, investigators reported that the concentration-response relationship was linear and “clearly continuing below the level” of the current annual standard (U.S. EPA, 2009a, p. 7-92; Schwartz et al., 2008).

Cohort studies conducted since the last review provided additional evidence of mortality associated with air quality distributions that are generally lower than those reported in the ACS and Harvard Six Cities studies, with effect estimates that were similar or, in some studies, significantly greater in magnitude than in the ACS and Harvard Six Cities studies (see also, section III.D.1.a of the proposal, 77 FR 38918 to 28919; U.S. EPA, 2011a, pp. 2-32 to 2-33). The Women's Health Initiative (WHI) study reported positive and most often statistically significant associations between long-term PM 2.5 exposure and cardiovascular-related mortality as well as morbidity effects, with much larger relative risk estimates for mortality than in the ACS and Harvard Six Cities studies, at an aggregate long-term mean PM 2.5 concentration of 12.9 μg/m [3] for 2000 (Miller et al., 2007). [39]

Using the Medicare cohort, Eftim et al. (2008) reported somewhat higher effect estimates than in the ACS and Harvard Six Cities studies with aggregate long-term mean concentrations of 13.6 μg/m [3] and 14.1 μg/m [3] , respectively, for 2000 to 2002. Zeger et al. (2008) reported associations between long-term PM 2.5 exposure and mortality for the eastern region of the U.S. at an aggregated long-term PM 2.5 median concentration of 14.0 μg/m [3] , although no association was reported for the western region with an aggregate long-term PM 2.5 median concentration of 13.1 μg/m [3] (U.S. EPA, 2009a, p. 7-88). [40]

Premature mortality in children reported in a national infant mortality study as well as mortality in a cystic fibrosis cohort including both children and adults reported positive but statistically nonsignificant effects associated with long-term aggregate mean concentrations of 14.8 μg/m [3] and 13.7 μg/m [3] , respectively (Woodruff et al., 2008; Goss et al., 2004).

With respect to respiratory morbidity effects associated with long-term PM 2.5 exposure, the across-city mean of 2-week average PM 2.5 concentrations reported in the initial Southern California Children's Health Study was approximately 15.1 µg/m [3] (Peters et al., 1999). These results were found to be consistent with results of cross-sectional analyses of the 24-Cities Study (Dockery et al., 1996; Raizenne et al., 1996), which reported a long-term cross-city mean PM 2.5 concentration of 14.5 μg/m [3] . [41] In this review, extended analyses of the Southern California Children's Health Study provide stronger evidence of PM 2.5-related respiratory effects, at lower air quality concentrations than had previously been reported, with a four-year aggregate mean concentration of 13.8 μg/m [3] across the 12 study communities (McConnell et al., 2003; Gauderman et al., 2004, U.S. EPA, 2009a, Figure 7-4).

In also considering health effects for which the Integrated Science Assessment concluded evidence was suggestive of a causal relationship, the Policy Assessment noted a limited number of birth outcome studies that reported positive and statistically significant effects related to aggregate long-term mean PM 2.5 concentrations down to approximately 12 μg/m [3] (U.S. EPA, 2011a, p. 2-33).

Collectively, the Policy Assessment concluded that currently available evidence provided support for associations between long-term PM 2.5 exposure and mortality and morbidity effects that extend to distributions of PM 2.5 concentrations that are lower than those that had previously been associated with such effects, with aggregate long-term mean PM 2.5 concentrations extending to well below the level of the current annual standard.

The Policy Assessment also considered the long-term mean PM 2.5 concentrations in short-term exposure studies in assessing the appropriateness of the level of the current annual standard. See American Farm Bureau Federation v. EPA, 559 F. 3d at 522, 523-24 (remanding 2006 standard because the EPA had not adequately explained its choice not to consider long-term means of short-term exposure studies in assessing adequacy of primary annual PM 2.5 standard). In light of the mixed findings reported in single-city, short-term exposure studies, the Policy Assessment placed comparatively greater weight on the results from multi-city studies in considering the adequacy of the current suite of standards (U.S. EPA, 2011a, pp. 2-34 to 2-35).

With regard to associations reported in short-term PM 2.5 exposure studies, the Policy Assessment recognized that long-term mean concentrations reported in new multi-city U.S. and Canadian studies provided evidence of associations between short-term PM 2.5 exposure and mortality at similar air quality distributions to those that had previously been observed in an 8-cities Canadian study (Burnett and Goldberg, 2003; aggregate long-term mean PM 2.5 concentration of 13.3 μg/m [3] ). In a multi-city time-series analysis of 112 U.S. cities, Zanobetti and Schwartz (2009) reported a positive and statistically significant association with all-cause, cardiovascular-related (e.g., heart attacks, stroke), and respiratory-related mortality and short-term PM 2.5 exposure, in which the aggregate long-term mean PM 2.5 concentration was 13.2 μg/m [3] (U.S. EPA, 2009a, Figure 6-24). Furthermore, city-specific effect estimates indicated the association between short-term exposure to PM 2.5 and total mortality and cardiovascular- and respiratory-related mortality was consistently positive for an overwhelming majority (99 percent) of the 112 cities across a wide range of air quality concentrations (long-term mean concentrations ranging from 6.6 μg/m [3] to 24.7 μg/m [3] ; U.S. EPA, 2009a, Figure 6-24, p. 6-178 to 179). The EPA staff noted that for all-cause mortality, city-specific effect estimates were statistically significant for 55 percent of the 112 cities, with long-term city-mean PM 2.5 concentrations ranging from 7.8 μg/m [3] to 18.7 μg/m [3] and 24-hour PM 2.5 city-mean 98th percentile concentrations ranging from 18.4 to 64.9 μg/m [3] (personal communication with Dr. Antonella Zanobetti, 2009). [42]

With regard to cardiovascular and respiratory morbidity effects, in the first analysis of the Medicare cohort conducted by Dominici et al. (2006a) across 204 U.S. counties, investigators reported a statistically significant association with hospitalizations for cardiovascular and respiratory diseases and short-term PM 2.5 exposure, in which the aggregate long-term mean PM 2.5 concentration was 13.4 μg/m [3] . Furthermore, a sub-analysis restricted to days with 24-hour average concentrations of PM 2.5 at or below 35 μg/m [3] indicated that, in spite of a reduced statistical power from a smaller number of study days, statistically significant associations were still observed between short-term exposure to PM 2.5 and hospital admissions for cardiovascular and respiratory diseases (Dominici, 2006b). [43] In an extended analysis of this cohort, Bell et al. (2008) reported a positive and statistically significant increase in cardiovascular hospitalizations associated with short-term PM 2.5 exposure, in which the aggregate long-term mean PM 2.5 concentration was 12.9 μg/m [3] . These results, along with the observation that approximately 50 percent of the 204 county-specific mean 98th percentile PM 2.5 concentrations in the study aggregated across all years were below the 24-hour standard of 35 μg/m [3] , not only indicated that effects are occurring in areas that would meet the current standards but also suggested that the overall health effects observed across the U.S. are not primarily driven by the higher end of the PM 2.5 air quality distribution (Bell, 2009a, personal communication from Dr. Michelle Bell regarding air quality data for Bell et al., 2008 and Dominici et al., 2006a).

Collectively, the Policy Assessment concluded that the findings from short-term PM 2.5 exposure studies provided evidence of PM 2.5-associated health effects occurring in areas that would likely have met the current suite of PM 2.5 standards (U.S. EPA, 2011a, p. 2-35). These findings were further bolstered by evidence of statistically significant PM 2.5-related health effects occurring in analyses restricted to days in which 24-hour average PM 2.5 concentrations were below 35 μg/m [3] (Dominici, 2006b).

In evaluating the currently available scientific evidence, as summarized in section III.B of the proposal, the Policy Assessment first concluded that there was stronger and more consistent and coherent support for associations between long- and short-term PM 2.5 exposures and a broad range of health outcomes than was available in the last review, providing the basis for fine particle standards at least as protective as the current PM 2.5 standards (U.S. EPA, 2011a, p. 2-26). Having reached this initial conclusion, the Policy Assessment addressed the question of whether the available evidence supported consideration of standards that were more protective than the current standards. In so doing, the Policy Assessment considered whether there was now evidence that health effect associations have been observed in areas that likely met the current suite of PM 2.5 standards. As discussed above, long- and short-term PM 2.5 exposure studies provided evidence of associations with mortality and cardiovascular and respiratory effects both at lower ambient PM 2.5 concentrations than had been observed in the previous review and at concentrations allowed by the current standards (U.S. EPA, 2011a, p. 2-35).

In reviewing this information, the Policy Assessment recognized that important limitations and uncertainties associated with this expanded body of scientific evidence, as discussed in section III.B.2 of the proposal, needed to be carefully considered in determining the weight to be placed on the body of studies available in this review. Taking these limitations and uncertainties into consideration, the Policy Assessment concluded that the currently available evidence clearly calls into question whether the current suite of primary PM 2.5 standards protects public health with an adequate margin of safety from effects associated with long- and short-term exposures. Furthermore, the Policy Assessment concluded this evidence provides strong support for considering fine particle standards that would afford increased protection beyond that afforded by the current standards (U.S. EPA, 2011a, p. 2-35).

In addition to evidence-based consideration, the Policy Assessment also considered the extent to which health risks estimated to occur upon simulating just meeting the current PM 2.5 standards may be judged to be important from a public health perspective, taking into account key uncertainties associated with the quantitative health risk estimates. In so doing, the Policy Assessment first noted that the quantitative risk assessment addresses: (1) The core PM 2.5-related risk estimates; (2) the related uncertainty and sensitivity analyses, including additional sets of reasonable risk estimates generated to supplement the core analysis; (3) an assessment of the representativeness of the urban study areas within a national context; [44] and (4) consideration of patterns in design values and air quality monitoring data to inform interpretation of the risk estimates, as discussed in section III.C above.

In considering the health risks estimated to remain upon simulation of just meeting the current suite of standards and considering both the qualitative and quantitative assessment of uncertainty completed as part of the assessment, the Policy Assessment concluded these risks are important from a public health standpoint and provided strong support for consideration of alternative standards that would provide increased protection beyond that afforded by the current PM 2.5 (U.S. EPA, 2011a, pp. 2-47 to 2-48). This conclusion reflected consideration of both the severity and the magnitude of the effects. For example, the Risk Assessment indicated the possibility that premature deaths related to ischemic heart disease associated with long-term PM 2.5 exposure alone would likely be on the order of thousands of deaths per year in the 15 urban study areas upon simulating just meeting the current standards [45] (U.S. EPA, 2011a, pp. 2-46 to 2-47). Moreover, additional risks were anticipated for premature mortality related to cardiopulmonary effects and lung cancer associated with long-term PM 2.5 exposure as well as mortality and cardiovascular- and respiratory-related morbidity effects (e.g., hospital admissions, emergency department visits) associated with short-term PM 2.5 exposures. Based on the consideration of both qualitative and quantitative assessments of uncertainty completed as part of the quantitative risk assessment, the Risk Assessment concluded that it was unlikely that the estimated risks are over-stated, particularly for mortality related to long-term PM 2.5 exposure, and may well be biased low based on consideration of alternative model specifications evaluated in the sensitivity analyses (U.S. EPA, 2010a, p. 5-16; U.S. EPA, 2011a, p. 2-41). Furthermore, the currently available scientific information summarized in section III.B of the proposal provided evidence for a broader range of health endpoints and at-risk populations beyond those included in the quantitative risk assessment (U.S. EPA, 2011a, p. 2-47).

b. CASAC Advice

The CASAC, based on its review of drafts of the Integrated Science Assessment, the Risk Assessment, and the Policy Assessment, provided an array of advice both with regard to interpreting the scientific evidence and quantitative risk assessment, as well as with regard to consideration of the adequacy of the current PM 2.5 standards (Samet, 2009a,b,c,d,e,f; Samet 2010a,b,c,d). With regard to the adequacy of the current standards, CASAC concluded that the “currently available information clearly calls into question the adequacy of the current standards” (Samet, 2010d, p. i) and that the current standards are “not protective” (Samet, 2010d, p. 1). Further, in commenting on the first draft Policy Assessment, CASAC noted:

With regard to the integration of evidence-based and risk-based considerations, CASAC concurs with EPA's conclusion that the new data strengthens the evidence available on associations previously considered in the last round of the assessment of the PM 2.5 standard. CASAC also agrees that there are significant public health consequences at the current levels of the standard that justify consideration of lowering the PM 2.5 NAAQS further (Samet, 2010c, p. 12).

c. Administrator's Proposed Conclusions Concerning the Adequacy of the Current Primary PM 2.5 Standards

At the time of the proposal, in considering the body of scientific evidence, the Administrator concluded there was stronger and more consistent and coherent support for associations between long- and short-term PM 2.5 exposure and a broader range of health outcomes than was available in the last review, providing the basis for fine particle standards at least as protective as the current PM 2.5 standards. In particular, the Administrator recognized in section III.D.4 of the proposal that the Integrated Science Assessment concluded that the results of epidemiological and experimental studies form a plausible and coherent data set that supports a causal relationship between long- and short-term PM 2.5 exposures and mortality and cardiovascular effects and a likely causal relationship between long- and short-term PM 2.5 exposures and respiratory effects. Furthermore, the Administrator reflected that effects had been observed at lower ambient PM 2.5 concentrations than what had been observed in the last review, including at ambient PM 2.5 concentrations in areas that likely met the current PM 2.5 NAAQS. With regard to the results of the quantitative risk assessment, the Administrator noted that the Risk Assessment concluded that the risks estimated to remain upon simulation of just meeting the current standards were important from a public health standpoint in terms of both the severity and magnitude of the effects.

At the time of the proposal, in considering whether the current suite of PM 2.5 standards should be revised to provide requisite public health protection, the Administrator carefully considered the staff conclusions and rationales presented in the Policy Assessment, the advice and recommendations from CASAC, and public comments to date on this issue. In so doing, the Administrator placed primary consideration on the evidence obtained from the epidemiological studies and provisionally found the evidence of serious health effects reported in long- and short-term exposure studies conducted in areas that would have met the current standards to be compelling, especially in light of the extent to which such studies are part of an overall pattern of positive and frequently statistically significant associations across a broad range of studies that collectively represent a strong and robust body of evidence.

As discussed in the Integrated Science Assessment and Policy Assessment, the Administrator recognized that much progress has been made since the last review in addressing some of the key uncertainties that were important considerations in establishing the current suite of PM 2.5 standards. For example, progress made since the last review provides increased confidence in the long- and short-term exposure studies as a basis for considering whether any revision of the annual standard is appropriate and increased confidence in the short-term exposure studies as a basis for considering whether any revision of the 24-hour standard is appropriate. [46]

Based on her consideration of these conclusions, as well as consideration of CASAC's conclusion that the evidence and risk assessment clearly called into question the adequacy of the public health protection provided by the current PM 2.5 NAAQS and public comments on the proposal, the Administrator provisionally concluded that the current primary PM 2.5 standards, taken together, were not requisite to protect public health with an adequate margin of safety and that revision was needed to provide increased public health protection. The Administrator provisionally concluded that the scientific evidence and information on risk provided strong support for consideration of alternative standards that would provide increased public health protection beyond that afforded by the current PM 2.5 standards.

2. Comments on the Need for Revision

This section addresses general comments based on relevant facts that either support or oppose any change to the current suite of primary PM 2.5 standards. Comments on specific long- and short-term exposure studies that relate to consideration of the appropriate levels of the annual and 24-hour standards are addressed in section III.E.4 below. Many public comments asserted that the current PM 2.5 standards are insufficient to protect public health with an adequate margin of safety and that revisions to the standards are therefore appropriate, indeed necessitated.

Among those calling for revisions to the current standards were the Children's Health Protection Advisory Committee (CHPAC); major medical and public health groups including the American Heart Association (AHA), American Lung Association (ALA), American Public Health Association (APHA), American Thoracic Society (ATS); the Physicians for Social Responsibility (PSR); major environmental groups such as the Clean Air Council, Clean Air Task Force, Earthjustice, Environmental Defense Fund (EDF), National Resources Defense Council (NRDC), and Sierra Club; many environmental justice organizations as well as medical doctors, academic researchers, health professionals, and many private citizens. For example, the American Heart Association and other major national public health and medical organizations stated that, “[o]ur organizations are keenly aware of the public health and medical threats from particulate matter” and called on the EPA to “significantly strengthen” both the annual and 24-hour PM 2.5 standards “to help us protect the health of our patients and our nation” (AHA et al., 2012, pp. 1 and 13). AHA et al. and ALA et al., as well as a group of more than 350 physicians, environmental health researchers, and public health and medical professionals articulated similar comments on the available evidence:

Ample scientific evidence supports adopting tighter standards to protect the health of people who are most susceptible to the serious health effects of these pollutants. More than 10,000 peer-reviewed scientific studies have been published since 1997 when EPA adopted the current annual standard. These studies validate and extend earlier epidemiologic research linking both acute and chronic fine particle pollution with serious morbidity and mortality. The newer research has also expanded our understanding of the range of health outcomes associated with PM and has identified adverse respiratory and cardiovascular health effects at lower exposure levels than previously reported. As discussed and interpreted in the EPA's 2009 Integrated Science Assessment for Particulate Matter, the new evidence reinforces already strong existing studies and supports the conclusion that PM 2.5 is causally associated with numerous adverse health effects in humans at exposure levels far below the current standard. Such a conclusion demands prompt action to protect human health. (AHA et al., 2012, pp. 1 to 2; ALA et al., pp. 4 to 5; similar comment submitted by Rom et al., 2012, p. 1).

All of these medical and public health commenters stated that the current PM 2.5 standards need to be revised, and that even more protective standards than those proposed by the EPA are needed to adequately protect public health, particularly for at-risk populations. Many environmental justice organizations and individual commenters also expressed such views.

The National Association of Clean Air Agencies (NACAA), the Northeast States for Coordinated Air Use Management (NESCAUM), and many State and local air agencies and health departments who commented on the PM 2.5 standards supported revision of the suite of current PM 2.5 standards, as did five state attorneys general (Schneiderman et al., 2012) and the National Tribal Air Association (NTAA).

These commenters based their views chiefly on the body of evidence and technical analyses presented and discussed in the Integrated Science Assessment, the Risk Assessment, and the Policy Assessment finding the available scientific information to be stronger and more compelling than in the last review. These commenters generally placed much weight on CASAC's recommendation to revise the PM 2.5 standards to provide increased public health protection and on the EPA staff conclusions presented in the final Policy Assessment.

Some of these commenters specifically mentioned extended analyses of seminal long-term exposure studies—the ACS (Krewski et al., 2009), Harvard Six Cities (Laden et al., 2006), and Southern California Children's Health (Gauderman et al., 2004) studies. These commenters also highlighted the availability of additional long-term exposure studies in this review, specifically a study of premature mortality in older adults (Eftim et al., 2008) and the WHI study of cardiovascular morbidity and mortality effects in women (Miller et al., 2007) providing stronger evidence of mortality and morbidity effects associated with long-term PM 2.5 exposures at lower concentrations than had previously been observed, including studies of effects in at-risk populations. For example, some commenters asserted:

Evidence during the last review showed clearly that the annual average standard needed to be much lower than the standard of 15 µg/m [3] that was first set in 1997. The evidence has only grown since then. Multiple, multi-city studies over long periods of time have shown clear evidence of premature death, cardiovascular and respiratory harm as well as reproductive and developmental harm at contemporary concentrations far below the level of the current (annual) standard (ALA et al., 2012, p. 39; AHA et al., 2012, p. 10).

These commenters also highlighted the availability of a number of short-term PM 2.5 exposure studies as providing evidence of mortality and morbidity effects at concentrations below the level of the current 24-hour PM 2.5 standard. Specifically, these commenters made note of multi-city studies of premature mortality (Zanobetti and Schwartz, 2009) and increased hospitalizations for cardiovascular and respiratory-related effects in older adults (Bell et al., 2008). These commenters also asserted the importance of many of the single-city studies, arguing that these studies “provide valuable information regarding impacts on susceptible populations and on health risk in areas with high peak to mean concentration ratios” (ALA, et al., 2012, p. 65). Collectively, considering the multi- and single-city short-term exposure studies, these commenters asserted “the record clearly supports a more stringent 24-hour standard of 25 µg/m [3] to provide uniform protection in all regions of the country particularly from short-term spikes in pollution and from the sub-daily exposures that trigger heart attacks and strokes” (ALA et al., 2012, p. 62). A group of more than 350 physicians, environmental health researchers, and public health and medical professionals argued, “[s]tudies of short-term exposure demonstrate that PM 2.5 air pollution increases the risk of hospital admissions for heart and lung problems even when you exclude days with pollution concentrations at or above the current daily standard of 35 µg/m [3] . Daily concentrations must be capped at lower levels to protect against peak exposure days that occur due to local and seasonal sources of emissions” (Rom et al., 2012, p. 2).

In addition, many of these commenters generally concluded that progress had been made in reducing many of the uncertainties identified in the last review, in better understanding mechanisms by which PM 2.5 may be causing the observed health effects, and in improving our understanding of at-risk populations. Further, a number of commenters argued that by making the standards more protective, the PM 2.5 NAAQS would be more consistent with other existing standards (e.g., California's annual average standard of 12 µg/m [3] ) (CARB, 2012; CA OEHHA, 2012). Other commenters argued that revising the primary PM 2.5 standards would be more consistent with the recommendations of the World Health Organization (WHO) and/or Canada (e.g., ALA et al., 2012, p. 62; ISEE, 2012, p. 2; MOE-Ontario, 2012, p. 1).

With regard to the scope of the literature reviewed for PM 2.5-related health effects, some commenters asserted that the EPA inappropriately narrowed the scope of the review by excluding a number of categories of relevant studies, specifically related to studies of diesel pollution and traffic-related pollution (ALA, et al., 2012, p. 17). These commenters argued that, based upon the exclusion of these types of studies, the Integrated Science Assessment “came to the erroneous conclusion that the causal relationship between PM and cancer is merely suggestive. This conclusion does not square with the International Agency Research on Cancer (IARC) finding that diesel emissions are a known human carcinogen nor with the conclusions of the extended analyses of the [Harvard] Six Cities and ACS cohort studies that report positive and statistically significant associations between PM 2.5 and lung cancer.”Id.

Some of these commenters also noted the results of the EPA's quantitative risk assessment, concluding that it showed that the risks estimated to remain when the current standards are met are large and important from a public health perspective and warrant increased protection. For example, ALA et al., noted that the Risk Assessment indicated the quantitative risk analyses likely underestimated PM 2.5-related mortality (U.S. EPA, 2010a, p. 5-16) and argued that “the measurements of risk should be treated conservatively” (ALA, et al., 2012, p. 73). These commenters also summarized an expanded analysis of alternative PM 2.5 standard levels that they argued documented the need for more protective standards (McCubbin, 2011).

In general, all of these commenters agreed on the importance of results from the large body of scientific studies reviewed in the Integrated Science Assessment and on the need to revise the suite of PM 2.5 standards as articulated in the EPA's proposal, while generally differing with the EPA's proposed judgments about the extent to which the standards should be revised based on this evidence, specifically for providing protection for at-risk populations.

The EPA generally agrees with these commenters' conclusion regarding the need to revise the current suite of PM 2.5 standards. The scientific evidence noted by these commenters was generally the same as that assessed in the Integrated Science Assessment and the Policy Assessment, and the EPA agrees that this evidence provides a strong basis for concluding that the current PM 2.5 standards, taken together, are not requisite to protect public health with an adequate margin of safety, and they need to be revised to provide increased protection. For reasons discussed in section III.E.4.c below, however, the EPA disagrees with aspects of these commenters' views on the level of protection that is appropriate.

The EPA disagrees with these commenters' views that diesel exhaust studies were excluded from the Integrated Science Assessment and were not considered when making the causality determination for cancer, mutagenicity, and genotoxicity. As discussed in section 7.5 of the Integrated Science Assessment, diesel exhaust studies were integrated within the broader body of scientific evidence that was considered in reaching the causality determination for these health endpoints. Additionally, as discussed in section 1.5.3 of the Integrated Science Assessment, the evidence from diesel exhaust studies was also considered as part of the collective evidence evaluated when making determinations for other, noncancer health outcomes (e.g., cardiovascular and respiratory effects). [47] Specifically, when evaluating this evidence, the focus was on understanding the effects of diesel exhaust particles.

It is important to recognize that the Integrated Science Assessment focused on experimental studies of diesel exhaust that evaluated exposures that were relevant to ambient concentrations, i.e., “within one or two orders of magnitude of ambient PM concentrations” (U.S. EPA, 2009a, section 1.3). The causal determination for cancer, mutagenicity, and genotoxicity presented in the Integrated Science Assessment represents an integration of experimental and observational evidence of exposures to ambient PM concentrations. The EPA fully considered the findings of studies that assessed these and other health effects associated with exposure to diesel particles in reaching causality determinations regarding health outcomes associated with PM 2.5 exposures. Furthermore, CASAC supported the EPA's change to the causal determination for cancer and long-term PM 2.5 concentrations from “inadequate” to “suggestive” (Samet, 2009f, p. 2).

With regard to traffic studies, the EPA disagrees with the commenters' views that traffic studies that focused on exposure indicators such as distance to roadways should have been included in the Integrated Science Assessment. These studies were excluded from consideration because they did not measure ambient concentrations of specific air pollutants, including PM 2.5, but instead were studies evaluating exposure to the undifferentiated “traffic related air pollution” mixture (ALA et al., 2012, p. 17) (U.S. EPA, 2009a, section 1.3). As a result, these studies do not add to the collective body of evidence on the relationship between long- or short-term exposure to ambient concentrations of PM 2.5 and health effects.

Some of these commenters also identified “new” studies that were not included in the Integrated Science Assessment as providing further support for the need to revise the primary PM 2.5 standards. As discussed in section II.B.3 above, the EPA notes that, as in past NAAQS reviews, the Agency is basing the final decisions in this review on the studies and related information included in the PM air quality criteria that have undergone CASAC and public review and will consider the “new” studies for purposes of decision making in the next PM NAAQS review. Nonetheless, in provisionally evaluating commenters' arguments (see Response to Comments document), the EPA notes that its provisional assessment of “new” science found that such studies did not materially change the conclusions in the Integrated Science Assessment (U.S. EPA, 2012b).

Another group of commenters opposed revising the current PM 2.5 standards. These views were most extensively presented in comments from the Utility Air Regulatory Group (UARG), representing a group of electric generating companies and organizations and several national trade associations; the American Petroleum Institute (API) representing more than 500 oil and natural gas companies; the National Association of Manufacturers (NAM), the American Chemistry Council (ACC), the American Fuel & Petroleum Manufacturers (AFPM), the Alliance of Automobile Manufacturers (AAM), and other manufacturing associations; the Electric Power Research Institute (EPRI); and the Texas Commission on Environmental Quality (Texas CEQ). These commenters generally mentioned many of the same studies that were cited by the commenters who supported revising the standards, as well as other studies, but highlighted different aspects of these studies in reaching substantially different conclusions about their strength and the extent to which progress has been made in reducing uncertainties in the evidence since the last review. Furthermore, they asserted that the evidence that has become available since the last review does not establish a more certain risk or a risk of effects that are significantly different in character to those that provided a basis for the current standards, nor does the evidence demonstrate that the risk to public health upon attainment of the current standards would be greater than was understood when the EPA established the current standards in 2006.

These commenters generally expressed the view that the current standards provide the requisite degree of public health protection. In supporting their view, these commenters generally argued that the EPA's conclusions are inconsistent with the current state of the science and questioned the underlying scientific evidence including the causal determinations reached in the Integrated Science Assessment. More specifically, this group of commenters argued that: (1) The EPA did not apply its framework for causal determination consistently across studies or health outcomes and, in the process, the EPA relied on a selective group of long- and short-term exposure studies to reach conclusions regarding causality; (2) toxicological and controlled human exposure studies do not provide supportive evidence that the health effects observed in epidemiological studies are biologically plausible; (3) uncertainties in the underlying health science are as great or greater than in 2006; (4) there is no evidence of greater risk since the last review to justify tightening the current annual PM 2.5 standard; and (5) “new” studies not included in the Integrated Science Assessment continue to increase uncertainty about possible health risks associated with exposure to PM 2.5. These comments are discussed in turn below.

(l) Some of these commenters asserted that the EPA did not apply its framework for causal determinations consistently across studies or health outcomes (e.g., ACC, 2012, Attachment A, pp. 1 to 2; API, 2012, Attachment 1, p. 30; NAM et al., 2012, pp. 22 to 25; Texas CEQ, 2012, pp 2 to 3). [48] These commenters argued that the EPA downplayed epidemiological studies with null or inconsistent results, inappropriately used the Hill criteria when evaluating the epidemiological evidence, and used the same study and the same underlying database to conclude that there was a causal association between mortality and multiple criteria pollutants.

The EPA disagrees with these commenters' views. First, the EPA recognizes that the evaluation of the scientific evidence and its application of the causal framework used in the current PM NAAQS review was the subject of exhaustive and detailed review by CASAC and the public. As summarized in section II.B.3 above, prior to finalizing the Integrated Science Assessment, two drafts were released for CASAC and public review to evaluate the scientific integrity of the documents. Evidence related to the substantive issues raised by CASAC and public commenters with regard to the content of the first and second draft Integrated Science Assessments were discussed at length during these public CASAC meetings and considered in developing the final Integrated Science Assessment. CASAC supported the development of the EPA's causality framework and its use in the current PM NAAQS review and concluded:

The five-level classification of strength of evidence for causal inference has been systematically applied; this approach has provided transparency and a clear statement of the level of confidence with regard to causation, and we recommend its continued use in future Integrated Science Assessments (Samet 2009f, p. 1).

These commenters asserted that during the application of the causal framework the EPA inappropriately relied on a selective group of long- and short-term exposure studies in reaching causal inferences (API, 2012, pp 12 to 17; ACC, 2012, Attachment A, pp 1 to 2; NAM et al., 2012, pp. 22 to 25; Texas CEQ, 2012, pp 2 to 3). Additionally, these commenters expressed the view that the EPA focused on a subset of epidemiological studies that reported positive and statistically significant results while ignoring other studies, especially those that reported no statistically significant associations, those that reported potential thresholds, or those that highlighted uncertainties and limitations in study design or results. Furthermore, some of these commenters argued that epidemiological studies are observational in nature and cannot provide evidence of a causal association.

The EPA disagrees with these commenters' views on assessing the health effects evidence and on the conclusions regarding the causality determinations reached in the Integrated Science Assessment. In conducting a comprehensive evaluation of the evidence in the Integrated Science Assessment, the EPA recognized the distinction between the evaluation of the relative scientific quality of individual study results and the evaluation of the pattern of results within the broader body of scientific evidence and considered both in reaching causality determinations. The more detailed characterizations of individual studies included an assessment of the quality of the study based on specific criteria as described in the Integrated Science Assessment (U.S. EPA, 2009a, section 1.5.3).

In developing an integrated assessment of the health effects evidence for PM, the EPA emphasized the importance of examining the pattern of results across various studies and did not focus solely on statistical significance [49] as a criterion of study strength. This approach is consistent with views clearly articulated throughout the epidemiological and causal inference literature, specifically, that it is important not to focus on results of statistical tests to the exclusion of other information. [50] The concepts underlying the EPA's approach to evaluating statistical associations have been discussed in numerous publications, including a report by the U.S. Surgeon General on the health consequences of smoking (Centers for Disease Control and Prevention, 2004). This report cautions against over-reliance on statistical significance in evaluating the overall evidence for an exposure-response relationship. Criteria characterized by Hill (1965) also addressed the value, or lack thereof, of statistical tests in the determination of cause:

No formal tests of significance can answer those [causal] questions. Such tests can, and should, remind us of the effects the play of chance can create, and they will instruct us in the likely magnitude of those effects. Beyond that, they contribute nothing to the `proof' of our hypothesis (Hill, 1965, p. 299).

The statistical significance of individual study findings has played an important role in the EPA's evaluation of the study's results and the EPA has placed greater emphasis on studies reporting statistically significant results. However, in the broader evaluation of the evidence from many epidemiological studies, and subsequently during the process of forming causality determinations in integrating evidence across epidemiological, controlled human exposure, and toxicological studies, the EPA has emphasized the pattern of results across epidemiological studies, and whether the effects observed were coherent across the scientific disciplines for drawing conclusions on the relationship between PM 2.5 and different health outcomes. Thus, the EPA did not limit its focus or consideration to just studies that reported positive associations or where the results were statistically significant.

In addition, some commenters asserted that the EPA inappropriately used the Hill criteria by failing to consider the limitations of studies with weak associations, thereby overstating the consistency of the observed associations (API, 2012, Attachment 1, pp. 30 to 35). These commenters argued that risk estimates greater than 3 to 4 reflect strong associations supportive of a causal link, while smaller risk estimates (i.e., 1.5 to 3) are considered to be weak and require other lines of evidence to demonstrate causality.

As discussed in section 1.5.3 of the Integrated Science Assessment, the EPA thoroughly considered the limitations of all studies during its evaluation of the scientific literature (U.S. EPA,, 2009a, p. 1-14). This collective body of evidence, including known uncertainties and limitations of the studies evaluated, were considered during the process of forming causality determinations as discussed in chapters 6 and 7 of the Integrated Science Assessment. For example, the EPA concluded that “a causal relationship exists between short-term PM 2.5 exposure and cardiovascular effects,” however, in reaching this conclusion, the Agency recognized and considered limitations of the current evidence that still requires further examination (U.S. EPA, 2009a., in section 6.2.12.1). Therefore, the EPA disagrees with these commenters' views that the Hill criteria were inappropriately used in that the limitations of studies were not considered.

The EPA also disagrees with the commenters' assertion that the magnitude of the association must be large to support a determination of causality. As discussed in the Integrated Science Assessment, the strength of the observed association is an important aspect to aid in judging causality and “while large effects support causality, modest effects therefore do not preclude it” (U.S. EPA, 2009a, Table 1-2, section 1.5.4). The weight of evidence approach used by the EPA encompasses a multitude of factors of which the magnitude of the association is only one component (U.S. EPA, 2009a, Table 1-3). An evaluation of the association across multiple investigators and locations supports the “reproducibility of findings [which] constitutes one of the strongest arguments for causality” (U.S. EPA, 2009a, Table 1-2). Even though the risk estimates for air pollution studies may be modest, the associations are consistent across hundreds of studies as demonstrated in the Integrated Science Assessment. Furthermore, the causality determinations rely on different lines of evidence, by integrating evidence across disciplines, including animal toxicological studies and controlled human exposure studies.

Furthermore, as summarized in section III.B above and discussed more fully in section III.B.3 of the proposal, the EPA recognizes that the population potentially affected by PM 2.5 is considerable, including large subgroups of the U.S. population that have been identified as at-risk populations (e.g., children, older adults, persons with underlying cardiovascular or respiratory disease). While individual effect estimates from epidemiological studies may be modest in size, the public health impact of the mortality and morbidity associations can be quite large given that air pollution is ubiquitous. Indeed, with the large population exposed, exposure to a pollutant causally associated at a population level with mortality and serious illness has significant public health consequences, virtually regardless of the relative risk. Taken together, this information indicates that exposure to ambient PM 2.5 concentrations has substantial public health impacts.

In addition, these commenters believed that the EPA downplayed null or inconsistent findings in numerous long-term mortality studies with reported PM 2.5 concentrations above and below the level of the current annual standard. The EPA disagrees that studies with null or inconsistent findings were not accurately presented and considered in the Integrated Science Assessment. For example, as discussed throughout section 7.6 and depicted in Figures 7-6 and 7-7 of the Integrated Science Assessment, the EPA presented the collective evidence from all studies that examined the association between long-term PM 2.5 exposure and mortality. Overall, across these studies there was evidence of consistent positive associations in different cohorts. That evidence, in combination with the biological plausibility provided by experimental and toxicological studies evaluated in sections 7.1 and 7.2 of the Integrated Science Assessment, supported a causal relationship exists between long-term PM 2.5 exposure and mortality.

Lastly, some of these commenters argued that in some cases, the EPA used the same study and the same underlying database to conclude that there is a causal association between mortality and multiple criteria pollutants. These commenters argued, “[i]n doing so, EPA attributes the cause of the mortality effects observed to whichever criteria pollutant it is reviewing at the time” (API, 2012, pp. 14 to 16).

The EPA strongly disagrees that the Agency “attributes the cause of mortality effects observed to whichever criteria pollutant it is reviewing at the time.” The EPA consistently recognizes that other pollutants are also associated with health outcomes, as is reflected in the fact that the EPA has established regulations to limit emissions of particulate criteria pollutants as well as other gaseous criteria pollutants. Epidemiological studies often examine the association between short- and long-term exposures to multiple air pollutants and mortality within a common dataset in an attempt to identify the air pollutant(s) of the complex mixture most strongly associated with mortality. In evaluating these studies, the EPA employs specific study selection criteria to identify those studies most relevant to the review of the NAAQS. In its assessment of the health evidence regarding PM 2.5, the EPA has carefully evaluated the potential for confounding, effect measure modification, and the role of PM 2.5 as a component of a complex mixture of air pollutants (U.S. EPA, 2009a, p. 1-9). The EPA used a rigorous weight of evidence approach to inform causality that evaluated consistency across studies within a discipline, evidence for coherence across disciplines, and biological plausibility. Additionally, during this process, the EPA assessed the limitations of each study in the context of the collective body of evidence. It was the collective evidence, not one individual study that ultimately determined whether a causal relationship exists between a pollutant and health outcome. In the Integrated Science Assessment, the combination of epidemiological and experimental evidence formed the basis for the Agency concluding for the first time that a causal relationship exists between short- or long-term exposure to a criteria pollutant and mortality (U.S. EPA, 2009, sections 2.3.1.1 and 2.3.1.2).

Additionally, while the EPA has evaluated some of the studies used to inform the causality determination for PM in the Integrated Science Assessments for other criteria air pollutants, the Agency has done so in the context of examining the collective body of evidence for each of the respective criteria air pollutants. As such, the body of evidence to inform causality has varied from pollutant to pollutant resulting in the association between each criteria air pollutant and mortality being classified at a different level of the five-level hierarchy used to inform causation (e.g., U.S. EPA, 2008e, U.S. EPA, 2008f, U.S. EPA, 2010k).

The EPA notes that the final causality determinations presented in the Integrated Science Assessment reflected CASAC's recommendations on the second draft Integrated Science Assessment (Samet, 2009f, pp. 2 to 3). Specifically, CASAC supported the EPA's changes (in the second versus first draft Integrated Science Assessment) from “likely causal” to “causal” for long-term exposure to PM 2.5 and cardiovascular effects and for cancer and PM 2.5 (from “inadequate” to “suggestive”). Id. Furthermore, CASAC recommended “upgrading” the causal classification for PM 2.5 and total mortality to “causal” for both the short- and long-term timeframes. Id. With regard to mortality, the “EPA carefully reevaluated the body of evidence, including the collective evidence for biological plausibility for mortality effects, and determined that a causal relationship exists for short- and long-term exposure to PM 2.5 and mortality, consistent with the CASAC comments” (Jackson, 2010).

(2) With regard to toxicological and controlled human exposure studies, these commenters argued that the available evidence does not provide coherence or biological plausibility for health effects observed in epidemiological studies (API, 2012, pp. 21 to 22, Attachment 1, pp. 25 to 29; AAM, 2012, pp. 15 to 16; Texas CEQ, 2012, p. 3). With regard to the issue of mechanisms, these commenters noted that although the EPA recognizes that new evidence is now available on potential mechanisms and plausible biological pathways, the evidence provided by toxicological and controlled human exposure studies still does not resolve all questions about how PM 2.5 at ambient concentrations could produce the mortality and morbidity effects observed in epidemiological studies. More specifically, for example, some of these commenters argued that:

A review of the Integrated Science Assessment, however, suggests that the experimental evidence is inconsistent and not coherent with findings in epidemiology studies. Specifically, the findings of mild and reversible effects in most experimental studies conducted at elevated exposures are not consistent with the more serious associations described in epidemiology studies (e.g., hospital admissions and mortality). Also, both animal studies and controlled human exposure studies have identified no effect levels for acute and chronic exposure to PM and PM constituents at concentrations considerably above ambient levels. EPA should consider the experimental findings in light of these higher exposure levels and what the relevance may be for ambient exposures (API, 2012, Attachment 1, p. 25).

The EPA notes that in the review completed in 1997, the Agency considered the lack of demonstrated biological mechanisms for the varying effects observed in epidemiological studies to be an important caution in its integrated assessment of the health evidence upon which the standards were based (71 FR 61157, October 17, 2006). In the review completed in 2006, the EPA recognized the findings from additional research that indicated that different health responses were linked with different particle characteristics and that both individual components and complex particle mixtures appeared to be responsible for many biologic responses relevant to fine particle exposures. Id. Since that review, there has been a great deal of research directed toward advancing our understanding of biologic mechanisms. While this research has not resolved all questions, and further research is warranted (U.S. EPA, 2011a, section 2.5), it has provided important insights as discussed in section III.B.1 of the proposal (77 FR at 38906 to 38909) and discussed more fully in the Integrated Science Assessment (U.S. EPA, 2009a, Chapter 5).

As noted in the proposal, toxicological studies provide evidence to support the biological plausibility of cardiovascular and respiratory effects associated with long- and short-term PM exposures observed in epidemiological studies (77 FR 38906) and provide supportive mechanistic evidence that the cardiovascular morbidity effects observed in long-term exposure epidemiological studies are coherent with studies of cardiovascular-related mortality (77 FR 38907). The Integrated Science Assessment concluded that the new evidence available in this review “greatly expands” upon the evidence available in the last review “particularly in providing greater understanding of the underlying mechanisms for PM 2.5 induced cardiovascular and respiratory effects for both short- and long-term exposures” (U.S. EPA, 2009a, p. 2-17). The mechanistic evidence now available, taken together with newly available epidemiological evidence, increases the Agency's confidence that a causal relationship exists between long- and short-term exposure to PM 2.5 and cardiovascular effects and mortality. [51] In addition, CASAC supported the Integrated Science Assessment approach and characterization of potential mechanisms or modes of action (Samet, 2009e, pp. 7 to 8; Samet, 2009f, p. 11), as well as the findings of a causal relationship at the population level between exposure to PM 2.5 and mortality and cardiovascular effects (Samet, 2009f, pp. 2 to 3). [52]

Additionally, the EPA disagrees with commenters that the mild and reversible effects observed in controlled human exposure studies are inconsistent with the more serious effects observed in epidemiological studies. Ethical considerations regarding the types of studies that can be performed with human subjects generally limit the effects that can be evaluated to those that are transient, reversible, and of limited short-term consequence. The relatively small number of subjects recruited for controlled exposure studies should also be expected to have less variability in health status and risk factors than occurring in the general population. [53] Consequently, the severity of health effects observed in controlled human exposure studies evaluating the effects of PM should be expected to be less than observed in epidemiologic studies. Nonetheless, that effects are observed in relatively healthy individuals participating in controlled exposure studies serves as an indicator that PM is initiating health responses and that more severe responses may reasonably be expected in a more diverse population.

It should also be noted that there is a small body of toxicological evidence demonstrating mortality in rodents exposed to PM (e.g., Killingsworth et al. 1997). Overall it is not surprising that lethality is not induced in more toxicological research, as these types of studies do not readily lend themselves to this endpoint. Epidemiological studies have observed associations between PM and mortality in communities with populations in the range of many thousands to millions of people. Clearly, it is not feasible to expose hundreds (if not thousands) of animals to ambient PM (potentially over many years) in a laboratory setting to induce enough lethalities to distinguish between natural deaths and those attributable to PM. Furthermore, the heterogeneous human populations sampled in epidemiological studies are comprised of individuals with different physical, genetic, health, and socioeconomic backgrounds which may impact the outcome. However, in toxicological studies, the rodent groups are typically inbred, such that inter-individual variability is minimized. Thus, if the rodent strain used is quite robust, PM-induced effects may not be observed at low exposure concentrations.

(3) In asserting that the uncertainties in the underlying health science are as great or greater than in the last review and therefore do not support revision to the standards at this time, commenters in this group variously discussed a number of issues related to: (a) Confounding, (b) heterogeneity in risk estimates, (c) exposure measurement error, (d) model specification, (e) the shape of the concentration-response relationship, and (f) understanding the relative toxicity of components within the mixture of fine particles. Each of these issues is addressed below and some are discussed in more detail in the Response to Comments document.

In summary, these commenters concluded that the substantial uncertainties present in the last review have not been resolved and/or that the uncertainty about the possible health risks associated with PM 2.5 exposure has not diminished. As discussed below, the EPA believes that the overall uncertainty about possible health risks associated with both long- and short-term PM 2.5 exposure has diminished to an important degree since the last review. While the EPA agrees that important uncertainties remain, and that future research directed toward addressing these uncertainties is warranted, the EPA disagrees with commenters' views that the remaining uncertainties in the scientific evidence are too great to warrant revising the current PM 2.5 NAAQS.

(a) Confounding

Some commenters have criticized the EPA for not adequately addressing the issue of confounding in both long- and short-term exposure studies of mortality and morbidity. This includes confounding due to copollutants, as well as unmeasured confounding. [54]

With regard to copollutant confounding, these commenters asserted that the EPA has not adequately interpreted the results from studies that examined the effect of copollutants on the relationship between long- and short-term PM 2.5 exposures and mortality and morbidity outcomes. These commenters contend that the EPA has inappropriately concluded that PM 2.5-related mortality and morbidity associations are generally robust to confounding. The commenters stated that statistically significant PM 2.5 associations in single-pollutant models in epidemiological studies do not remain statistically significant in copollutant models.

The loss of statistical significance or the reduction in the magnitude of the effect estimate when a co-pollutant model is used may be the result of factors other than confounding. These changes do not prove either the existence or absence of confounding. These impacts must be evaluated in a broader context that considers the entire body of evidence. The broader examination of this issue in the Integrated Science Assessment included a focus on evaluating the stability of the size of the effect estimates in epidemiological studies conducted by a number of research groups using single- and copollutant models (U.S. EPA, 2009a, sections 6.2.10.9, 6.3.8.5, and 6.5, Figures 6-5, 6-9, and 6-15). This examination found that, for most epidemiological studies, there was little change in effect estimates based on single- and copollutant models, although the Integrated Science Assessment recognized that in some cases, the PM 2.5 effect estimates were markedly reduced in size and lost statistical significance. Additionally, the EPA notes that these comments do not adequately reflect the complexities inherent in assessing the issue of copollutant confounding. As discussed in the proposal (77 FR 38907, 38909, and 38910) and more fully in the Integrated Science Assessment (U.S.EPA, 2009a, sections 6.2, 6.3, and 6.5), although copollutant models may be useful tools for assessing whether gaseous copollutants may be potential confounders, such models alone cannot determine whether copollutants are in fact confounders. Interpretation of the results of copollutant models is complicated by correlations that often exist among air pollutants, by the fact that some pollutants play a role in the atmospheric reactions that form other pollutants such as secondary fine particles, and by the statistical power of the studies in question inherent in the study methodology. For example, the every-third or sixth-day sampling schedule often employed for PM 2.5 measurements compared to daily measurements of gaseous copollutants drastically reduces the overall sample size to assess the effect of copollutants on the PM 2.5-morbidity or mortality relationship, such that the reduced sample size can lead to less precise effect estimates (e.g., wider confidence intervals).

The EPA recognizes that when PM 2.5 is correlated with gaseous pollutants it can be difficult to identify the effect of individual pollutants in the ambient mixture (77 FR 38910). However, based on the available evidence, the EPA concludes epidemiological studies continue to support the conclusion that PM 2.5 associations with mortality and morbidity outcomes are robust to the inclusion of gaseous copollutants in statistical models. The EPA evaluated the potential confounding effects of gaseous copollutants and, although it is recognized that uncertainties and limitations still remain, the Agency concluded the collective body of scientific evidence is “stronger and more consistent than in previous reviews providing a strong basis for decision making in this review” (77 FR 38910/1).

Several commenters offered detailed comments on the long-term PM 2.5 exposure studies arguing that associations from mortality studies are subjected to unmeasured confounding and as a result are not appropriately characterized as providing evidence of a causal relationship between long-term PM 2.5 exposure and mortality (e.g., UARG, 2012, pp. 10 to 11, Attachment A, pp. 17 to 23; API, 2012, pp. 13 to 14, Attachment 1, pp. 11 to 14, Attachment 7, pp. 2-10; ACC, 2012, p. 18 to 21; AFPM, 2012, p. 8; Texas CEQ, 2012, p. 4). Specifically, commenters cited two studies (i.e., Janes et al., 2007 and Greven et al., 2011) that used a new type of statistical analysis to examine associations between annual (long-term) and monthly (sub-chronic) PM 2.5 exposure and mortality. The commenters interpreted the results of these analyses as evidence of unmeasured confounding in the long-term PM 2.5 exposure-mortality relationship. These commenters interpreted these studies as raising fundamental questions regarding the EPA's determination that a causal relationship exists between long-term PM 2.5 exposure and mortality. In addition to the commenters mentioned above, all of the authors of the publications by Janes et al. (2007) and Greven et al. (2011) (i.e., Francesca Dominici, Scott Zeger, Holly Janes, and Sonja Greven) submitted a joint comment to the public docket in order to clarify specific points regarding these two studies (Dominici et al., 2012).

The first study, Janes et al. (2007), was evaluated in the Integrated Science Assessment (U.S. EPA, 2009a, p. 7-88). The second study, Greven et al. (2011), an extension of the Janes et al. (2007) study adding three more years of data, is a “new” study discussed in the Provisional Science Assessment (U.S. EPA, 2012). Both studies used nationwide Medicare mortality data to examine the association between monthly average PM 2.5 concentrations over the preceding 12 months and monthly mortality rates in 113 U.S. counties and examined whether community-specific trends in monthly PM 2.5 concentrations and mortality declined at the same rate as the national rate. The investigators examined this by decomposing the association between PM 2.5 and mortality into two components: (1) National trends, defined as the association between the national average trend in monthly PM 2.5 concentrations averaged over the previous 12 months and the national average trend in monthly mortality rates, and (2) local trends, defined as county-specific deviations in monthly PM 2.5 concentrations and monthly mortality rates from national trends.

The EPA does not question the results of the national trends analyses conducted by Janes et al. (2007) and Greven et al. (2011). [55] Both Janes et al. (2007) and Greven et al. (2011) observed positive and statistically significant associations between long-term exposure to PM 2.5 and mortality in their national analyses. However, Janes et al. (2007) and Greven et al. (2011) eliminated all of the spatial variation in air pollution and mortality in their data set when estimating the national effect, focusing instead on both chronic (yearly) and sub-chronic (monthly) temporal differences in the data (Dominici et al. 2012). Janes et al. (2007) (Table 1) highlighted that over 90 percent of the variance in the data set used for the analyses conducted by both Janes et al. (2007) and Greven et al. (2011) was attributable to spatial variability, which the authors chose to discard. As noted above, the focus of the analyses by Janes et al. (2007) and Greven et al. (2011) was on two components: (1) A temporal or time component, i.e., the “national” trends analysis, which examined the association between the national average trend in monthly PM 2.5 concentrations averaged over the previous 12 months and the national average trend in monthly mortality rates and (2) a space-by-time component, i.e., the “local” trends analysis, which examined county-specific deviations in monthly PM 2.5 concentrations and monthly mortality rates from national trends. These two components combined comprised less than 10 percent of the variance in the data set. The authors included a focus on the space-by-time component, which represented approximately 5 percent of the variance in the data set, in an attempt to identify, absent confounding, if PM 2.5 was associated with mortality at this unique exposure window. Thus, these studies are not directly comparable to other cohort studies investigating the relationship between long-term exposure to PM 2.5 and mortality, which make use of spatial variability in air pollution and mortality data. [56] This point was highlighted by the study authors who stated that “when one considers that this wealth of information is not accounted for in [Janes 2007], it is not as surprising that * * * vastly different estimates of the PM 2.5/mortality relationship [were observed] than in other studies that do exploit that variability” (Dominici et al., 2012, p. 2).

The EPA notes that the results of the local trends analyses conducted by Janes et al. (2007) and Greven et al. (2011) are limited by the monthly timescale used in these analyses. This view is consistent with comments on the Janes et al. (2007) study articulated in Pope and Burnett (2007), [57] which noted that an important limitation of the local scale analysis conducted by Janes et al. (2007) and subsequently by Greven et al. (2011) was the subchronic exposure window considered in these analyses. Both studies used annual average PM 2.5 concentrations to characterize long-term national trends which was consistent with exposure windows considered in other studies of long-term exposure to PM 2.5 and mortality. [58] However, the local scale analyses used monthly average PM 2.5 concentrations to characterize county-specific deviations from national trends (the local scale). The use of monthly average data likely does not provide enough exposure contrast to observe temporal changes in mortality at the local scale. It also represents a different exposure window than considered in the large body of evidence of health effects related to short-term (from less than one day to up to several days) and chronic (one or more years) measures of PM 2.5.

Furthermore, the EPA disagrees with commenters that studies by Janes et al. (2007) and Greven et al. (2011) provide evidence that other studies of long-term exposure to PM 2.5 and mortality are affected by unmeasured confounding. As noted above, the design of the studies conducted by Janes et al. (2007) and Greven et al. (2011) are fundamentally different than those used in other studies of long-term exposure to PM 2.5 and mortality, including the ACS cohort and the Harvard Six Cities study. Studies, such as the ACS and Harvard Six Cities studies, used the spatial variation between cities to measure the effect of long-term (annual) exposures to PM 2.5 on mortality risk, and did not conduct any analyses relying on the temporal variation in PM 2.5. The opposite is true of the Janes et al. (2007) and Greven et al. (2011) studies which first removed the spatial variability in PM 2.5 and then examined the temporal variation at both the national and local scale to measure the effects of temporal differences in PM 2.5 on mortality risk. Janes et al. (2007) and Greven et al. (2011) focus on changes in PM 2.5 concentrations over time and, therefore, control for confounders would be based on including variables that vary over time rather than over space. As a result, any evidence of potential confounding of the PM 2.5-mortality risk relationship derived from Janes et al. (2007) and Greven et al. (2011) cannot be extrapolated to draw conclusions related to potential spatial confounding in studies based on the spatial variation in PM 2.5 concentrations.

As detailed in the Integrated Science Assessment (U.S. EPA, 2009a, section 7.6), and recognized by the authors of Janes et al. (2007) and Greven et al. (2011), the cohort studies that informed the causality determination for long-term PM 2.5 exposure and mortality “have developed approaches to adjust for measured and unmeasured confounders” (Dominici et al., 2012, p. 2). These approaches were specifically designed to adjust for spatial confounding. The hypothesis that the authors of Janes et al. (2007) and Greven et al. (2011) chose to examine was that differences in the local and national effects indicated unmeasured temporal confounding in either the local or national effect estimate. This hypothesis was specific to these two studies that examined temporal variability in exposure to air pollution and did not include known potential confounders at either the national or local scale as time-varying covariates in the statistical model. The authors acknowledged that the interpretation of either the national or local estimates needs to occur with an appreciation of the potential confounding effects of national and local scale covariates that were omitted from the model (Dominici et al., 2012).

It is important to recognize that because Janes et al. (2007) and Greven et al. (2011) focused on variations in PM 2.5 over time and not space, the results from these two studies do not provide any indication that other studies of long-term exposure to PM 2.5 and mortality exhibit spatial confounding, or that PM 2.5 does not cause mortality. [59] The authors of Janes et al. (2007) and Greven et al. (2011) recognized that “it is entirely possible that these papers are looking for an association at a timescale for which no association truly exists” (Dominici et al., 2012, p. 3). Furthermore, as highlighted in the Integrated Science Assessment and discussed by Pope and Burnett (2007), the conclusions of Janes et al. (2007) “are overstated * * * [T]heir analysis tells us little or nothing about unmeasured confounding in those and related studies because the methodology of Janes et al. largely excludes the sources of variability that are exploited in those other studies. By using monthly mortality counts and lagged 12-month average pollution concentrations, the authors eliminate the opportunity to exploit short-term or day-to-day variability.”

In conclusion, the EPA interprets the results of the analyses conducted by Janes et al. (2007) and Greven et al. (2011) as being consistent with prior knowledge of examining associations with long-term exposure to PM 2.5 at the national scale using long-term average PM 2.5 concentrations. For the reasons presented above and discussed in more detail in the Response to Comments document, the Agency disagrees with the commenters' assumption that the results of Janes et al. (2007) and Greven et al. (2011) indicate unmeasured confounding in the results of other cohort studies of long-term exposure to PM 2.5 and mortality. Therefore, the EPA concludes that these studies do not invalidate the large body of epidemiological evidence that supports the EPA's determination that a causal relationship exists between long-term PM 2.5 exposure and mortality. [60]

(b) Heterogeneity in Risk Estimates

Some commenters argued that the heterogeneity in risk estimates observed in multi-city epidemiological studies and the lack of statistical significance in many regional or seasonal estimates highlights a potential bias associated with combined multi-city epidemiological study results (e.g., API, 2012, Attachment 1, pp. 15 to 19). These commenters further argued that more refined intra-urban exposure estimates conducted for two of the largest cities included in the ACS study, Los Angeles and New York City, based on land-use regression models and/or kriging methods (Krewski et al., 2009) “underscore the importance of considering city-specific health estimates, which may account for heterogeneity in PM 2.5 concentrations or other differences among cities, rather than relying on pooled nationwide results from multi-city studies” (API, 2012, Attachment 1, p. 17).

With respect to understanding the nature and magnitude of PM 2.5-related risks, the EPA agrees that epidemiological studies evaluating health effects associated with long- and short-term PM 2.5 exposures have reported heterogeneity in responses between cities and effect estimates across geographic regions of the U.S. (U.S. EPA, 2009a, sections 6.2.12.1, 6.3.8.1, 6.5.2, and 7.6.1; U.S. EPA, 2011a, p. 2-25). For example, when focusing on short-term PM 2.5 exposure, the Integrated Science Assessment found that multi-city studies that examined associations with mortality and cardiovascular and respiratory hospital admissions and emergency department visits demonstrated greater cardiovascular effects in the eastern versus the western U.S. (Dominici, et al., 2006a; Bell et al., 2008; Franklin et al. (2007, 2008)).

In addition, the Integrated Science Assessment evaluated studies that provided some evidence for seasonal differences in PM 2.5 risk estimates, specifically in the northeast. The Integrated Science Assessment found evidence indicating that individuals may be at greater risk of dying from higher exposures to PM 2.5 in the warmer months, and at greater risk of PM 2.5 associated hospitalization for cardiovascular and respiratory diseases during colder months of the year. The limited influence of seasonality on PM risk estimates in other regions of the U.S. may be due to a number of factors including varying PM composition by season, exposure misclassification due to regional tendencies to spend more or less time outdoors and air conditioning usage, and the prevalence of infectious diseases during the winter months (U.S. EPA, 2009a, p. 3-182).

Overall, the EPA took note in the proposal that uncertainties still remain regarding various factors that contribute to heterogeneity observed in epidemiological studies (77 FR 38909/3). Nonetheless, the EPA recognizes that this heterogeneity could be attributed, at least in part, to differences in PM 2.5 composition across the U.S., as well as to exposure differences that vary regionally such as personal activity patterns, microenvironmental characteristics, and the spatial variability of PM 2.5 concentrations in urban areas (U.S. EPA, 2009a, section 2.3.2; 77 FR 38910).

As recognized in the Policy Assessment, the current epidemiological evidence and the limited amount of city-specific speciated PM 2.5 data do not allow conclusions to be drawn that specifically differentiate effects of PM 2.5 in different locations (U.S. EPA, 2011a, p. 2-25). Furthermore, the Integrated Science Assessment concluded “that many constituents of PM 2.5 can be linked with multiple health effects, and the evidence is not yet sufficient to allow differentiation of those constituents or sources that are more closely related to specific health outcomes” (U.S. EPA, 2009a, p. 2-17). CASAC thoroughly reviewed the EPA's presentation of the scientific evidence indicating heterogeneity in PM 2.5 effect estimates in epidemiological studies and concurred with the overall conclusions presented in the Integrated Science Assessment.

(c) Exposure Measurement Error

Some commenters argued that the EPA did not adequately consider exposure measurement error, which they asserted is an important source of bias in epidemiological studies that can bias effect estimates in either direction (e.g., API, 2012, Attachment 1, pp. 19 to 20).

The EPA agrees that exposure measurement error is an important source of uncertainty and that the variability in risk estimates observed in multi-city studies could be attributed, in part, to exposure error due to measurement-related issues (77 FR 38910). However, the Agency disagrees with the commenters' assertion that exposure measurement error was not adequately considered in this review. The Integrated Science Assessment included an extensive discussion that addresses issues of exposure measurement error (U.S. EPA, 2009a, sections 2.3.2 and 3.8.6). Exposure measurement error may lead to bias in effect estimates in epidemiological studies. A number of studies evaluated in the last review (U.S. EPA, 2004, section 8.4.5) and in the current review (U.S. EPA, 2009a, section 3.8.6) have discussed the direction and magnitude of bias resulting from specified patterns of exposure measurement error (Armstrong 1998; Thomas et al. 1993; Carroll et al. 1995) and have generally concluded “classical” (i.e., random, within-person) exposure measurement error can bias effect estimates towards the null. Therefore, consistent with conclusions reached in the last review, the Integrated Science Assessment concluded “in most circumstances, exposure error tends to bias a health effect estimate downward” (U.S. EPA, 2009a, sections 2.3.2 and 3.8.6) (emphasis added). Thus, the EPA has both considered and accounted for the possibility of exposure measurement error, and the possible bias would make it more difficult to detect true associations, not less difficult.

(d) Model Specification

Commenters contended that the EPA did not account for the fact that “selecting an appropriate statistical model for epidemiologic studies of air pollution involves several choices that involve much ambiguity, scant biological evidence, and a profound impact on analytic results, given that many estimated associations are weak” (ACC, 2012, p. 5). For short-term exposure studies, the EPA recognizes, as summarized in the HEI review panel commentary that selecting a level of control to adjust for time-varying factors, such as temperature, in time-series epidemiological studies involves a trade-off (HEI, 2003). For example, if the model does not sufficiently adjust for the relationship between the health outcome and temperature, some effects of temperature could be falsely ascribed to the pollution variable. Conversely, if an overly aggressive approach is used to control for temperature, the result would possibly underestimate the pollution-related effect and compromise the ability to detect a small but true pollution effect (U.S. EPA, 2004, p. 8-236; HEI, 2003, p. 266). The selection of approaches to address such variables depends in part on prior knowledge and judgments made by the investigators, for example, about weather patterns in the study area and expected relationships between weather and other time-varying factors and health outcomes considered in the study. As demonstrated in section 6.5 of the Integrated Science Assessment, the EPA thoroughly considered each of these issues and the overall effect of different model specifications on the association between short-term PM 2.5 exposure and mortality. Regardless of the model employed, consistent positive associations were observed across studies that controlled for the potential confounding effects of time and weather using different approaches (U.S. EPA 2009a, Figure 6-27). The EPA also considered the influence of model specification in the examination of long-term PM 2.5 exposure studies. For example, in section 7.6 of the Integrated Science Assessment, Figures 7-6 and 7-7 summarize the collective evidence that evaluated the association between long-term PM 2.5 exposure and mortality. Regardless of the model used, these studies collectively found evidence of consistent positive associations between long-term PM 2.5 exposure and mortality.

The EPA, therefore, disagrees with commenters that model specification was not considered when evaluating the epidemiological evidence used to form causality determinations. The EPA specifically points out that the process of assessing the scientific quality and relevance of epidemiological studies includes examining “important methodological issues (e.g., lag or time period between exposure and effects, model specifications, thresholds, mortality displacement) related to interpretation of the health evidence (U.S. EPA, 2009, p. 1-9).” Consistent with the conclusions of the 2004 PM Air Quality Criteria Document, the EPA recognizes that there is still no clear consensus at this time as to what constitutes appropriate control of weather and temporal trends in short-term exposure studies, and that no single statistical modeling approach is likely to be most appropriate in all cases (U.S. EPA, 2004, p. 8-238). However, the EPA believes that the available evidence interpreted in light of these remaining uncertainties does provide increased confidence relative to the last review in the reported associations between short- and long-term PM 2.5 exposures and mortality and morbidity effects, alone and in combination with other pollutants.

(e) Concentration-Response Relationship

Additionally, commenters questioned the interpretation of the shape of the concentration-response relationship, specifically stating that multiple studies have demonstrated that there is a threshold in the PM-health effect relationship and that the log-linear model is not biologically plausible (API, 2012, Attachment 9; ACC, 2012, Appendix A, pp. 7 to 8). The EPA disagrees with this assertion due to the number of studies evaluated in the Integrated Science Assessment that continue to support the use of a no-threshold, log-linear model to most appropriately represent the PM concentration-response relationship (U.S. EPA, 2009a, section 2.4.3). While recognizing that uncertainties remain, the EPA believes that our understanding of this issue for both long- and short-term exposure studies has advanced since the last review. As discussed in the Integrated Science Assessment, both long- and short-term exposure studies have employed a variety of statistical approaches to examine the shape of the concentration-response function and whether a threshold exists. While the EPA recognizes that there likely are individual biological thresholds for specific health responses, the Integrated Science Assessment concluded the overall evidence from existing epidemiological studies does not support the existence of thresholds at the population level, for effects associated with either long-term or short-term PM exposures within the ranges of air quality observed in these studies (U.S. EPA, 2009a, section 2.4.3). [61] The Integrated Science Assessment concluded that this evidence collectively supported the conclusion that a no-threshold, log-linear model is most appropriate (U.S. EPA, 2009a, sections 6.2.10.10, 6.5.2.7, and 7.6.4). CASAC likewise advised that “[a]lthough there is increasing uncertainty at lower levels, there is no evidence of a threshold” (Samet, 2010d, p. ii).

The EPA recognizes that some short-term exposure studies have examined the PM 2.5 concentration-response relationship in individual cities or on a city-to-city basis and observed heterogeneity in the shape of the concentration-response curve across cities. As discussed in (b) above, these findings are a source of uncertainty that the EPA agrees requires further investigation. Nonetheless, the Integrated Science Assessment concluded that “the studies evaluated further support the use of a no-threshold, log-linear model, but additional issues such as the influence of heterogeneity in estimates between cities and the effects of seasonal and regional differences in PM on the concentration-response-relationship still require further investigation” (U.S. EPA, 2009a, p. 2-25).

(f) Relative Toxicity of PM 2.5 Components

Some commenters highlighted uncertainties in understanding the role of individual constituents within the mix of fine particles. These commenters asserted that a mass-based standard may not be appropriate due to the growing body of evidence indicating that certain PM 2.5 components may be more closely related to specific health outcomes (e.g., EC and OC) (EPRI, 2012, p. 2).

With regard to questions about the role of individual constituents within the mix of fine particles, as a general matter, the EPA recognizes that although new research directed toward this question has been conducted since the last review, important questions remain and the issue remains an important element in the Agency's ongoing research program. At the time of the last review, the Agency determined that it was appropriate to continue to control fine particles as a group, as opposed to singling out any particular component or class of fine particles (71 FR 61162 to 61164, October 17, 2006). This distinction was based largely on epidemiological evidence of health effects using various indicators of fine particles in a large number of areas that had significant contributions of differing components or sources of fine particles, together with some limited experimental studies that provided some evidence suggestive of health effects associated with high concentrations of numerous fine particle components.

In this review, as discussed in the proposal (77 FR 38922 to 38923) and in section III.E.1 below, while most epidemiological studies continue to be indexed by PM 2.5 mass, several recent epidemiological studies included in the Integrated Science Assessment have used PM 2.5 speciation data to evaluate health effects associated with fine particle exposures. In the Integrated Science Assessment, the EPA thoroughly evaluated the scientific evidence that examined the effect of different PM 2.5 components and sources on a variety of health outcomes (U.S. EPA, 2009a, section 6.6) and observed that the available information continues to suggest that many different chemical components of fine particles and a variety of different types of source categories are all associated with, and probably contribute to, effects associated with PM 2.5. The Integrated Science Assessment concluded that the current body of scientific evidence indicated that “many constituents of PM can be linked with differing health effects and the evidence is not yet sufficient to allow differentiation of those constituents or sources that are more closely related to specific health outcomes” (U.S. EPA, 2009a, p. 2-26 and 6-212). Furthermore, the Policy Assessment concluded that the evidence is not sufficient to support eliminating any component or group of components associated with any specific source categories from the mix of fine particles included in the PM 2.5 indicator (U.S. EPA, 2009a, p. 2-56). CASAC agreed that it was reasonable to retain PM 2.5 as an indicator for fine particles in this review as “[t]here was insufficient peer-reviewed literature to support any other indicator at this time” (Samet, 2010c, p. 12).

This information is relevant to the Agency's decision to retain PM 2.5 as the indicator for fine particles as discussed in section III.E.1 below. The EPA also believes that it is relevant to the Agency's conclusion as to whether revision of the suite of primary PM 2.5 standards is appropriate. While there remain uncertainties about the role and relative toxicity of various components of fine PM, the current evidence continues to support the view that fine particles should be addressed as a group for purposes of public health protection.

In summary, in considering the above issues related to uncertainties in the underlying health science, on balance, the EPA believes that the available evidence interpreted in light of these remaining uncertainties does provide increased confidence relative to the last review in the reported associations between long- and short-term PM 2.5 exposures and mortality and morbidity effects, alone and in combination with other pollutants, and supports stronger inferences as to the causal nature of the associations. The EPA also believes that this increased confidence, when taken in context of the entire body of available health effects evidence and in light of the evidence from epidemiological studies of associations observed in areas meeting the current primary PM 2.5 standards, specifically in areas meeting the current primary annual PM 2.5 standard, adds support to its conclusion that the current suite of PM 2.5 standards needs to be revised to provide increased public health protection.

(4) In asserting that there is no evidence of greater risk since the 2006 review to justify lowering the current annual PM 2.5 standard, some commenters argued that, “if the current primary PM 2.5 annual standard of 15 μg/m [3] was considered to be adequately protective of public health in 2006, given relative risk estimates that EPA was using at that time, then that standard would surely still be adequately protective of the public health if relative risk estimates remain at the same level (or lower)” (UARG, 2012, Attachment 1, p. 24). These commenters compared risk coefficients used for mortality in the EPA's risk assessment done in the last review with those from the Agency's core risk assessment done as part of this review, and they concluded that “the entire range of the core relative risk for long-term mortality is lower now than it was in the prior review” (UARG, 2012, Attachment 1, p. 24). These commenters used this conclusion as the basis for a claim that there is no reason to revise the current annual PM 2.5 standard.

The EPA believes that this claim is fundamentally flawed. In comparing the scientific understanding of the risk presented by exposure to PM 2.5 between the last and current reviews, one must examine not only the quantitative estimate of risk from those exposures (e.g., the numbers of premature deaths or increased hospital admissions at various concentrations), but also the degree of confidence that the Agency has that the observed health effects are causally linked to PM 2.5 exposure at those concentrations. As documented in the Integrated Science Assessment and in the recommendations and conclusions of CASAC, the EPA recognizes significant advances in our understanding of the health effects of PM 2.5, based on evidence that is stronger than in the last review. As a result of these advances, the EPA is now more certain that fine particles, alone or in combination with other pollutants, present a significant risk to public health at concentrations allowed by the current primary PM 2.5 standards. From this more comprehensive perspective, since the risks presented by PM 2.5 are more certain, similar or even somewhat lower relative risk estimates would not be a basis to conclude that no revision to the suite of PM 2.5 standards is “requisite” to protect public health with an adequate margin of safety. This also ignores that the relative risk estimate is only one factor considered by the Administrator, e.g. it ignores that epidemiological studies since the last review indicate associations between PM 2.5 and mortality and morbidity in areas meeting the current annual standard.

In any case, the commenters' reliance on the flawed 2006 review is misplaced. As discussed in section III.A.2 above, the D.C. Circuit remanded Administrator Johnson's 2006 decision to retain the primary annual PM 2.5 standard because the Agency failed to adequately explain why the annual standard provided the requisite protection from both short- and long-term exposure to fine particles including protection for at-risk populations. The 2006 standard was also at sharp odds with CASAC advice and recommendations as to the requisite level of protection (Henderson, 2006a,b). In other words, the 2006 primary annual PM 2.5 standard is not an appropriate benchmark for comparison.

(5) Some of these commenters also identified “new” as well as older studies that had been included in prior reviews as providing additional evidence that the causality determinations presented in the Integrated Science Assessment did not consider the totality of the scientific literature, further supporting their view that a revision of the PM 2.5 is unwarranted. As discussed in section II.B.3 above, the EPA notes that, as in past NAAQS reviews, the Agency is basing the final decisions in this review on the studies and related information included in the Integrated Science Assessment that have undergone CASAC and public review, and will consider newly published studies for purposes of decisionmaking in the next PM NAAQS review. In provisionally evaluating commenters' arguments (see Response to Comments document), the EPA notes that its provisional assessment of “new” science found that such studies did not materially change the conclusions reached in the Integrated Science Assessment (U.S. EPA, 2012b).

3. Administrator's Final Conclusions Concerning the Adequacy of the Current Primary PM 2.5 Standards

Having carefully considered the public comments, as discussed above, the Administrator believes the fundamental scientific conclusions on the effects of PM 2.5 reached in the Integrated Science Assessment, and discussed in the Policy Assessment, are valid. In considering whether the suite of primary PM 2.5 standards should be revised, the Administrator places primary consideration on the evidence obtained from the epidemiological studies. The Administrator believes that this literature, combined with the other scientific evidence discussed in the Integrated Science Assessment, collectively represents a strong and generally robust body of evidence of serious health effects associated with both long- and short-term exposures to PM 2.5. As discussed in the Integrated Science Assessment and Policy Assessment, the EPA believes that much progress has been made since the last review in reducing some of the major uncertainties that were important considerations in establishing the current suite of PM 2.5 standards. In that context, the Administrator finds the evidence of serious health effects reported in exposure studies conducted in areas with long-term mean concentrations ranging from approximately at or above the level of the annual standard to long-term mean concentrations significantly below the level of the annual standard to be compelling, especially in light of the extent to which such studies are part of an overall pattern of positive and frequently statistically significant associations across a broad range of studies. The information in the quantitative risk assessment lends support to this conclusion.

There has been extensive critical review of this body of evidence, the quantitative risk assessment, and related uncertainties, including review by CASAC and the public. The public comments on the basis for the EPA's proposed decision to revise the suite of primary PM 2.5 standards have identified a number of issues about which different parties disagree including issues for which additional research is warranted. Having weighed all comments and the advice of CASAC, the Administrator believes that since the last review the overall uncertainty about the public health risks associated with both long- and short-term exposure to PM 2.5 has been diminished to an important degree. The remaining uncertainties in the available evidence do not diminish confidence in the associations between exposure to fine particles and mortality and serious morbidity effects. Based on her increased confidence in the association between exposure to PM 2.5 and serious public health effects, combined with evidence of such an association in areas that would meet the current standards, the Administrator agrees with CASAC that revision of the current suite of PM 2.5 standards to provide increased public health protection is necessary. Based on these considerations, the Administrator concludes that the current suite of primary PM 2.5 standards is not sufficient, and thus not requisite, to protect public health with an adequate margin of safety, and that revision is needed to increase public health protection.

It is important to note that this conclusion, and the reasoning on which it is based, do not resolve the question of what specific revisions are appropriate. That requires looking specifically at the current 24-hour and annual PM 2.5 standards, including their indicator, averaging times, forms, and levels, and evaluating the scientific evidence and other information relevant to determining the appropriate revision of the suite of standards.

E. Conclusions on the Elements of the Primary Fine Particle Standards

1. Indicator

In initially setting standards for fine particles in 1997, the EPA concluded it was appropriate to control fine particles as a group, rather than singling out any particular component or class of fine particles. The EPA noted that community health studies had found significant associations between various indicators of fine particles, and that health effects in a large number of areas had significant mass contributions of differing components or sources of fine particles. In addition, a number of toxicological and controlled human exposure studies had reported health effects associations with high concentrations of numerous fine particle components. It was also not possible to rule out any component within the mix of fine particles as not contributing to the fine particle effects found in the epidemiologic studies (62 FR 38667, July 18, 1977). In establishing a size-based indicator in 1977 to distinguish fine particles from particles in the coarse mode, the EPA noted that the available epidemiological studies of fine particles were based largely on PM 2.5 and also considered monitoring technology that was generally available. The selection of a 2.5 µm size cut reflected the regulatory importance of defining an indicator that would more completely capture fine particles under all conditions likely to be encountered across the U.S., especially when fine particle concentrations and humidity are likely to be high, while recognizing that some small coarse particles would also be captured by current methods to monitor PM 2.5 (62 FR 38666 to 38668, July 18, 1997). In the last review, based on the same considerations, the EPA again recognized that the available information supported retaining the PM 2.5 indicator and remained too limited to support a distinct standard for any specific PM 2.5 component or group of components associated with any source categories of fine particles (71 FR 61162 to 61164, October 17, 2006).

In this current review, the same considerations continue to apply for selection of an appropriate indicator for fine particles. As an initial matter, the Policy Assessment recognizes that the available epidemiological studies linking mortality and morbidity effects with long- and short-term exposures to fine particles continue to be largely indexed by PM 2.5. For the same reasons discussed in the last two reviews, the Policy Assessment concluded that it was appropriate to consider retaining a PM 2.5 indicator to provide protection from effects associated with long- and short-term fine particle exposures (U.S. EPA, 2011a, p. 2-50).

The Policy Assessment also considered the expanded body of evidence available in this review to consider whether there was sufficient evidence to support a separate standard for ultrafine particles [62] or whether there was sufficient evidence to establish distinct standards focused on regulating specific PM 2.5 components or a group of components associated with any source categories of fine particles (U.S. EPA, 2011a, section 2.3.1).

A number of studies available in this review have evaluated potential health effects associated with short-term exposures to ultrafine particles. As noted in the Integrated Science Assessment, the enormous number and larger, collective surface area of ultrafine particles are important considerations for focusing on this particle size fraction in assessing potential public health impacts (U.S. EPA, 2009a, p. 6-83). Per unit mass, ultrafine particles may have more opportunity to interact with cell surfaces due to their greater surface area and their greater particle number compared with larger particles (U.S. EPA, 2009a, p. 5-3). Greater surface area also increases the potential for soluble components (e.g., transition metals, organics) to adsorb to ultrafine particles and potentially cross cell membranes and epithelial barriers (U.S. EPA, 2009a, p. 6-83). In addition, evidence available in this review suggests that the ability of particles to enhance allergic sensitization is associated more strongly with particle number and surface area than with particle mass (U.S. EPA, 2009a, p. 6-127).

New evidence, primarily from controlled human exposure and toxicological studies, expands our understanding of cardiovascular and respiratory effects related to short-term ultrafine particle exposures. However, the Policy Assessment concluded that this evidence was still very limited and largely focused on exposure to diesel exhaust, for which the Integrated Science Assessment concluded it was unclear whether the effects observed are due to ultrafine particles, larger particles within the PM 2.5 mixture, or the gaseous components of diesel exhaust (U.S. EPA, 2009a, p. 2-22). In addition, the Integrated Science Assessment noted uncertainties associated with the controlled human exposure studies using concentrated ambient particle systems which have been shown to modify the composition of ultrafine particles (U.S. EPA, 2009a, p. 2-22, see also section 1.5.3).

The Policy Assessment recognized that there are relatively few epidemiological studies that have examined potential cardiovascular and respiratory effects associated with short-term exposures to ultrafine particles (U.S. EPA, 2011a, p. 2-51). These studies have reported inconsistent and mixed results (U.S. EPA, 2009a, section 2.3.5).

Collectively, in considering the body of scientific evidence available in this review, the Integrated Science Assessment concluded that the currently available evidence was suggestive of a causal relationship between short-term exposures to ultrafine particles and cardiovascular and respiratory effects. Furthermore, the Integrated Science Assessment concluded that evidence was inadequate to infer a causal relationship between short-term exposure to ultrafine particles and mortality as well as long-term exposure to ultrafine particles and all outcomes evaluated (U.S. EPA, 2009a, sections 2.3.5, 6.2.12.3, 6.3.10.3, 6.5.3.3, 7.2.11.3, 7.3.9, 7.4.3.3, 7.5.4.3, and 7.6.5.3; Table 2-6).

With respect to our understanding of ambient ultrafine particle concentrations, at present, there is no national network of ultrafine particle samplers; thus, only episodic and/or site-specific data sets exist (U.S. EPA, 2009a, p. 2-2). Therefore, the Policy Assessment recognized a national characterization of concentrations, temporal and spatial patterns, and trends was not possible at this time, and the availability of ambient ultrafine measurements to support health studies was extremely limited (U.S. EPA, 2011a, p. 2-51). In general, measurements of ultrafine particles are highly dependent on monitor location and, therefore, more subject to exposure error than accumulation mode particles (U.S. EPA, 2009a, p. 2-22). Furthermore, the number of ultrafine particles generally decreases sharply downwind from sources, as ultrafine particles may grow into the accumulation mode by coagulation or condensation (U.S. EPA, 2009a, p. 3-89). Limited studies of ambient ultrafine particle measurements have suggested that these particles exhibit a high degree of spatial and temporal heterogeneity driven primarily by differences in nearby source characteristics (U.S. EPA, 2009a, p. 3-84). Internal combustion engines and, therefore, roadways are a notable source of ultrafine particles, so concentrations of these particles near roadways are generally expected to be elevated (U.S. EPA, 2009a, p. 2-3). Concentrations of ultrafine particles have been reported to drop off much more quickly with distance from roadways than fine particles (U.S. EPA, 2009a, p. 3-84).

In considering both the currently available health effects evidence and the air quality data, the Policy Assessment concluded that this information was still too limited to provide support for consideration of a distinct PM standard for ultrafine particles (U.S. EPA, 2011a, p. 2-52).

In addressing the issue of particle composition, the Integrated Science Assessment concluded that, “[f]rom a mechanistic perspective, it is highly plausible that the chemical composition of PM would be a better predictor of health effects than particle size” (U.S. EPA, 2009a, p. 6-202). Heterogeneity of ambient concentrations of PM 2.5 constituents (e.g., elemental carbon, organic carbon, sulfates, nitrates) observed in different geographical regions as well as regional heterogeneity in PM 2.5-related health effects reported in a number of epidemiological studies are consistent with this hypothesis (U.S. EPA, 2009a, section 6.6).

With respect to the availability of ambient measurement data for fine particle components in this review, the Policy Assessment noted that there were now more extensive ambient PM 2.5 speciation measurement data available through the Chemical Speciation Network (CSN) than in previous reviews (U.S. EPA, 2011a, section 1.3.2 and Appendix B, section B.1.3). The Integrated Science Assessment observed that data from the CSN provided further evidence of spatial and seasonal variation in both PM 2.5 mass and composition among cities and geographic regions (U.S. EPA, 2009a, pp. 3-50 to 3-60; Figures 3-12 to 3-18; Figure 3-47). Some of this variation may be related to regional differences in meteorology, sources, and topography (U.S. EPA, 2009a, p. 2-3).

The currently available epidemiological, toxicological, and controlled human exposure studies evaluated in the Integrated Science Assessment on the health effects associated with ambient PM 2.5 constituents and categories of fine particle sources used a variety of quantitative methods applied to a broad set of PM 2.5 constituents, rather than selecting a few constituents a priori (U.S. EPA, 2009a, p. 2-26). Epidemiological studies have used measured ambient PM 2.5 speciation data, including monitoring data from the CSN, while all of the controlled human exposure and most of the toxicological studies have used concentrated ambient particles and analyzed the constituents therein (U.S. EPA, 2009a, p. 6-203). [63] The CSN provides PM 2.5 speciation measurements generally on a one-in-three or one-in-six day sampling schedule and, thus, does not capture data every day at most sites. [64]

The Policy Assessment recognized that several new multi-city studies evaluating short-term exposures to fine particle constituents are now available. These studies continued to show an association between mortality and cardiovascular and/or respiratory morbidity effects and short-term exposures to various PM 2.5 components including nickel, vanadium, elemental carbon, organic carbon, nitrates, and sulfates (U.S. EPA, 2011a, section 2.3.1; U.S. EPA, 2009a, sections 6.5.2.5 and 6.6).

Limited evidence is available to evaluate the health effects associated with long-term exposures to PM 2.5 components (U.S. EPA, 2009a, section 7.6.2). The Policy Assessment noted the most significant new evidence was provided by a study that evaluated multiple PM 2.5 components and an indicator of traffic density in an assessment of health effects related to long-term exposure to PM 2.5 (Lipfert et al., 2006a). Using health data from a cohort of U.S. military veterans and PM 2.5 measurement data from the CSN, Lipfert et al. (2006a) reported positive associations between mortality and long-term exposures to nitrates, elemental carbon, nickel, and vanadium as well as traffic density and peak ozone concentrations (U.S. EPA, 2011a, p. 2-54; U.S. EPA, 2009a, pp. 7-89 to 7-90).

With respect to source categories of fine particles potentially associated with a range of health endpoints, the Integrated Science Assessment reported that the currently available evidence suggests associations between cardiovascular effects and a number of specific PM 2.5-related source categories, including oil combustion, wood or biomass burning, motor vehicle emissions, and crustal or road dust sources (U.S. EPA, 2009a, section 6.6; Table 6-18). In addition, a few studies have evaluated associations between PM 2.5-related source categories and mortality. For example, one study reported an association between mortality and a PM 2.5 coal combustion factor (Laden et al., 2000), while other studies linked mortality to a secondary sulfate long-range transport PM 2.5 source (Ito et al., 2006; Mar et al., 2006) (U.S. EPA, 2009a, section 6.6.2.1). Other studies have looked at different components of particulate matter. There was less consistency in associations observed between selected sources of fine particles and respiratory health endpoints, which may be partially due to the fact that fewer studies have evaluated respiratory-related outcomes and measures. However, there was some evidence for PM 2.5-related associations with secondary sulfate and decrements in lung function in asthmatic and healthy adults (U.S. EPA, 2009a, p. 6-211; Gong et al., 2005; Lanki et al., 2006). A couple of studies have observed an association between respiratory endpoints in children and adults with asthma and surrogates for the crustal/soil/road dust and traffic sources of PM (U.S. EPA, 2009a, p. 6-205; Gent et al., 2009; Penttinen et al., 2006).

Recent studies have shown that source apportionment methods have the potential to add useful insights into which sources and/or PM constituents may contribute to different health effects. Of particular interest are several epidemiological studies that compared source apportionment methods and reported consistent results across research groups (U.S. EPA, 2009a, p. 6-211; Hopke et al., 2006; Ito et al., 2006; Mar et al., 2006; Thurston et al., 2005). These studies reported associations between total mortality and secondary sulfate in two cities for two different lag times. The sulfate effect was stronger for total mortality in Washington, DC and for cardiovascular-related mortality in Phoenix (U.S. EPA, 2009a, p. 6-204). These studies also found some evidence for associations with mortality and a number of source categories (e.g., biomass/wood combustion, traffic, copper smelter, coal combustion, sea salt) at various lag times (U.S. EPA, 2009a, p. 6-204). Sarnat et al. (2008) compared three different source apportionment methods and reported consistent associations between emergency department visits for cardiovascular diseases with mobile sources and biomass combustion as well as increased respiratory-related emergency department visits associated with secondary sulfate (U.S. EPA, 2009a, pp. 6-204 and 6-211).

Collectively, in considering the currently available evidence for health effects associated with specific PM 2.5 components or groups of components associated with any source categories of fine particles as presented in the Integrated Science Assessment, the Policy Assessment concluded that additional information available in this review continues to provide evidence that many different constituents of the fine particle mixture as well as groups of components associated with specific source categories of fine particles are linked to adverse health effects (U.S. EPA, 2011a, p. 2-55). However, as noted in the Integrated Science Assessment, while “[t]here is some evidence for trends and patterns that link particular ambient PM constituents or sources with specific health outcomes * * * there is insufficient evidence to determine whether these patterns are consistent or robust” (U.S. EPA, 2009a, p. 6-210). Assessing this information, the Integrated Science Assessment concluded that “the evidence is not yet sufficient to allow differentiation of those constituents or sources that are more closely related to specific health outcomes” (U.S. EPA, 2009a, pp. 2-26 and 6-212). Therefore, the Policy Assessment concluded that the currently available evidence is not sufficient to support consideration of a separate indicator for a specific PM 2.5 component or group of components associated with any source category of fine particles. Furthermore, the Policy Assessment concluded that the evidence is not sufficient to support eliminating any component or group of components associated with any source categories of fine particles from the mix of fine particles included in the PM 2.5 indicator (U.S. EPA, 2011a, p. 2-56).

The CASAC agreed with the EPA staff conclusions presented in the Policy Assessment and concluded that it is appropriate to consider retaining PM 2.5 as the indicator for fine particles and further asserted, “There [is] insufficient peer-reviewed literature to support any other indicator at this time” (Samet, 2010c, p. 12). CASAC expressed a strong desire for the EPA to “look ahead to future review cycles and reinvigorate support for the development of evidence that might lead to newer indicators that may correlate better with the health effects associated with ambient air concentrations of PM * * *” (Samet, 2010c, p 2).

Consistent with the staff conclusions presented in the Policy Assessment and CASAC advice, the Administrator proposed to retain PM 2.5 as the indicator for fine particles. Further, the Administrator provisionally concluded that currently available scientific information does not provide a sufficient basis for supplementing mass-based, primary fine particle standards with standards using a separate indicator for ultrafine particles or a separate indicator for a specific PM 2.5 component or group of components associated with any source categories of fine particles. In addition, the Administrator also provisionally concluded that the currently available scientific information did not provide a sufficient basis for eliminating any individual component or group of components associated with any source categories from the mix of fine particles included in the PM 2.5 mass-based indicator.

The EPA received comparatively few public comments on issues related to the indicator for fine particles. [65] Some commenters emphasized the need to conduct additional research to more fully understand the effect of specific PM 2.5 components and/or sources on public health. These commenters expressed views about the importance of evaluating health effect associations with various fine particle components and types of source categories as a basis for focusing ongoing and future research to reduce uncertainties in this area and for considering whether alternative indicator(s) may be appropriate to consider in future PM NAAQS reviews for standards intended to protect against the array of health effects that have been associated with fine particles as indexed by PM 2.5. For example, the PSR encouraged more research and monitoring related to PM 2.5 components and noted the importance of components associated with coal combustion (PSR, 2012, pp. 5 to 6). EPRI asserted that “new” studies support focusing on EC and OC and encouraged the EPA to seriously consider the mass-based approach (EPRI, 2012, p. 2). Likewise, Georgia Mining Association supported additional monitoring and research efforts related to PM 2.5 composition and specifically encouraged the evaluation of using particle number (e.g., particle count) (GMA, 2012, pp. 2 to 3).

The Administrator agrees with CASAC as well as these commenters that the results of additional research and monitoring efforts will be helpful for informing future PM NAAQS reviews. Information from such studies could also help inform the development of strategies that emphasize control of specific types of emission sources so as to address particles of greatest concern to public health. However, based upon the scientific information considered in the Integrated Science Assessment as well as the public comments summarized above, the Administrator continues to take note there is evidence that many different constituents of the fine particle mixture as well as groups of components associated with specific sources of fine particles are linked to adverse health effects. Furthermore, she recognizes that the evidence is not yet sufficient to differentiate those constituents or sources that are most closely related to specific health outcomes nor to exclude any PM 2.5 components or sources of fine particles from the mix of particles included in the PM 2.5 indicator.

Having considered the public comments on this issue, the Administrator concurs with the Policy Assessment conclusions and CASAC recommendations and concludes that it is appropriate to retain PM 2.5 as the indicator for fine particles.

2. Averaging Time

In 1997, the EPA initially set both an annual standard, to provide protection from health effects associated with both long- and short-term exposures to PM 2.5, and a 24-hour standard to supplement the protection afforded by the annual standard (62 FR 38667 to 38668, July, 18, 1997). In the last review, the EPA retained both annual and 24-hour averaging times (71 FR 61164, October 17, 2006). These decisions were based, in part, on evidence of health effects related to both long-term (from a year to several years) and short-term (from less than one day to up to several days) measures of PM 2.5.

The overwhelming majority of studies conducted since the last review continue to utilize annual (or multi-year) and 24-hour averaging times, reflecting the averaging times of the current PM 2.5 standards. These studies continue to provide evidence that health effects are associated with annual and 24-hour averaging times. Therefore, the Policy Assessment concluded it is appropriate to retain the current annual and 24-hour averaging times to provide protection from effects associated with both long- and short-term PM 2.5 exposures (U.S. EPA, 2011a, p. 2-57).

In considering whether the information available in this review supports consideration of different averaging times for PM 2.5 standards specifically with regard to considering a standard with an averaging time less than 24 hours to address health effects associated with sub-daily PM 2.5 exposures, the Policy Assessment noted there continues to be a growing body of studies that provide additional evidence of effects associated with exposure periods less than 24-hours (U.S. EPA, 2011a, p. 2-57). Relative to information available in the last review, recent studies provide additional evidence for cardiovascular effects associated with sub-daily (e.g., one to several hours) exposure to PM, especially effects related to cardiac ischemia, vasomotor function, and more subtle changes in markers of systemic inflammation, hemostasis, thrombosis and coagulation (U.S. EPA, 2009a, section 6.2). Because these studies have used different indicators (e.g., PM 2.5, PM 10, PM 10-2.5, ultrafine particles), averaging times (e.g., 1, 2, and 4 hours), and health outcomes, it is difficult to draw conclusions about cardiovascular effects associated specifically with sub-daily exposures to PM 2.5.

With regard to respiratory effects associated with sub-daily PM 2.5 exposures, the currently available evidence was much sparser than for cardiovascular effects and continues to be very limited. The Integrated Science Assessment concluded that for several studies of hospital admissions or medical visits for respiratory diseases, the strongest associations were observed with 24-hour average or longer exposures, not with less than 24-hour exposures (U.S. EPA, 2009a, section 6.3).

Collectively, the Policy Assessment concluded that this information, when viewed as a whole, is too unclear, with respect to the indicator, averaging time and health outcome, to serve as a basis for consideration of establishing a primary PM 2.5 standard with an averaging time shorter than 24-hours at this time (U.S. EPA, 2011a, p. 2-57).

With regard to health effects associated with PM 2.5 exposure across varying seasons in this review, Bell et al. (2008) reported higher PM 2.5 risk estimates for hospitalization for cardiovascular and respiratory diseases in the winter compared to other seasons. In comparison to the winter season, smaller statistically significant associations were also reported between PM 2.5 and cardiovascular morbidity for spring and autumn, and a positive, but statistically non-significant association was observed for the summer months. In the case of mortality, Zanobetti and Schwartz (2009) reported a 4-fold higher effect estimate for PM 2.5-associated mortality for the spring as compared to the winter. Taken together, these results provided emerging but limited evidence that individuals may be at greater risk of dying from higher exposures to PM 2.5 in the warmer months and may be at greater risk of PM 2.5-associated hospitalization for cardiovascular and respiratory diseases during colder months of the year (U.S. EPA, 2011a, p. 2-58).

Overall, the Policy Assessment observed that there are few studies presently available to deduce a general pattern in PM 2.5-related risk across seasons. In addition, these studies utilized 24-hour exposure periods within each season to assess the PM 2.5-associated health effects and do not provide information on health effects associated with a season-long exposure to PM 2.5. Due to these limitations in the currently available evidence, the Policy Assessment concluded that there was no basis to consider a seasonal averaging time separate from a 24-hour averaging time.

Based on the above considerations, the Policy Assessment concluded that the currently available information provided strong support for consideration of retaining the current annual and 24-hour averaging times but does not provide support for considering alternative averaging times (U.S. EPA, 2011a, p. 2-58). In addition, CASAC considered it appropriate to retain the current annual and 24-hour averaging times for the primary PM 2.5 standards (Samet, 2010c, pp. 2 to 3). At the time of the proposal, the Administrator concurred with the staff conclusions and CASAC advice and proposed that the averaging times for the primary PM 2.5 standards should continue to include annual and 24-hour averages to protect against health effects associated with long- and short-term exposures. Furthermore, the Administrator provisionally concluded, consistent with conclusions reached in the Policy Assessment and by CASAC, that the currently available information was too limited to support consideration of alternative averaging times to establish a national standard with a shorter-than 24-hour averaging time or with a seasonal averaging time.

The EPA received no significant public comments on the issue of averaging time for the PM 2.5 primary standards. The Administrator concurs with recommendations made by CASAC and the staff conclusions presented in the Policy Assessment and concludes, as proposed, that it is appropriate to retain the current annual and 24-hour averaging times for the primary PM 2.5 standards to protect against health effects associated with long- and short-term exposure periods.

3. Form

The “form” of a standard defines the air quality statistic that is to be compared to the level of the standard in determining whether an area attains the standard. In this review, the EPA considers whether currently available information supports retaining or revising the forms for the annual or 24-hour PM 2.5 standards.

a. Annual Standard

In 1997, the EPA established the form of the annual PM 2.5 standard as an annual arithmetic mean, averaged over 3 years, from single or multiple community-oriented monitors. This form was intended to represent a relatively stable measure of air quality and to characterize longer-term area-wide PM 2.5 concentrations, in conjunction with a 24-hour standard designed to provide adequate protection against localized peak or seasonal PM 2.5 concentrations. The level of the standard was to be compared to measurements made at each community-oriented monitoring site, or, if specific criteria were met, measurements from multiple community-oriented monitoring sites could be averaged (i.e., spatial averaging) [66] (62 FR 38671 to 38672, July 18, 1997). The constraints were intended to ensure that spatial averaging would not result in inequities in the level of protection provided by the standard (62 FR 38672, July 18, 1997). This approach was consistent with the epidemiological studies on which the PM 2.5 standard was primarily based, in which air quality data were generally averaged across multiple monitors in an area or were taken from a single monitor that was selected to represent community-wide exposures.

In the last review, the EPA tightened the criteria for use of spatial averaging to provide increased protection for vulnerable populations exposed to PM 2.5. This change was based in part on an analysis of the potential for disproportionate impacts on potentially at-risk populations, which found that the highest concentrations in an area tend to be measured at monitors located in areas where the surrounding population is more likely to have lower education and income levels and higher percentages of minority populations (71 FR 61166/2, October 17, 2006; U.S. EPA, 2005, section 5.3.6.1).

In this review, as outlined in section III.B above and discussed more fully in section III.B.3 of the proposal, there now exist more health data such that the Integrated Science Assessment has identified persons from lower socioeconomic strata as an at-risk population (U.S. EPA, 2009a, section 8.1.7; U.S. EPA, 2011a, section 2.2.1). Moreover, there now exist more years of PM 2.5 air quality data than were available in the last review. Consideration in the Policy Assessment of the spatial variability across urban areas that was revealed by this expanded data base has raised questions as to whether an annual standard that allows for spatial averaging, even within specified constraints as narrowed in 2006 (71 FR 61165 to 61167, October 17, 2006), would provide appropriate public health protection.

In considering the potential for disproportionate impacts on at-risk populations, the Policy Assessment considered an update of an air quality analysis conducted for the last review (U.S. EPA, 2011a, pp. 2-59 to 60; Schmidt, 2011, Analysis A). This analysis focused on determining whether the spatial averaging provisions, as modified in 2006, could introduce inequities in protection for at-risk populations exposed to PM 2.5. Specifically, the Policy Assessment considered whether persons of lower socioeconomic status, minority groups, or different age groups (i.e., children or older adults) are more likely than the general population to live in areas in which the monitors recording the highest air quality values in an area are located. Data used in this analysis included demographic parameters measured at the Census Block or Census Block Group level, including percent minority population, percent minority subgroup population, percent of persons living below the poverty level, percent of persons 18 years of age or older, and percent of persons 65 years of age and older. In each candidate geographic area, data from the Census Block(s) or Census Block Group(s) surrounding the location of the monitoring site (as delineated by radii buffers of 0.5, 1.0, 2.0, and 3.0 miles) in which the highest air quality value was monitored were compared to the average of monitored values in the area. This analysis looked beyond areas that would meet the current spatial averaging criteria and considered all urban areas (i.e., Core Based Statistical Areas or CBSAs) with at least two valid annual design value monitors (Schmidt, 2011, Analysis A). Recognizing the limitations of such cross-sectional analyses, the Policy Assessment observed that the highest concentrations in an area tend to be measured at monitors located in areas where the surrounding populations are more likely to live below the poverty line and to have higher percentage of minorities (U.S. EPA, 2011a, p. 2-60).

Based upon the analysis described above, the Policy Assessment concluded that the existing constraints on spatial averaging, as modified in 2006, may be inadequate to avoid substantially greater exposures in some areas, potentially resulting in disproportionate impacts on at-risk populations of persons with lower SES levels as well as minorities. Therefore, the Policy Assessment concluded that it was appropriate to consider revising the form of the annual PM 2.5 standard such that it did not allow for the use of spatial averaging across monitors. In doing so, the level of the annual PM 2.5 standard would be compared to measurements made at the monitoring site that represents area-wide air quality recording the highest PM 2.5 concentrations [67] (U.S. EPA, 2011a, p. 2-60).

The CASAC agreed with staff conclusions that it was “reasonable” for the EPA to eliminate the spatial averaging provisions (Samet, 2010d, p. 2). Further, in CASAC's comments on the first draft Policy Assessment, it noted, “Given mounting evidence showing that persons with lower SES levels are a susceptible group for PM-related health risks, CASAC recommends that the provisions that allow for spatial averaging across monitors be eliminated for the reasons cited in the (first draft) Policy Assessment” (Samet, 2010c, p. 13). In its review of the second draft Policy Assessment, CASAC recognized “although much of the epidemiological research has been conducted using community-wide averages, several key studies reference the nearest measurement site, so that some risk estimates are not necessarily biased by the averaging process. Further, the number of such studies is likely to expand in the future” (Samet, 2010d, pp. 1 to 2).

Only two areas in the country used the initial spatial averaging provisions for demonstrating attainment with the primary annual PM 2.5 standard set in 1997 (70 FR 19847, April 14, 2005; U.S. EPA, 2006c). Since these provisions were tightened in 2006, no area has used spatial averaging to demonstrate attainment. No areas in the country are currently using the spatial averaging provisions to demonstrate attainment with the current primary annual PM 2.5 standard.

In considering the Policy Assessment's conclusions based on the results of the analysis discussed above and concern over the evidence of potential disproportionate impacts on at-risk populations as well as CASAC advice, the Administrator proposed to revise the form of the annual PM 2.5 standard to eliminate the use of spatial averaging. Thus, the Administrator proposed revising the form of the annual PM 2.5 standard to compare the level of the standard with measurements from each “appropriate” monitor in an area [68] with no allowance for spatial averaging. Thus, for an area with multiple monitors, the appropriate reporting monitor with the highest design value would determine the attainment status for that area.

Of the commenters noted in section III.D.2 above who supported a more stringent annual PM 2.5 standard, those who commented on the form of the annual PM 2.5 standard supported the EPA's proposal to eliminate the spatial averaging provisions. These commenters contended that the EPA's analyses of the potential impacts of spatial averaging, discussed above and in the proposal (77 FR 38924), demonstrated that the current form results in uneven public health protection leading to disproportionate impacts on at-risk populations. Specifically, the ALA and other environmental and public health commenters contended that “spatial averaging allows exposure of people to unhealthy levels of pollution at specific locales even within an area meeting the standard” (ALA et al., 2012, p. 23). These commenters particularly focused on the importance for low-income and minority populations of eliminating the spatial averaging provisions. They concluded that spatial averaging “is an environmental justice concern because poor people are more likely to live near roads, depots, factories, ports, and other pollution sources.”Id. p. 24.

Other commenters (e.g., AAM, 2012; Dow, 2012) also supported the elimination of spatial averaging in order to “avoid potential disproportionate impacts on at-risk populations” and to maximize “the benefits to public health of reducing the annual PM 2.5 standard.” However, these groups expressed concern that the elimination of spatial averaging, in combination with the requirement for near road monitors (as discussed in section VIII.B.3.b.i of the proposal), would effectively and inappropriately increase the stringency of the annual PM 2.5 standard.

This concern was also shared by other commenters who disagreed with the elimination of spatial averaging. For example, the Class of '85 RRG emphasized concerns about increasing the stringency of the standard while providing few health benefits if spatial averaging is eliminated, particularly in combination with the requirement for near-road monitors. These commenters contended that “[b]ecause EPA proposes to use the readings from the highest single worst case monitor (rather than the average of all community area monitors), and since roadway monitoring locations will likely be worst case monitors, the proposed NAAQS will become more stringent without targeting the PM 2.5 species most harmful to human health” (Class of '85 RRG, 2012, p. 6).

Several commenters also maintained that because spatial averaging is consistent with how air quality data are considered in the underlying epidemiological studies, such averaging should not be eliminated. Specifically, commenters including NAM et al., AFPM, and ACC pointed out that PM 2.5 epidemiological studies use spatially averaged multi-monitor concentrations, rather than the single highest monitor, when evaluating health effects. Therefore, these commenters contended that allowing spatial averaging would make the PM 2.5 standard more consistent with the approaches used in the epidemiological studies upon which the standard is based. In addition, some commenters also contended that the EPA failed to consider whether modifying, rather than eliminating, the constraints on spatial averaging would have been sufficient to protect the public health. If so, these commenters argued that “elimination of spatial averaging would go beyond what is requisite to protect the public health” (NAM et al., 2012, p. 20).

In considering the public comments on the form of the annual standard, the EPA recognizes a number of commenters agreed with the basis for the EPA's proposal to eliminate spatial averaging. While other commenters expressed disagreement or concern with the proposed decision to eliminate the spatial averaging provisions, the Agency notes that these commenters did not challenge the analyses or considerations that provided the fundamental basis for the Administrator's proposed decision. Rather, these commenters generally raised concerns that eliminating the option for spatial averaging would increase the stringency of the standard, especially in light of additional monitoring sites in near-road environments (as discussed in section VIII.B.3.b.1 below).

The EPA does not agree with the comment that siting some monitors in near roadway environments makes the standard more stringent or impermissibly more stringent. As discussed in section VIII.B.3.b.i below, a significant fraction of the population lives in proximity to major roads, and these exposures occur in locations that represent ambient air. Monitoring in such areas does not make the standard more stringent than warranted, but rather affords the intended protection to the exposed populations, among them at-risk populations, exposed to fine particles in these areas. Thus, in cases where monitors in near roadway environments are deemed to be representative of area-wide air quality they would be compared to the annual standard (as discussed more fully in section VIII below). The 24-hour and annual NAAQS are designed to protect the public with an adequate margin of safety, and this siting provision is fully consistent with providing the protection the standard is designed to provide and does not make the standard more stringent or more stringent than necessary.

Monitors that are representative of area-wide air quality may be compared to the annual standard. This is consistent with the use of monitoring data in the epidemiological studies that provide the primary basis for determining the level of the annual standard. In addition, the EPA notes that the annual standard is designed to protect against both long- and short-term exposures through controlling the broad distribution of air quality across an area over time. [69] It is fully consistent with the protection the standard is designed to provide for near road monitors to be compared to the annual standard if the monitor is representative of area-wide air quality. This does not make the standard either more stringent or impermissibly more stringent.

In further considering these comments, the EPA notes that the stringency or level of protection provided by each NAAQS is not based solely on the form of the standard; rather, the four elements of the standard that together serve to define each standard (i.e., indicator, averaging time, form, and level) must be considered collectively in evaluating the protection afforded by each standard. Therefore, the EPA considers these comments are also appropriate to discuss collectively with other issues related to the appropriate level for annual standard, and are discussed below in sections III.E.4.c-d.

In reaching a final decision on the form of the annual standard, the Administrator considers the available analyses, CASAC advice, and public comments on form as discussed above. She also considers related issues in the public comments on the level of the annual standard as discussed in section III.E.4.c below. She notes that even when the annual PM 2.5 standard was first set in 1997, the spatial averaging provisions included constraints intended to ensure that inequities in the level of protection would not result. These constraints on spatial averaging were tightened in the last review, based on an analysis showing the potential for spatial averaging to allow higher PM 2.5 concentrations in locations where subgroups within the general population were potentially disproportionately exposed and hence, at disproportionate risk (e.g., low income and minority communities). The Administrator notes that in proposing to eliminate spatial averaging altogether in this review, she has relied on further analyses in the current review (Schmidt, 2011, Analysis A). As discussed above and in the proposal (77 FR 38924), these analyses showed that the current constraints on spatial averaging may be inadequate in some areas to avoid substantially greater exposures for people living near monitors recording the highest PM 2.5 concentrations. Such exposures could result in disproportionate impacts to at-risk populations, including low-income populations as well as minority groups.

On this basis, the Administrator concludes that public health would not be protected with an adequate margin of safety in all locations, as required by law, if disproportionately higher exposure concentrations in at-risk populations such as low income communities as well as minority communities were averaged together with lower concentrations measured at other sites in a large urban area. See ALA v. EPA, 134 F. 3d 388, 389 (D.C. Cir., 1998) (“this court has held that `NAAQS must protect not only average healthy individuals, but also sensitive citizens such as children,' and `if a pollutant adversely affects the health of these sensitive individuals, EPA must strengthen the entire national standard'”) and Coalition of Battery Recyclers Association v. EPA, 604 F 3d. 613, 617 (D.C. Cir., 2010) (“Petitioners' assertion that the revised lead NAAQS is overprotective because it is more stringent than necessary to protect the entire population of young U.S. children ignores that the Clean Air Act allows protection of sensitive subpopulations.”) In reaching this conclusion, the Administrator further notes that her concern over possible disproportionate PM 2.5-related health impacts in at-risk populations extends to populations living near important sources of PM 2.5, including the large populations that live near major roadways. [70]

In light of all of the above considerations, including consideration of available analyses, CASAC advice, and public comments, the Administrator concludes that the current form of the annual PM 2.5 standard should be revised to eliminate spatial averaging provisions. Thus, the level of the revised annual PM 2.5 standard established with this rule will be compared with measurements from each appropriate monitor in an area, with no allowance for spatial averaging. The Administrator's conclusions with regard to the appropriate level of the annual PM 2.5 standard to set in conjunction with this form are discussed below in section III.E.4.d.

b. 24-Hour Standard

In 1997, the EPA established the form of the 24-hour PM 2.5 standard as the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area, averaged over three years (62 FR at 38671 to 38674, July 18, 1997). The Agency selected the 98th percentile as an appropriate balance between adequately limiting the occurrence of peak concentrations and providing increased stability which, when averaged over 3 years, facilitated effective health protection through the development of more stable implementation programs. By basing the form of the standard on concentrations measured at population-oriented monitoring sites, the EPA intended to provide protection for people residing in or near localized areas of elevated concentrations. In the last review, in conjunction with lowering the level of the 24-hour standard, the EPA retained this form based in part on a comparison with the 99th percentile form. [71]

In revisiting the stability of a 98th versus 99th percentile form for a 24-hour standard intended to provide supplemental protection for a generally controlling annual standard, an analysis presented in the Policy Assessment considered air quality data reported in 2000 to 2008 to update our understanding of the ratio between peak-to-mean PM 2.5 concentrations. This analysis provided evidence that the 98th percentile value was a more stable metric than the 99th percentile (U.S. EPA, 2011a, Figure 2-2, p. 2-62).

At the time of the proposal, the Agency recognized that the selection of the appropriate form of the 24-hour standard includes maintaining adequate protection against peak 24-hour concentrations while also providing a stable target for risk management programs, which serves to provide for the most effective public health protection in the long run. [72] As in previous reviews, the EPA recognized that a concentration-based form, compared to an exceedance-based form, was more reflective of the health risks posed by elevated pollutant concentrations because such a form gives proportionally greater weight to days when concentrations are well above the level of the standard than to days when the concentrations are just above the level of the standard. Further, the Agency provisionally concluded that a concentration-based form, when averaged over three years, provided an appropriate balance between limiting peak pollutant concentrations and providing a stable regulatory target, thus facilitating the development of more stable implementation programs.

In considering the information provided in the Policy Assessment and recognizing that the degree of public health protection likely to be afforded by a standard is a result of the combination of the form and the level of the standard, the Administrator proposed to retain the 98th percentile form of the 24-hour standard. The Administrator provisionally concluded that the 98th percentile form represents an appropriate balance between adequately limiting the occurrence of peak concentrations and providing increased stability relative to an alternative 99th percentile form.

Few public commenters commented specifically on the form of the 24-hour standard. None of the public commenters raised objections to continuing the use of a concentration-based form for the 24-hour standard. Many of the individuals and groups who supported a more stringent 24-hour PM 2.5 standard noted in section III.D.2 above, however, recommended a more restrictive concentration-based percentile form, specifically a 99th percentile form. The limited number of these commenters who provided a specific rationale for this recommendation generally expressed their concern that the 98th percentile form could allow too many days where concentrations exceeded the level of the standard, and thus fail to adequately protect public health. Other public commenters representing state and local air agencies and industry groups generally supported retaining the current 98th percentile form. In most cases, these groups expressed the overall view that the current 24-hour PM 2.5 standard, including the form of the current standard, should be retained.

The EPA notes that the viewpoints represented in this review are similar to comments submitted in the last review and through various NAAQS reviews. The EPA recognizes that the selection of the appropriate form includes maintaining adequate protection against peak 24-hour values while also providing a stable target for risk management programs, which serves to provide for the most effective public health protection in the long run. [73] Nothing in the commenters' views has provided a reason to change the Administrator's previous conclusion regarding the appropriate balance represented in the proposed form of the 24-hour PM 2.5 standard. Therefore, the Administrator concurs with staff conclusions presented in the Policy Assessment and CASAC recommendations and concludes that it is appropriate to retain the 98th percentile form for the 24-hour PM 2.5 standard.

4. Level

In the last review, the EPA selected levels for the annual and the 24-hour PM 2.5 standards using evidence of effects associated with periods of exposure that were most closely matched to the averaging time of each standard. Thus, as discussed in section III.A.1, the EPA relied upon evidence from long-term exposure studies as the principal basis for selecting the level of the annual PM 2.5 standard that would protect against effects associated with long-term exposures. The EPA relied upon evidence from the short-term exposure studies as the principal basis for selecting the level of the 24-hour PM 2.5 standard that would protect against effects associated with short-term exposures. As summarized in section III.A.2 above, the 2006 decision to retain the level of the annual PM 2.5 standard at 15 μg/m [3] [74] was challenged and on judicial review, the DC Circuit remanded the primary annual PM 2.5 standard to the EPA, finding that EPA's explanation for its approach to setting the level of the annual standard was inadequate.

a. General Approach for Considering Standard Levels

Building upon the lessons learned in the previous PM NAAQS reviews, in considering alternative standard levels supported by the currently available scientific information, the Policy Assessment used an approach that integrated evidence-based and risk-based considerations, took into account CASAC advice, and considered the issues raised by the court in remanding the primary annual PM 2.5 standard. Following the general approach outlined in section III.A.3 above, for the reasons discussed below, the Policy Assessment concluded it was appropriate to consider the protection afforded by the annual and 24-hour standards taken together against mortality and morbidity effects associated with both long- and short-term PM 2.5 exposures. This was consistent with the approach taken in the review completed in 1997 rather than considering each standard separately, as was done in the review completed in 2006.

Beyond looking directly at the relevant epidemiologic evidence, the Policy Assessment considered the extent to which specific alternative PM 2.5 standard levels were likely to reduce the nature and magnitude of both long-term exposure-related mortality risk and short-term exposure-related mortality and morbidity risk (U.S. EPA, 2011a, section 2.3.4.2; U.S.EPA, 2010a, section 4.2.2). As noted in section III.C above, patterns of increasing estimated risk reductions were generally observed as either the annual or 24-hour standard, or both, were reduced below the level of the current standards (U.S. 2011a, Figures 2-11 and 2-12; U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and 5.2.3).

Based on the quantitative risk assessment, the Policy Assessment observed, as discussed in section III.A.3, that analyses conducted for this and previous reviews demonstrated that much, if not most, of the aggregate risk associated with short-term exposures results from the large number of days during which the 24-hour average concentrations are in the low-to mid-range, below the peak 24-hour concentrations (U.S. EPA, 2011a, p. 2-9). Furthermore, as discussed in section III.C above and in section III.C.3 of the proposal, the Risk Assessment observed that alternative annual standard levels, when controlling, resulted in more consistent risk reductions across urban study areas, thereby potentially providing a more consistent degree of public health protection (U.S. EPA, 2010a, pp. 5-15 to 5-16). In contrast, the Risk Assessment noted that the results of simulating alternative suites of PM 2.5 standards including different combinations of alternative annual and 24-hour standard levels suggested that an alternative 24-hour standard level can produce additional estimated risk reductions beyond that provided by an alternative annual standard alone. However, the degree of estimated risk reduction provided by alternative 24-hour standard levels was highly variable, in part due to the choice of rollback approached used (U.S. EPA, 2010a, p. 5-17).

Based on its review of the second draft Policy Assessment, CASAC agreed with the EPA staff's general approach for translating the available epidemiological evidence, risk information, and air quality information into the basis for reaching conclusions on alternative standards for consideration. Furthermore, CASAC agreed “that it is appropriate to return to the strategy used in 1997 that considers the annual and the short-term standards together, with the annual standard as the controlling standard, and the short-term standard supplementing the protection afforded by the annual standard” and “considers it appropriate to place the greatest emphasis” on health effects judged to have evidence supportive of a causal or likely causal relationship as presented in the Integrated Science Assessment (Samet, 2010d, p. 1).

Therefore, the Policy Assessment concluded, consistent with specific CASAC advice, that it was appropriate to set a “generally controlling” annual standard that will lower a wide range of ambient 24-hour concentrations. The Policy Assessment concluded this approach would likely reduce aggregate risks associated with both long- and short-term exposures with more consistency than a generally controlling 24-hour standard and would be the most effective and efficient way to reduce total PM 2.5-related population risk and so provide appropriate protection. The staff believed this approach, in contrast to one focusing on a generally controlling 24-hour standard, would likely reduce aggregate risks associated with both long- and short-term exposures with more consistency and would likely avoid setting national standards that could result in relatively uneven protection across the country due to setting standards that were either more or less stringent than necessary in different geographical areas.

The Policy Assessment recognized that an annual standard intended to serve as the primary means for providing protection against effects associated with both long- and short-term PM 2.5 exposures cannot be expected to offer an adequate margin of safety against the effects of all short-term PM 2.5 exposures. As a result, in conjunction with a generally controlling annual standard, the Policy Assessment concluded it was appropriate to consider setting a 24-hour standard to provide supplemental protection, particularly for areas with high peak-to-mean ratios possibly associated with strong local or seasonal sources, or PM 2.5-related effects that may be associated with shorter-than-daily exposure periods.

At the time of the proposal, the Administrator agreed with the approach discussed in the Policy Assessment as summarized in section III.A.3 above, and supported by CASAC, of considering the protection afforded by the annual and 24-hour standards taken together for mortality and morbidity effects associated with both long- and short-term exposures to PM 2.5. Furthermore, based on the evidence and quantitative risk assessment, the Administrator provisionally concluded it was appropriate to set a “generally controlling” annual standard that will lower a wide range of ambient 24-hour concentrations, with a 24-hour standard focused on providing supplemental protection, particularly for areas with high peak-to-mean ratios possibly associated with strong local or seasonal sources, or PM 2.5-related effects that may be associated with shorter-than daily exposure periods. The Administrator provisionally concluded this approach would likely reduce aggregate risks associated with both long- and short-term exposures more consistently than a generally controlling 24-hour standard and would be the most effective and efficient way to reduce total PM 2-5-related population risk.

The Administrator is mindful that considering what standards are requisite to protect public health with an adequate margin of safety requires public health policy judgments that neither overstate nor understate the strength and limitations of the evidence or the appropriate inferences to be drawn from the evidence. At the time of the proposal, in considering how to translate the available information into appropriate standard levels, the Administrator weighed the available scientific information and associated uncertainties and limitations. For the purpose of determining what standard levels were appropriate to propose, the Administrator recognized, as did the EPA staff in the Policy Assessment, that there was no single factor or criterion that comprised the “correct” approach to weighing the various types of available evidence and information, but rather there were various approaches that were appropriate to consider. The Administrator further recognized that different evaluations of the evidence and other information before the Administrator could reflect placing different weight on the relative strengths and limitations of the scientific information, and different judgments could be made as to how such information should appropriately be used in making public health policy decisions on standard levels. This recognition led the Administrator to consider various approaches to weighing the evidence so as to identify appropriate standard levels to propose. In so doing, the Administrator encouraged extensive public comment on alternative approaches to weighing the evidence and other information so as to inform her public health policy judgments before reaching final decisions on appropriate standard levels.

b. Proposed Decisions on Standard Levels

i. Consideration of the Alternative Standard Levels in the Policy Assessment

In recognizing the absence of a discernible population threshold below which effects would not occur, the Policy Assessment's general approach for identifying alternative annual standard levels that were appropriate to consider focused on characterizing the part of the distribution of PM 2.5 concentrations in which we had the most confidence in the associations reported in the epidemiological studies and conversely where our confidence in the association became appreciably lower. The most direct approach to address this issue, consistent with CASAC advice (Samet, 2010c, p. 10), was to consider epidemiological studies reporting confidence intervals around concentration-response relationships (U.S. EPA, 2011a, p. 2-63). Based on a thorough search of the available evidence, the Policy Assessment identified only one study (Schwartz et al., 2008) that conducted a multi-model analysis to characterize confidence intervals around the estimated concentration-response relationship. The Policy Assessment concluded that this single relevant analysis was too limited to serve as the principal basis for identifying alternative standard levels in this review (U.S. EPA, 2011a, p. 2-70).

The Policy Assessment explored other approaches to characterize the part of the distributions of long-term mean PM 2.5 concentrations that were most influential in generating health effect estimates in long- and short-term epidemiological studies, and placed greatest weight on those studies that reported positive and statistically significant associations (U.S. EPA, 2011a, p. 2-63). First, as discussed in section III.A.3 above, the Policy Assessment considered the statistical metric used in previous reviews. This approach recognized the EPA's views that the strongest evidence of associations occurs at concentrations around the long-term mean concentration. Thus, in earlier reviews, the EPA focused on identifying standard levels that were somewhat below the long-term mean concentrations reported in PM 2.5 epidemiological studies. The long-term mean concentrations represented air quality data typically used in epidemiological analyses and provided a direct link between PM 2.5 concentrations and the observed health effects. Further, these data were available for all long- and short-term exposure studies analyzed and, therefore, represented the data set available for the broadest set of epidemiological studies.

However, consistent with CASAC's comments on the second draft Policy Assessment [75] (Samet, 2010d, p. 2), in preparing the final Policy Assessment, the EPA staff explored ways to take into account additional information from epidemiological studies, when available (Rajan et al., 2011). These analyses focused on evaluating different statistical metrics, beyond the long-term mean concentration, to characterize the part of the distribution of PM 2.5 concentrations in which staff continued to have confidence in the associations observed in epidemiological studies and below which there was a comparative lack of data such that the staff's confidence in the relationship was appreciably less. This would also be the part of the distribution of PM 2.5 concentrations which had the most influence on generating the health effect estimates reported in epidemiological studies. As discussed in section III.A.3 above, the Policy Assessment recognized there was no one percentile value within a given distribution that was the most appropriate or “correct” way to characterize where our confidence in the associations becomes appreciably lower. The Policy Assessment concluded that focusing on concentrations within the lower quartile of a distribution, such as the range from the 25th to the 10th percentile, was reasonable to consider as a region within which we begin to have appreciably less confidence in the associations observed in epidemiological studies. [76] In the EPA staff's view, considering lower PM 2.5 concentrations, down to the lowest concentration observed in a study, would be a highly uncertain basis for selecting alternative standard levels (U.S. EPA, 2009a, p. 2-71).

As outlined in section III.A.3 above, the Policy Assessment recognized that there were two types of population-level information to consider in identifying the range of PM 2.5 concentrations which have the most influence on generating the health effect estimates reported in epidemiological studies. The most relevant information to consider was the number of health events (e.g., deaths, hospitalizations) occurring within a study population in relation to the distribution of PM 2.5 concentrations likely experienced by study participants. However, in recognizing that access to health event data may be restricted, and consistent with advice from CASAC (Samet 2010d, p. 2), EPA staff also considered the number of participants within each study area, in relation to the distribution of PM 2.5 concentrations (i.e., study population data), as a surrogate for health event data.

In applying this approach, the Policy Assessment focused on identifying the part of the distribution of PM 2.5 concentrations which had the most influence on generating health effect estimates in epidemiological studies, as discussed in section III.A.3 above. As discussed below, in working with study investigators, the EPA staff was able to obtain health event data for three large multi-city studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et al., 2008) and population data for the same three studies and one additional long-term exposure study (Miller et al., 2007), as documented in a staff memorandum (Rajan et al., 2011). [77] For the three studies for which both health event and study population data were available, the EPA staff analyzed the reliability of using study population data as a surrogate for health event data. Based on these analyses, the EPA staff recognized that the 10th and 25th percentiles of the health event and study population distributions are nearly identical and concluded that the distribution of population data can be a useful surrogate for event data, providing support for consideration of the study population data for Miller et al. (2007), for which health event data were not available (Rajan et al., 2011, Analysis 1 and Analysis 2, in particular, Table 1 and Figures 1 and 2).

With regard to the long-term mean PM 2.5 concentrations which are relevant to the first approach, Figures 1 through 3 (U.S. EPA, 2011a, Figures 2-4, 2-5, 2-6, and 2-8) summarize data available for multi-city, long- and short-term exposure studies that evaluated endpoints classified in the Integrated Science Assessment as having evidence of a causal or likely causal relationship or evidence suggestive of a causal relationship, showing the studies with long-term mean PM 2.5 concentrations below 17 μg/m [3] . [78] As discussed in more detail in section III.E.4.b of the proposal, Figures 1 and 3 summarize the health outcomes evaluated, relative risk estimates, air quality data, and geographic scope for long- and short-term exposure studies, respectively, that evaluated mortality (evidence of a causal relationship); cardiovascular effects (evidence of a causal relationship); and respiratory effects (evidence of a likely causal relationship) in the general population, as well as in older adults, an at-risk population. Figure 2 provides this same summary information for long-term exposure studies that evaluated respiratory effects (evidence of a likely causal relationship) in children, an at-risk population, as well as developmental effects (evidence suggestive of a causal relationship).

With regard to consideration of additional information from epidemiological studies which was relevant to the second approach, the EPA staff compiled a summary of the range of PM 2.5 concentrations corresponding with the 25th to 10th percentiles of health event or study population data from the four multi-city studies, for which distributional statistics are available [80] (U.S. EPA, 2011a, Figure 2-7; Rajan et al., 2011, Table 1). By considering this approach, one could focus on the range of PM 2.5 concentrations below the long-term mean ambient concentrations over which we continue to have confidence in the associations observed in epidemiological studies (e.g., above the 25th percentile) where commensurate public health protection could be obtained for PM 2.5-related effects and, conversely, identify the range in the distribution below which our confidence in the associations is appreciably less, to identify alternative annual standard levels.

The mean PM 2.5 concentrations associated with the studies summarized in Figures 1, 2, and 3 and with the distributional statistics analyses (Rajan et al., 2011) are based on concentrations averaged across ambient monitors within each area included in a given study and then averaged across study areas to calculate an overall study mean concentration, as discussed above. Figure 4, discussed in more detail in section III.E.4.a of the proposal, summarizes statistical metrics for those key studies [81] included in Figures 1, 2, and 3 that provide evidence of positive and generally statistically significant PM 2.5-related effects, which are relevant to the two approaches for translating epidemiological evidence into potential standard levels as discussed above. The top of Figure 4 includes information for long-term exposure studies evaluating health outcomes classified as having evidence of a causal or likely causal relationship with PM 2.5 exposures (long-term mean PM 2.5 concentrations indicated by diamond symbols). The middle of Figure 4 includes information for short-term exposure studies evaluating health outcomes classified as having evidence of a causal or likely causal relationship with PM 2.5 exposures (long-term mean PM 2.5 concentrations indicated by triangle symbols). The bottom of Figure 4 includes information for long-term exposures studies evaluating health outcomes classified as having evidence suggestive of a causal relationship (long-term mean PM 2.5 concentrations indicated by square symbols). Figure 4 also summarizes the range of PM 2.5 concentrations corresponding with the 25th (indicated by solid circles) to 10th (indicated by open circles) percentiles of the health event or study population data from the four multi-city studies (highlighted in bold text) for which distributional statistics are available.

In considering the evidence, the Policy Assessment recognized that NAAQS are standards set so as to provide requisite protection, neither more nor less stringent than necessary to protect public health, with an adequate margin of safety. This judgment, ultimately made by the Administrator, involves weighing the strength of the evidence and the inherent uncertainties and limitations of that evidence. Therefore, depending on the weight placed on different aspects of the evidence and inherent uncertainties, consideration of different alternative standard levels could be supported.

Given the currently available evidence discussed in more detail in section III.E.4.b of the proposal and considering the various approaches discussed above, the Policy Assessment concluded it was appropriate to focus on an annual standard level within a range of about 12 to 11 μg/m [3] (U.S. EPA, 2011a, pp. 2-82, 2-101, and 2-106). As illustrated in Figure 4, the Policy Assessment recognized that a standard level of 12 μg/m [3] , at the upper end of this range, was somewhat below the long-term mean PM 2.5 concentrations reported in all the multi-city, long- and short-term exposure studies that provided evidence of positive and statistically significant associations with health effects classified as having evidence of a causal or likely causal relationship, including premature mortality and hospitalizations and emergency department visits for cardiovascular and respiratory effects as well as respiratory effects in children. Further, a level of 12 μg/m [3] would reflect consideration of additional population-level information from such epidemiological studies in that it generally corresponded with approximately the 25th percentile of the available distributions of health events data in the studies for which population-level information was available. In addition, a level of 12 μg/m [3] would reflect some consideration of studies that provided more limited evidence of reproductive and developmental effects, which were suggestive of a causal relationship, in that it was about at the same level as the lowest long-term mean PM 2.5 concentrations reported in such studies (see Figure 4).

Alternatively the Policy Assessment recognized that an annual standard level of 11 μg/m [3] , at the lower end of this range, was well below the lowest long-term mean PM 2.5 concentrations reported in all multi-city long- and short-term exposure studies that provide evidence of positive and statistically significant associations with health effects classified as having evidence of a causal or likely causal relationship. A level of 11 μg/m [3] would reflect placing more weight on the distributions of health event and population data, in that this level was within the range of PM 2.5 concentrations corresponding to the 25th and 10th percentiles of all the available distributions of such data. In addition, a level of 11 μg/m [3] was somewhat below the lowest long-term mean PM 2.5 concentrations reported in reproductive and developmental effects studies that are suggestive of a causal relationship. Thus, a level of 11 μg/m [3] would reflect an approach to translating the available evidence that places relatively more emphasis on margin of safety considerations and less certain causal relationships than would a standard set at a higher level. Such a policy approach would tend to weigh uncertainties in the evidence in such a way as to avoid potentially underestimating PM 2.5-related risks to public health. Further, recognizing the uncertainties inherent in identifying any particular point at which our confidence in reported associations becomes appreciably less, the Policy Assessment concluded that the available evidence did not provide a sufficient basis to consider alternative annual standard levels below 11 μg/m [3] (U.S. EPA, 2011a, p. 2-81).

The Policy Assessment also considered the extent to which the available evidence provided a basis for considering alternative annual standard levels above 12 μg/m [3] . As discussed below, the Policy Assessment concluded that it could be reasonable to consider a standard level up to 13 μg/m [3] based on a policy approach that weighed uncertainties in the evidence in such a way as to avoid potentially overestimating PM 2.5-related risks to public health, especially to the extent that primary emphasis was placed on long-term exposure studies as a basis for an annual standard level. A level of 13 μg/m [3] was somewhat below the long-term mean PM 2.5 concentrations reported in all but one of the long-term exposure studies providing evidence of positive and statistically significant associations with PM 2.5-related health effects classified as having a causal or likely causal relationship. As shown in Figure 4, the one long-term exposure study with a long-term mean PM 2.5 concentration just below 13 μg/m [3] was the Miller et al., (2007) study. However, as noted in section III.D.1.a of the proposal and discussed in more detail in the Response to Comments document, the Policy Assessment observed that in comparison to other long-term exposure studies, the Miller et al. study was more limited in that it was based on only one year of air quality data and the one year was after the health outcomes were reported (U.S. EPA, 2011a, pp. 2-81 to 2-82). Thus, to the extent that less weight was placed on the Miller et al. study than on other long-term exposure studies with more robust air quality data, a level of 13 μg/m [3] could be considered as being protective of long-term exposure related effects classified as having a causal or likely causal relationship. In also considering short-term exposure studies, however, the Policy Assessment noted that a level of 13 μg/m [3] was below the long-term mean PM 2.5 concentrations reported in most but not all such studies. In particular, two studies—Burnett et al. (2004) and Bell et al. (2008)—reported long-term mean PM 2.5 concentrations of 12.8 and 12.9 μg/m [3] , respectively. In considering these studies, the Policy Assessment found no basis to conclude that these two studies were any more limited or uncertain than the other short-term exposure studies shown in Figures 3 and 4 (U.S. EPA, 2011a, p. 2-82). On this basis, as discussed below, the Policy Assessment concluded that consideration of an annual standard level of 13 μg/m [3] would have implications for the degree of protection that would need to be provided by the 24-hour standard, in order that the suite of PM 2.5 standards, taken together, would provide appropriate protection from effects on public health related to short-term exposure to PM 2.5 (U.S. EPA, 2011a, p. 2-82).

The Policy Assessment also noted that a standard level of 13 μg/m [3] would reflect a judgment that the uncertainties in the epidemiological evidence as summarized in section III.B above and discussed in more detail in section III.B.2 of the proposal, including uncertainties related to the heterogeneity observed in the epidemiological studies in the eastern versus western parts of the U.S., the relative toxicity of PM 2.5 components, and the potential role of co-pollutants, are too great to warrant placing any weight on the distributions of health event and population data that extend down below the long-term mean concentrations into the lower quartile of the data. This level would also reflect a judgment that the evidence from reproductive and developmental effects studies that is suggestive of a causal relationship was too uncertain to support consideration of any lower level.

Beyond evidence-based considerations, the Policy Assessment also considered the extent to which the quantitative risk assessment supported consideration of these alternative standard levels or provided support for lower levels. In considering simulations of just meeting alternative annual standard levels within the range of 13 to 11 μg/m [3] (in conjunction with the current 24-hour standard level of 35 μg/m [3] ), the Policy Assessment concluded that important public health improvements are associated with risk reductions estimated for standard levels of 13 and 12 μg/m [3] and noted that the level of 11 μg/m [3] was not included in the quantitative risk assessment. The Policy Assessment noted that the overall confidence in the quantitative risk estimates varied for the different alternative standard levels evaluated and was stronger for the higher levels and substantially lower for the lowest level evaluated (i.e., 10 μg/m [3] ). Based on the above considerations, the Policy Assessment concluded that the quantitative risk assessment provided support for considering alternative annual standard levels within a range of 13 to 11 μg/m [3] , but did not provide strong support for considering lower alternative standard levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concluded that consideration of alternative annual standard levels in the range of 13 to 11 μg/m [3] may be appropriate. Furthermore, the Policy Assessment concluded that the currently available evidence most strongly supported consideration of an alternative annual standard level in the range of 12 to 11 μg/m [3] (U.S. EPA, 2011a, p. 2-82). The Policy Assessment concluded that an alternative level within the range of 12 to 11 μg/m [3] would more fully take into consideration the available information from all long- and short-term PM 2.5 exposure studies, including studies of at-risk populations, than would a higher level. This range also reflected placing weight on information from studies that helped to characterize the range of PM 2.5 concentrations over which we continue to have confidence in the associations observed in epidemiological studies, as well as the extent to which our confidence in the associations was appreciably less at lower concentrations.

As recognized in sections III.A.3 and III.E.4.a above, an annual standard intended to serve as the primary means for providing protection from effects associated with both long- and short-term PM 2.5 exposures is not expected to provide appropriate protection against the effects of all short-term PM 2.5 exposures (unless established at a level so low as to undoubtedly provide more protection than necessary for long-term exposures). Of particular concern are areas with high peak-to-mean ratios possibly associated with strong local or seasonal sources, or PM 2.5-related effects that may be associated with shorter-than-daily exposure periods. As a result, the Policy Assessment concluded that it was appropriate to consider alternative 24-hour PM 2.5 standard levels that would supplement the protection provided by an annual standard.

As outlined in section III.A.3 above, the Policy Assessment considered the available evidence from short-term PM 2.5 exposure studies, as well as the uncertainties and limitations in that evidence, to assess the degree to which alternative annual and 24-hour PM 2.5 standards can be expected to reduce the estimated risks attributed to short-term fine particle exposures. In considering the available epidemiological evidence, the Policy Assessment took into account information from multi-city studies as well as single-city studies. The Policy Assessment considered the distributions of 24-hour PM 2.5 concentrations reported in short-term exposure studies, focusing on the 98th percentile concentrations to match the form of the 24-hour standard as discussed in section III.E.3.b above. In recognizing that the annual and 24-hour standards work together to provide protection from effects associated with short-term PM 2.5 exposures, the Policy Assessment also considered information on the long-term mean PM 2.5 concentrations from these studies.

In addition to considering the epidemiological evidence, the Policy Assessment considered air quality information, specifically peak-to-mean ratios using county-level 24-hour and annual design values, to characterize air quality patterns in areas possibly associated with strong local or seasonal sources. These patterns helped in understanding the extent to which different combinations of annual and 24-hour standards would be consistent with the policy goal of setting a generally controlling annual standard with a 24-hour standard that provides supplemental protection especially for areas with high peak-to-mean ratios (U.S. EPA, 2011a, p. 2-14).

In considering the information provided by the short-term exposure studies, the Policy Assessment recognized that to the extent these studies were conducted in areas that likely did not meet one or both of the current standards, such studies did not help inform the characterization of the potential public health improvements of alternative standards set at lower levels. Therefore, in considering the short-term exposure studies to inform staff conclusions regarding levels of the 24-hour standard that are appropriate to consider, the Policy Assessment placed greatest weight on studies conducted in areas that likely met both the current annual and 24-hour standards.

With regard to multi-city studies that evaluated effects associated with short-term PM 2.5 exposures, as summarized in Figure 3 above and discussed in more detail in section III.E.4.c of the proposal, the Policy Assessment noted that, to the extent air quality distributions were reduced to reflect just meeting the current 24-hour standard, additional protection would be anticipated for the effects observed in the three multi-city studies with 98th percentile values greater than 35 μg/m [3] (Burnett et al., 2004; Burnett and Goldberg, 2003; Franklin et al., 2008). In the three additional studies with 98th percentile values below 35 μg/m [3] , specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 μg/m [3] , the Policy Assessment noted that these studies reported long-term mean PM 2.5 concentrations of 12.9, 13.2, and 13.4 μg/m [3] , respectively (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a). To the extent that consideration was given to revising the level of the annual standard, as discussed in section III.E.4.b of the proposal, the Policy Assessment recognized that potential changes associated with meeting such an alternative annual standard would result in lowering risks associated with both long- and short-term PM 2.5 exposures. Consequently, in considering a 24-hour standard that would operate in conjunction with an annual standard to provide appropriate public health protection, the Policy Assessment noted that to the extent that the level of the annual standard was revised to within a range of 13 to 11 μg/m [3] , in particular in the range of 12 to 11 μg/m [3] , additional protection would be provided for the long-term effects observed in these multi-city studies (U.S. EPA, 2011a, p. 2-84).

Based on this information, the Policy Assessment concluded that the multi-city, short-term exposure studies generally provided support for retaining the 24-hour standard level at 35 μg/m [3] so long as the standard is in conjunction with an annual standard level revised to within a range of 12 to 11 μg/m [3] (U.S. EPA, 2011a, p. 2-84). Alternatively, in conjunction with an annual standard level of 13 μg/m [3] , the Policy Assessment concluded that the multi-city studies provided limited support for revising the 24-hour standard level somewhat below 35 μg/m [3] , such as down to 30 μg/m [3] , based on one study (Bell et al., 2008) that reported positive and statistically significant effects with an overall 98th percentile value below the level of the current 24-hour standard and an overall long-term mean concentration slightly less than 13 μg/m [3] (Figure 3; U.S. EPA, 2011a, p. 2-84).

In reaching staff conclusions regarding alternative 24-hour standard levels that were appropriate to consider, the Policy Assessment also took into account relevant information from single-city studies that evaluated effects associated with short-term PM 2.5 exposures. The Policy Assessment recognized that these studies may provide additional insights regarding impacts on at-risk populations and/or on areas with isolated peak concentrations.

As discussed in more detail in section III.E.4.c of the proposal, although a number of single-city studies reported effects at appreciably lower PM 2.5 concentrations than multi-city short-term exposure studies, the uncertainties and limitations associated with the single-city studies were considerably greater than those associated with the multi-city studies and, thus, the Policy Assessment concluded there was less confidence in using these studies as a basis for setting the level of a standard. Therefore, the Policy Assessment concluded that the multi-city short-term exposure studies provided the strongest evidence to inform decisions on the level of the 24-hour standard, and the single-city studies did not warrant consideration of 24-hour standard levels different from those supported by the multi-city studies (U.S. EPA, 2011a, p. 2-88).

In addition to considering the epidemiological evidence, the Policy Assessment took into account air quality information based on county-level 24-hour and annual design values to understand the public health implications of the alternative standard levels supported by the currently available scientific evidence, as discussed in this section. Consistent with the general approach discussed in section III.A.3 above, the Policy Assessment considered the extent to which different combinations of alternative annual and 24-hour standard levels based on the evidence would support the policy goal of lowering annual and 24-hour air quality distributions by using the annual standard to be the “generally controlling” standard in conjunction with setting the 24-hour standard to provide supplemental protection (U.S. EPA, 2011a, pp 2-88 to 2-91, Figure 2-10).

Using information on the relationship of the 24-hour and annual design values, the Policy Assessment examined the implications of three alternative suites of PM 2.5 standards identified as appropriate to consider based on the currently available scientific evidence, as discussed above. The Policy Assessment concluded that an alternative suite of PM 2.5 standards that would include an annual standard level of 11 or 12 μg/m [3] and a 24-hour standard with a level of 35 μg/m [3] (i.e., 11/35 or 12/35) would result in the annual standard being the generally controlling standard in most areas although the 24-hour standard would continue to be the generally controlling standard in the Northwest (U.S. EPA, 2011a, pp. 2-89 to 2-91 and Figure 2-10). These Northwest counties generally represented areas where the annual mean PM 2.5 concentrations have historically been low but where relatively high 24-hour concentrations occur, often related to seasonal wood smoke emissions. Alternatively, combining an alternative annual standard of 13 μg/m [3] with a 24-hour standard of 30 μg/m [3] would result in many more areas across the country in which the 24-hour standard would likely become the controlling standard (the standard driving air quality distributions lower) than if an alternative annual standard of 12 or 11 μg/m [3] were paired with the current level of the 24-hour standard (i.e., 35 μg/m [3] ).

The Policy Assessment concluded that consideration of retaining the 24-hour standard level at 35 μg/m [3] would reflect placing greatest weight on evidence from multi-city studies that reported positive and statistically significant associations with health effects classified as having a causal or likely causal relationship. In conjunction with lowering the annual standard level, especially within a range of 12 to 11 μg/m [3] , this alternative recognized additional public health protection against effects associated with short-term PM 2.5 exposures which would be provided by lowering the annual standard such that revision to the 24-hour standard would not be warranted (U.S. EPA, 2011a, p. 2-91).

Beyond evidence-based considerations, the Policy Assessment also considered the extent to which the quantitative risk assessment supported consideration of retaining the current 24-hour standard level or provided support for lower standard levels. In considering simulations of just meeting the current 24-hour standard level of 35 μg/m [3] or alternative levels of 30 or 25 μg/m [3] (in conjunction with alternative annual standard levels within a range of 13 to 11 μg/m [3] ), the Policy Assessment noted that the overall confidence in the quantitative risk estimates varied for the different standard levels evaluated and was stronger for the higher levels and substantially lower for the lowest level evaluated (i.e., 25 μg/m [3] ). Based on this information, the Policy Assessment concluded that the quantitative risk assessment provided support for considering a 24-hour standard level of 35 or 30 μg/m [3] (in conjunction with an alternative standard level within a range of 13 to 11 μg/m [3] ) but did not provide strong support for considering lower alternative 24-hour standard levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concluded that while it was appropriate to consider an alternative 24-hour standard level within a range of 35 to 30 μg/m [3] , the currently available evidence most strongly supported consideration for retaining the current 24-hour standard level at 35 μg/m [3] in conjunction with lowering the level of the annual standard within a range of 12 to 11 μg/m [3] (U.S. EPA, 2011a, p. 2-92).

ii. CASAC Advice

Based on its review of the second draft Policy Assessment, CASAC agreed with the general approach for translating the available epidemiological evidence, risk information, and air quality information into the basis for reaching conclusions on alternative standards for consideration. Furthermore, CASAC agreed “that it is appropriate to return to the strategy used in 1997 that considers the annual and the short-term standards together, with the annual standard as the controlling standard, and the short-term standard supplementing the protection afforded by the annual standard” and “considers it appropriate to place the greatest emphasis” on health effects judged to have evidence supportive of a causal or likely causal relationship as presented in the Integrated Science Assessment (Samet, 2010d, p. 1).

CASAC concluded that the range of levels presented in the second draft Policy Assessment (i.e., alternative annual standard levels within a range of 13 to 11 μg/m [3] and alternative 24-hour standard levels within a range of 35 to 30 μg/m [3] ) “are supported by the epidemiological and toxicological evidence, as well as by the risk and air quality information compiled” in the Integrated Science Assessment, Risk Assessment, and second draft Policy Assessment. CASAC further noted that “[a]lthough there is increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a level below which there is no risk for adverse health effects)” (Samet, 2010d, p. ii).

Although CASAC supported the alternative standard level ranges presented in the second draft Policy Assessment, it did not express support for any specific levels or combinations of standards. Rather, CASAC encouraged the EPA to develop a clearer rationale in the final Policy Assessment for staff conclusions regarding annual and 24-hour standards that were appropriate to consider, including consideration of the combination of these standards supported by the available information (Samet, 2010d, p. ii). Specifically, in commenting on a distributional statistical analysis of air quality and associated population data presented in the second draft Policy Assessment, CASAC encouraged staff to focus on information related to the concentrations that were most influential in generating the health effect estimates in individual studies to inform alternative standard levels. CASAC urged that the EPA redo that analysis using health event or study population data (Samet, 2010d, p. 2). CASAC also commented that the approach presented in the second draft Policy Assessment to identify alternative 24-hour standard levels which focused on peak-to-mean ratios was not relevant for informing the actual level (Samet 2010d, p. 4). Further, they expressed the concern that the combinations of annual and 24-hour standard levels discussed in the second draft Policy Assessment (i.e., in the range of 13 to 11 μg/m [3] for the annual standard, in conjunction with retaining the current 24-hour PM 2.5 standard level of 35 μg/m [3] ; alternatively, revising the level of the 24-hour standard to 30 μg/m [3] in conjunction with an annual standard level of 11 μg/m [3] ) “may not be adequately inclusive” and “[i]t was not clear why, for example a daily standard of 30 μg/m [3] should only be considered in combination with an annual level of 11 μg/m [3] ” (Samet, 2010d, p. ii). CASAC encouraged the EPA to more clearly explain its rationale for identifying the 24-hour/annual combinations that are appropriate for consideration (Samet 2010d, p. ii).

In considering CASAC's advice as well as public comment on the second draft Policy Assessment, the EPA staff conducted additional analyses and modified their conclusions regarding alternative standard levels that were appropriate to consider. The staff conclusions in the final Policy Assessment (U.S. EPA, 2011a, section 2.3.4.4) differed somewhat from the alternative standard levels discussed in the second draft Policy Assessment (U.S. EPA, 2010f, section 2.3.4.3), upon which CASAC based its advice. Changes made in the final Policy Assessment were primarily focused on improving and clarifying the approach for translating the epidemiological evidence into a basis for staff conclusions on the broadest range of alternative standard levels supported by the available scientific information and more clearly articulating the rationale for the staff's conclusions (Wegman, 2011, pp. 1 to 2). Consistent with CASAC's advice to consider more information from epidemiological studies, as discussed in section III.E.4.b.1 above, the EPA analyzed additional population-level data obtained from several study authors (Rajan et al., 2011). In transmitting the final Policy Assessment to CASAC, the Agency notified CASAC that the final staff conclusions reflected consideration of CASAC's advice and that those staff conclusions were based, in part, on the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 2011, p.2). Thus, CASAC had an opportunity to comment on the final Policy Assessment, but chose not to provide any additional comments or advice after receiving it.

iii. Administrator's Proposed Decisions on the Primary PM 2.5 Standard Levels

In reaching her conclusions regarding appropriate alternative standard levels to consider, the Administrator considered the epidemiological and other scientific evidence, estimates of risk reductions associated with just meeting alternative annual and/or 24-hour standards, air quality analyses, related limitations and uncertainties, staff conclusions as presented in the Policy Assessment, and the advice of CASAC. As an initial matter, the Administrator agreed with the general approach discussed in the Policy Assessment as summarized in sections III.A.3 and III.E.4.a above, and supported by CASAC, of considering the protection afforded by the annual and 24-hour standards taken together for mortality and morbidity effects associated with both long- and short-term exposures to PM 2.5 (77 FR 38939). Furthermore, based on the evidence and quantitative risk assessment, the Administrator provisionally concluded it is appropriate to set a “generally controlling” annual standard that will lower a wide range of ambient 24-hour concentrations, with a 24-hour standard focused on providing supplemental protection, particularly for areas with high peak-to-mean ratios possibly associated with strong local or seasonal sources, or PM 2.5-related effects that may be associated with shorter-than daily exposure periods. The Administrator provisionally concluded this approach would likely reduce aggregate risks associated with both long- and short-term exposures more consistently than a generally controlling 24-hour standard and would be the most effective and efficient way to reduce total PM 2.5-related population risk. Id.

In reaching decisions on alternative standard levels to propose, the Administrator judged that it was most appropriate to examine where the evidence of associations observed in the epidemiological studies was strongest and, conversely, where she had appreciably less confidence in the associations observed in the epidemiological studies. Based on the characterization and assessment of the epidemiological and other studies presented and assessed in the Integrated Science Assessment and the Policy Assessment, the Administrator recognized the substantial increase in the number and diversity of studies available in this review including extended analyses of the seminal studies of long-term PM 2.5 exposures (i.e., ACS and Harvard Six Cities studies) as well as important new long-term exposure studies (as summarized in Figures 1 and 2). Collectively, the Administrator noted that these studies, along with evidence available in the last review, provided consistent and stronger evidence of an association with premature mortality, with the strongest evidence related to cardiovascular-related mortality, at lower ambient concentrations than previously observed. The Administrator also recognized the availability of stronger evidence of morbidity effects associated with long-term PM 2.5 exposures, including evidence of cardiovascular effects from the WHI study and respiratory effects, including decreased lung function growth, from the extended analyses for the Southern California Children's Health Study. Furthermore, the Administrator recognized new U.S. multi-city studies that greatly expanded and reinforced our understanding of mortality and morbidity effects associated with short-term PM 2.5 exposures, providing stronger evidence of associations at ambient concentrations similar to those previously observed (as summarized in Figure 3). Id. at 38939-40.

The newly available scientific evidence built upon the previous scientific data base to provide evidence of generally robust associations and to provide a basis for greater confidence in the reported associations than in the last review. The Administrator recognized that the weight of evidence, as evaluated in the Integrated Science Assessment, was strongest for health endpoints classified as having evidence of a causal relationship. These relationships included those between long- and short-term PM 2.5 exposures and mortality and cardiovascular effects. She recognized that the weight of evidence was also strong for health endpoints classified as having evidence of a likely causal relationship, which included those between long- and short-term PM 2.5 exposures and respiratory effects. In addition, the Administrator made note of the much more limited evidence for health endpoints classified as having evidence suggestive of a causal relationship, including developmental, reproductive and carcinogenic effects. Id. at 38940.

Based on information discussed and presented in the Integrated Science Assessment, the Administrator recognized that health effects may occur over the full range of concentrations observed in the long- and short-term epidemiological studies and that no discernible threshold for any effects can be identified based on the currently available evidence (U.S. EPA, 2009a, section 2.4.3). She also recognized, in taking note of CASAC advice and the distributional statistics analysis discussed in section III.E.4.b.i above and in the Policy Assessment, that there was significantly greater confidence in observed associations over certain parts of the air quality distributions in the studies, and conversely, that there was significantly diminished confidence in ascribing effects to concentrations toward the lower part of the distributions.

Consistent with the general approach summarized in section III.A.3 above, and supported by CASAC as discussed in section III.E.4.a above, the Administrator generally agreed that it was appropriate to consider a level for an annual standard that was somewhat below the long-term mean PM 2.5 concentrations reported in long- and short-term exposure studies. In recognizing that the evidence of an association in any such study was strongest at and around the long-term average where the data in the study are most concentrated, she understood that this approach did not provide a bright line for reaching decisions about appropriate standard levels. The Administrator noted that long-term mean PM 2.5 concentrations were available for each study considered and, therefore, represented the most robust data set to inform her decisions on appropriate annual standard levels. She also noted that the overall study mean PM 2.5 concentrations were generally calculated based on monitored concentrations averaged across monitors in each study area with multiple monitors, referred to as a composite monitor concentration, in contrast to the highest concentration monitored in each study area, referred to as a maximum monitor concentration, which are used to determine whether an area meets a given standard. In considering such long-term mean concentrations, the Administrator understood that it was appropriate to consider the weight of evidence for the health endpoints evaluated in such studies in giving weight to this information. Id.

Based on the information summarized in Figure 4 above and presented in more detail in the Policy Assessment (U.S. EPA, 2011a, chapter 2) for effects classified in the Integrated Science Assessment as having a causal or likely causal relationship with PM 2.5 exposures, the Administrator observed an overall pattern of statistically significant associations reported in studies of long-term PM 2.5 exposures with long-term mean concentrations ranging from somewhat above the current standard level of 15 μg/m [3] down to the lowest mean concentration in such studies of 12.9 μg/m [3] (in Miller et al., 2007). [83] She observed a similar pattern of statistically significant associations in studies of short-term PM 2.5 exposures with long-term mean concentrations ranging from around 15 μg/m [3] down to 12.8 μg/m [3] (in Burnett et al., 2004). With regard to effects classified as providing evidence suggestive of a causal relationship, the Administrator observed a small number of long-term exposure studies related to developmental and reproductive effects that reported statistically significant associations with overall study mean PM 2.5 concentrations down to 11.9 μg/m [3] (in Bell et al., 2007). [84] Id.

The Administrator also considered additional information from epidemiological studies, consistent with CASAC advice, to take into account the broader distribution of PM 2.5 concentrations and the degree of confidence in the observed associations over the broader air quality distribution. In considering this additional information, she understood that the Policy Assessment presented information on the 25th and 10th percentiles of the distributions of PM 2.5 concentrations available from four multi-city studies to provide a general frame of reference as to the part of the distribution in which the data become appreciably more sparse and, thus, where her confidence in the associations observed in epidemiological studies would become appreciably less.

As summarized in Figure 4 above, the Administrator took note of additional population-level data that were available for four studies (Krewski et al., 2009; Miller et al., 2007; Bell et al., 2008; Zanobetti and Schwartz, 2009), each of which reported statistically significant associations with health endpoints classified as having evidence of a causal relationship. In considering the long-term PM 2.5 concentrations associated with the 25th percentile values of the population-level data for these four studies, she observed that these values ranged from somewhat above to somewhat below 12 μg/m [3] . The Administrator recognized that these studies include some of the strongest evidence available within the overall body of scientific evidence and noted that three of these studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009) were used as the basis for concentration-response functions used in the quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3).

In considering this information, the Administrator noted that CASAC advised that information about the long-term PM 2.5 concentrations that were most influential in generating the health effect estimates in epidemiological studies can help to inform selection of an appropriate annual standard level. However, the Administrator also recognized that additional population-level data were available for only these four studies and, therefore, she believed that these studies comprised a more limited data set than one based on long-term mean PM 2.5 concentrations for which data were available for all studies considered, as discussed above.

The Administrator recognized, as summarized in section III.B above, that important uncertainties remain in the evidence and information considered in this review of the primary fine particle standards. These uncertainties are generally related to understanding the relative toxicity of the different components in the fine particle mixture, the role of PM 2.5 in the complex ambient mixture, exposure measurement errors inherent in epidemiological studies based on concentrations measured at fixed monitor sites, and the nature, magnitude, and confidence in estimated risks related to increasingly lower ambient PM 2.5 concentrations. Furthermore, the Administrator noted that epidemiological studies have reported heterogeneity in responses both within and between cities and geographic regions across the U.S. She recognized that this heterogeneity may be attributed, in part, to differences in fine particle composition in different regions and cities. The Administrator also recognized that there are additional limitations associated with evidence for reproductive and developmental effects, identified as being suggestive of a causal relationship with long-term PM 2.5 exposures, including: the limited number of studies evaluating such effects; uncertainties related to identifying the relevant exposure time periods of concern; and limited toxicological evidence providing little information on the mode of action(s) or biological plausibility for an association between long-term PM 2.5 exposures and adverse birth outcomes. Id. at 38941.

The Administrator was mindful that considering what standards were requisite to protect public health with an adequate margin of safety required public health policy judgments that neither overstated nor understated the strength and limitations of the evidence or the appropriate inferences to be drawn from the evidence. In considering how to translate the available information into appropriate standard levels, the Administrator weighed the available scientific information and associated uncertainties and limitations. For the purpose of determining what standard levels were appropriate to propose, the Administrator recognized, as did EPA staff in the Policy Assessment, that there was no single factor or criterion that comprised the sole “correct” approach to weighing the various types of available evidence and information, but rather there were various approaches that are appropriate to consider. The Administrator further recognized that different evaluations of the evidence and other information before the Administrator could reflect placing different weight on the relative strengths and limitations of the scientific information, and different judgments could be made as to how such information should appropriately be used in making public health policy decisions on standard levels. This recognition led the Administrator to consider various approaches to weighing the evidence so as to identify appropriate standard levels to propose. In so doing, the Administrator encouraged extensive public comment on alternative approaches to weighing the evidence and other information so as to inform her public health policy judgments before reaching final decisions on appropriate standard levels.

In considering the available information, the Administrator noted the advice of CASAC that the currently available scientific information, including epidemiological and toxicological evidence as well as risk and air quality information, provided support for considering an annual standard level within a range of 13 to 11 μg/m [3] and a 24-hour standard level within a range of 35 to 30 μg/m [3] . In addition, the Administrator recognized that the Policy Assessment concluded that the available evidence and risk-based information support consideration of annual standard levels in the range of 13 to 11 μg/m [3] , and that the Policy Assessment also concluded that the evidence most strongly supported consideration of an annual standard level in the range of 12 to 11 μg/m [3] . In considering how the annual and 24-hour standards work together to provide appropriate public health protection, the Administrator observed that CASAC did not express support for any specific levels or combinations of standards within these ranges. Nor did CASAC choose to comment on additional information and analyses presented in the final Policy Assessment prepared in response to CASAC's recommendations on the second draft Policy Assessment (Wegman, 2011).

In considering the extent to which the currently available evidence and information provided support for specific standard levels within the ranges identified by CASAC and the Policy Assessment as appropriate for consideration, the Administrator initially considered standard levels within the range of 13 to 11 μg/m [3] for the annual standard. In so doing, the Administrator first considered the long-term mean PM 2.5 concentrations reported in studies of effects classified as having evidence of a causal or likely causal relationship, as summarized in Figure 4 above and discussed more broadly above. She noted that a level at the upper end of this range would be below most but not all the overall study mean concentrations from the multi-city studies of long- and short-term exposures, whereas somewhat lower levels within this range would be below all such overall study mean concentrations. In considering the appropriate weight to place on this information, the Administrator again noted that the evidence of an association in any such study was strongest at and around the long-term average where the data in the study are most concentrated, and that long-term mean PM 2.5 concentrations were available for each study considered and, therefore, represented the most robust data set to inform her decisions on appropriate annual standard levels. Further, she was mindful that this approach did not provide a bright line for reaching decisions about appropriate standard levels. Id.

In considering the long-term mean PM 2.5 concentrations reported in studies of effects classified as having evidence suggestive of a causal relationship, as summarized in Figure 4 for reproductive and developmental effects, the Administrator noted that a level at the upper end of this range would be below the overall study mean concentration in one of the three studies, while levels in the mid- to lower part of this range would be below the overall study mean concentrations in two or three of these studies. In considering the appropriate weight to place on this information, the Administrator noted the very limited nature of this evidence of such effects and the additional uncertainties in these epidemiological studies relative to the studies that provide evidence of causal or likely causal relationships.

The Administrator also considered the distributional analyses of population-level information that were available from four of the epidemiological studies that provide evidence of effects identified as having a causal relationship with long- or short-term PM 2.5 concentrations for annual standard levels within the same range of 13 to 11 μg/m [3] . In so doing, the Administrator first noted that a level in the mid-part of this range generally corresponds with approximately the 25th percentile of the distributions of health events data available in three of these studies. The Administrator also noted that standard levels toward the upper part of this range would reflect placing substantially less weight on this information, whereas standard levels toward the lower part of this range would reflect placing substantially more weight on this information. In considering this information, the Administrator noted that there was no bright line that delineates the part of the distribution of PM 2.5 concentrations within which the data become appreciably more sparse and, thus, where her confidence in the associations observed in epidemiological studies became appreciably less.

In considering mean PM 2.5 concentrations and distributional analyses from the various sets of epidemiological studies noted above, the Administrator was mindful, as noted above, that such studies typically report concentrations based on composite monitor distributions, in which concentrations may be averaged across multiple ambient monitors that may be present within each area included in a given study. Thus, a policy approach that used data based on composite monitors to identify potential alternative standard levels would inherently build in a margin of safety of some degree relative to an alternative standard level based on measurements at the monitor within an area that records the highest concentration, or the maximum monitor, since once a standard was set, concentrations at appropriate maximum monitors within an area were generally used to determine whether an area meets a given standard.

The Administrator also recognized that judgments about the appropriate weight to place on any of the factors discussed above should reflect consideration not only of the relative strength of the evidence but also on the important uncertainties that remained in the evidence and information being considered in this review. The Administrator noted that the extent to which these uncertainties influenced judgments about appropriate annual standard levels within the range of 13 to 11 μg/m [3] would likely be greater for standard levels in the lower part of this range which would necessarily be based on fewer available studies than would higher levels within this range.

Based on the above considerations, the Administrator concluded that it was appropriate to propose to set a level for the primary annual PM 2.5 standard within the range of 12 to 13 μg/m [3] . The Administrator provisionally concluded that a standard set within this range would reflect alternative approaches to appropriately placing the most weight on the strongest available evidence, while placing less weight on much more limited evidence and on more uncertain analyses of information available from a relatively small number of studies. Further, she provisionally concluded that a standard level within this range would reflect alternative approaches to appropriately providing an adequate margin of safety for the populations at risk for the serious health effects classified as having evidence of a causal or likely causal relationship, depending in part on the emphasis placed on margin of safety considerations. The Administrator recognized that setting an annual standard level at the lower end of this range would reflect an approach that placed more emphasis on the entire body of the evidence, including the analysis of the distribution of air quality concentrations most influential in generating health effect estimates in the studies, and on margin of safety considerations, than would setting a level at the upper end of the range. Conversely, an approach that would support a level at the upper end of this range would generally support a view that the uncertainties remaining in the evidence are such that the evidence does not warrant setting a lower annual standard level. Id. at 38942.

At the time of the proposal, while the Administrator recognized that CASAC advised, and the Policy Assessment concluded, that the available scientific information provided support for considering a range that extended down to 11 μg/m [3] , she concluded that proposing such an extended range would reflect a public health policy approach that placed more weight on relatively limited evidence and more uncertain information and analyses than she considered appropriate at this time. Nonetheless, the Administrator solicited comment on a level down to 11 μg/m [3] as well as on approaches for translating scientific evidence and rationales that would support such a level. Such an approach might reflect a view that the uncertainties associated with the available scientific information warrant a highly precautionary public health policy response that would incorporate a large margin of safety.

The Administrator recognized that potential air quality changes associated with meeting an annual standard set at a level within the range of 12 to 13 μg/m [3] will result in lowering risks associated with both long- and short-term PM 2.5 exposures. However, the Administrator recognized that such an annual standard intended to serve as the primary means for providing protection from effects associated with both long- and short-term PM 2.5 exposures would not by itself be expected to offer requisite protection with an adequate margin of safety against the effects of all short-term PM 2.5 exposures. As a result, in conjunction with proposing an annual standard level in the range of 12 to 13 μg/m [3] , the Administrator provisionally concluded that it was appropriate to continue to provide supplemental protection by means of a 24-hour standard set at the appropriate level, particularly for areas with high peak-to-mean ratios possibly associated with strong local or seasonal sources, or for PM 2.5-related effects that may be associated with shorter-than-daily exposure periods.

Based on the approach discussed in section III.A.3 above, at the time of the proposal the Administrator relied upon evidence from the short-term exposure studies as the principal basis for selecting the level of the 24-hour standard. In considering these studies as a basis for the level of a 24-hour standard, and having selected a 98th percentile form for the standard, the Administrator agreed with the focus in the Policy Assessment of looking at the 98th percentile values, as well as at the long-term mean PM 2.5 concentrations in these studies.

In considering the information provided by the short-term exposure studies, the Administrator recognized that to the extent these studies were conducted in areas that likely did not meet one or both of the current standards, such studies did not help inform the characterization of the potential public health improvements of alternative standards set at lower levels. By reducing the PM 2.5 concentrations in such areas to just meet the current standards, the Administrator anticipated that additional public health protection would occur. Therefore, the Administrator focused on studies that reported positive and statistically significant associations in areas that would likely have met both the current 24-hour and annual standards. She also considered whether or not these studies were conducted in areas that would likely have met an annual standard level of 12 to 13 μg/m [3] to inform her decision regarding an appropriate 24-hour standard level. As discussed in section III.E.4.a, consistent with the Policy Assessment, the Administrator concluded that multi-city, short-term exposure studies provided the strongest data set for informing her decisions on appropriate 24-hour standard levels. The Administrator viewed the single-city, short-term exposure studies as a much more limited data set providing mixed results and, therefore, she had less confidence in using those studies as a basis for setting the level of a 24-hour standard. With regard to the limited number of single-city studies that reported positive and statistically significant associations for a range of health endpoints related to short-term PM 2.5 concentrations in areas that would likely have met the current suite of PM 2.5 standards, the Administrator recognized that many of those studies had significant limitations (e.g., limited statistical power, limited exposure data) or equivocal results (mixed results within the same study area) that made them unsuitable to form the basis for setting the level of a 24-hour standard.

With regard to multi-city studies that evaluated effects associated with short-term PM 2.5 exposures, the Administrator observed an overall pattern of positive and statistically significant associations in studies with 98th percentile values averaged across study areas in the range of 45.8 to 34.2 μg/m [3] (Burnett et al., 2004; Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and Goldberg, 2003; Franklin et al., 2008). The Administrator noted that, to the extent air quality distributions were reduced to reflect just meeting the current 24-hour standard, additional protection would be anticipated for the effects observed in the three multi-city studies with 98th percentile values greater than 35 μg/m [3] (Burnett et al., 2004; Burnett and Goldberg, 2003; Franklin et al., 2008). In the three additional studies with 98th percentile values below 35 μg/m [3] , specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 μg/m [3] , the Administrator noted that these studies reported long-term mean PM 2.5 concentrations of 12.9, 13.2, and 13.4 μg/m [3] , respectively (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a).

In proposing to revise the level of the annual standard to within the range of 12 to 13 μg/m [3] , as discussed above, the Administrator recognized that additional protection would be provided for the short-term effects observed in these multi-city studies in conjunction with an annual standard level of 12 μg/m [3] , and in two of these three studies in conjunction with an annual standard level of 13 μg/m [3] . She noted that the study-wide mean concentrations were based on averaging across monitors within study areas and that compliance with the standard would be based on concentrations measured at the monitor reporting the highest concentration within each area. The Administrator believed it would be reasonable to conclude that revision to the 24-hour standard would not be appropriate in conjunction with an annual standard within this range. Based on the above considerations related to the epidemiological evidence, the Administrator provisionally concluded that it was appropriate to retain the level of the 24-hour standard at 35 μg/m [3] , in conjunction with a revised annual standard level in the proposed range of 12 to 13 μg/m [3] .

In addition to considering the epidemiological evidence, the Administrator also took into account air quality information based on county-level 24-hour and annual design values to understand the public health implications of retaining the 24-hour standard level at 35 μg/m [3] in conjunction with an annual standard level within the proposed range of 12 to 13 μg/m [3] . She considered whether these suites of standards would meet a public health policy goal which included setting the annual standard to be the “generally controlling” standard in conjunction with setting the 24-hour standard to provide supplemental protection to the extent that additional protection is warranted. As discussed above, the Administrator provisionally concluded that this approach was the most effective and efficient way to reduce total population risk associated with both long- and short-term PM 2.5 exposures, resulting in more uniform protection across the U.S. than the alternative of setting the 24-hour standard to be the controlling standard.

In considering the air quality information, the Administrator first recognized that there was no annual standard within the proposed range of levels, when combined with a 24-hour standard at the proposed level of 35 μg/m [3] , for which the annual standard would be the generally controlling standard in all areas of the country. She further observed that such a suite of PM 2.5 standards with an annual standard level of 12 μg/m [3] would result in the annual standard as the generally controlling standard in most regions across the country, except for certain areas in the Northwest, where the annual mean PM 2.5 concentrations have historically been low but where relatively high 24-hour concentrations occur, often related to seasonal wood smoke emissions (U.S. EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). Although not explicitly delineated on Figure 2-10 in the Policy Assessment, an annual standard of 13 μg/m [3] would be somewhat less likely to be the generally controlling standard in some regions of the U.S. outside the Northwest in conjunction with a 24-hour standard level of 35 μg/m [3] .

Taking the above considerations into account, the Administrator proposed to revise the level of the primary annual PM 2.5 standard from 15.0 μg/m [3] to within the range of 12.0 to 13.0 μg/m [3] and to retain the 24-hour standard level at 35 μg/m [3] . In the Administrator's judgment, such a suite of primary PM 2.5 standards and the rationale supporting such levels could reasonably be judged to reflect alternative approaches to the appropriate consideration of the strength of the available evidence and other information and their associated uncertainties and the advice of CASAC.

The Administrator recognized that the final suite of standards selected from within the proposed range of annual standard levels, or the broader range of annual standard levels on which public comment was solicited, must be clearly responsive to the issues raised by the DC Circuit's remand of the 2006 primary annual PM 2.5 standard. Furthermore, at the time of the proposal, she recognized that the final suite of standards will reflect her ultimate judgment in the final rulemaking as to the suite of primary PM 2.5 standards that would be requisite to protect the public health with an adequate margin of safety from effects associated with fine particle exposures. The final judgment to be made by the Administrator will appropriately consider the requirement for a standard that is neither more nor less stringent than necessary and will recognize that the CAA does not require that primary standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety.

At the time of the proposal, having reached her provisional judgment to propose revising the annual standard level from 15.0 to within a range of 12.0 to 13.0 μg/m [3] and to propose retaining the 24-hour standard level at 35 μg/m [3] , the Administrator solicited public comment on this range of levels and on approaches to considering the available evidence and information that would support the choice of levels within this range. The Administrator also solicited public comment on alternative annual standard levels down to 11 μg/m [3] and on the combination of annual and 24-hour standards that commenters may believe is appropriate, along with the approaches and rationales used to support such levels. In addition, given the importance the evidence from epidemiologic studies played in considering the appropriate annual and 24-hour levels, the Administrator solicited public comment on issues related to translating epidemiological evidence into standards, including approaches for addressing the uncertainties and limitations associated with this evidence.

c. Comments on Standard Levels

This section addresses comments that relate to consideration of the appropriate levels of the primary annual and 24-hour PM 2.5 standards, including comments on the general approach used by the EPA to translate the available scientific information into standard levels and how specific PM 2.5 exposure studies should be considered as a basis for the standard levels. These comments on standard levels expand upon the more general comments that either supported or opposed any change to the current suite of primary PM 2.5 standards, which are addressed above in section III.D.2. [85] As explained there, one group of commenters generally opposed any change to the current primary PM 2.5 standards and more specifically disagreed with the basis for the EPA's proposal to revise the annual standard level. Another group of commenters supported revising the current suite of primary PM 2.5 standards to provide increased public health protection. Some commenters in this second group argued that both the annual and 24-hour standard levels should be lowered while other commenters in this group agreed with the EPA's proposal to retain the level of the 24-hour standard in conjunction with revising the level of the annual standard. While generally supporting the EPA's proposal to lower the level of the annual standard, many commenters in this group disagreed that a level within the EPA's proposed range was adequately protective and supported a level of 11 μg/m [3] or below.

i. Annual Standard Level

The group of commenters opposed to any change to the current suite of primary PM 2.5 standards generally raised questions regarding the underlying scientific evidence, including the causal determinations reached in the Integrated Science Assessment, and focused strongly on the uncertainties they saw in the scientific evidence as a basis for their conclusion that no changes to the current standard levels were warranted. In commenting on the proposed standard levels, these commenters typically relied on the arguments summarized and addressed above in section III.D.2 as to why they believed it was inappropriate for the EPA to make any revisions to the suite of primary PM 2.5 standards. That is, they asserted that the EPA's causal determinations were not adequately supported by the underlying scientific information; the biological plausibility of health effects observed in epidemiological studies has not been demonstrated in controlled human exposure and toxicological studies; uncertainties in the underlying health science are as great or greater than in 2006; there is no evidence of greater risk since the last review to justify tightening the current annual PM 2.5 standard; and “new” studies not included in the Integrated Science Assessment continue to increase uncertainty about possible health risks associated with exposure to PM 2.5.

With regard to the level of the annual standard, these commenters strongly disagreed with the Agency's proposed decision to revise the level to within a range of 12 to 13 μg/m [3] and argued that the current standard level of 15 μg/m [3] should be retained. For example, UARG, API, and other commenters in this group raised a number of issues that they asserted called into question the EPA's interpretation of the epidemiological evidence to support revising the annual standard level. These commenters raised specific questions related to the general approach used by the EPA to translate the air quality and other information from specific epidemiological studies into standard levels, including: (1) The EPA's approach for using composite monitor air quality distributions reported in epidemiological studies to select a standard level that would be compared to measurements at the monitor recording the highest value in an area to determine compliance with the standard; (2) the appropriate exposure period for effects observed in long-term exposure mortality studies; and (3) the use of the EPA's analysis of distributions of underlying population-level data (i.e., health event and study population data) for those epidemiological studies for which such information was available. These commenters also raised questions regarding the EPA's consideration of specific scientific evidence as a basis for setting a standard level, including: (4) evidence of respiratory morbidity effects in long-term exposure studies and (5) more limited evidence of health effects which have been categorized in the Integrated Science Assessment as suggestive of a causal relationship (i.e., developmental and reproductive outcomes). These comments are discussed in turn below.

(1) Some commenters in this group argued that one reason why they believe there is no basis for setting a standard level below 15 μg/m [3] is that the air quality metric from epidemiological studies that the EPA relied on in the proposal is not the same metric that will be compared to the level of the standard to determine compliance with the standard. That is, commenters noted that the long-term mean PM 2.5 concentrations that the EPA considered, shown in Figure 4 above, are composite monitor mean concentrations (i.e., concentrations averaged across multiple monitors within areas with more than one monitor), whereas the PM 2.5 concentrations that will be compared to the level of the standard are maximum monitor concentrations (i.e., the concentration measured by the monitor within an area reporting the highest concentration). This comment was presented most specifically in UARG's comments (UARG, 2012, Attachment 1, pp. 2 to 6), which raised two overarching issues as discussed below.

First, the commenter noted that the EPA's approach of considering composite monitor mean PM 2.5 concentrations in selecting a standard level, and then comparing the maximum monitor mean PM 2.5 concentration in each area to the standard level when the standard is implemented, was characterized in the proposal as inherently having the potential to build in a margin of safety (UARG, 2012, Attachment 1, p. 4, citing 77 FR 38905). The commenter asserted that the Administrator is ignoring this distinction between composite and maximum monitor concentrations, and that this approach creates an unwarranted case for lowering the standard level, since in the commenter's view, it would result in a margin of safety that would be arbitrary, not based on evidence, and unquantified (UARG, 2012, Attachment 1, p. 4). In support of this view, the commenter asserted that there is a significant difference between composite monitor mean PM 2.5 concentrations and maximum monitor mean PM 2.5 concentrations. The commenter asserted that the maximum monitor value will always be higher than the composite monitor value (except in areas that contain only a single monitor), such that when an area just attains the NAAQS, that area's composite monitor long-term mean PM 2.5 concentration will be lower than the level of the standard (UARG, 2012, Attachment 1, p. 3).

Second, the commenter asserted that a more “reasoned and consistent approach would be to decide on a mean composite monitor PM 2.5 level that should be achieved and then identify the maximum monitor level that would result in that composite value” (UARG, 2012, Attachment 1, p. 4). The commenter conducted an analysis of maximum monitor versus composite monitor annual mean PM 2.5 concentrations using monitoring data [86] from 2006 to 2008 and presented results averaged across areas within two groups (i.e., those with design values [87] above the current standard level and those with design values just below the current standard level) to illustrate their suggested alternative approach. The commenter interpreted this analysis as showing that the composite monitor long-term mean PM 2.5 concentrations from the subset of the epidemiological studies shown in Figure 4 (of the proposal and above) that the commenter considered to be an appropriate focus for this analysis would be achieved across the U.S. if the current annual NAAQS of 15 μg/m [3] is retained and attained. The commenter considered the subset of epidemiological studies that included only long-term exposures studies of effects for which the evidence is categorized as causal or likely causal, but did not consider short-term exposure studies. On this basis, the commenter asserted that attaining the current annual PM 2.5 standard would result in composite monitor long-term mean concentrations in all areas that would be generally within or below the range of the composite monitor long-term mean concentrations from such studies and, as a result, there is no reason to lower the level of the current annual NAAQS.

In considering the first issue related to the EPA's approach, the EPA notes that in proposing to revise both the form and level of the annual standard, the Administrator clearly took into account the distinction between the composite monitor long-term mean PM 2.5 concentrations from the epidemiological studies, considered as a basis for selecting an annual standard level, and maximum monitor long-term mean PM 2.5 concentrations. In deciding to focus on the composite monitor long-term mean concentrations in selecting the standard level, and on the maximum monitor concentrations in selecting the form of the standard (i.e., consistent with proposing to eliminate the option for spatial averaging across monitors within an area when implementing the standard [88] ), the Administrator reasonably considered the distinction between these metrics in a manner that was consistent with advice from CASAC (Samet et al., 2010d, pp. 2 to 3).

As noted above in section III.A.3, the EPA recognizes that a statistical metric (e.g., the mean of a distribution) based on maximum monitor concentrations may be identical to or above the same statistical metric based on composite monitor concentrations. More specifically, many areas have only one monitor, in which case the composite and maximum monitor concentrations are identical. Based on the most recent data from the EPA's AQS from 2009 to 2011 in the 331 CBSAs in which valid PM 2.5 data are available, as discussed in Frank (2012a, Table 5), there were 208 such areas (with design values ranging up to about 15 μg/m [3] ). Frank (2012a) also observed that other areas have multiple monitors with composite and maximum monitor mean PM 2.5 concentrations that were the same or relatively close, with 57 areas in which the maximum monitor mean concentration was no more than 0.5 μg/m [3] higher than the composite monitor mean concentration and 56 areas in which the difference was between 0.6 and 2 μg/m [3] . Further, there were only a few other areas in which the maximum monitor mean concentration was appreciably higher than the composite monitor mean concentration, such as areas in which some monitors may be separately impacted by local sources. There were only 10 such areas in the country in which the maximum monitor mean concentration was between 2 to 6 μg/m [3] higher than the composite monitor concentration (Frank, 2012a, Table 4). [89] Thus, the EPA does not agree that there is a significant difference between composite monitor mean PM 2.5 concentrations and maximum monitor mean PM 2.5 concentrations in the large majority of areas across the country.

In proposing to revise the form of the annual PM 2.5 standard, as discussed above in section III.E.3.a, the EPA noted that when an annual PM 2.5 standard was first set in 1997, the form of the standard included the option for averaging across measurements at appropriate monitoring sites within an area, generally consistent with the composite monitor approach used in epidemiological studies, with some constraints intended to ensure that spatial averaging would not result in inequities in the level of protection for communities within large metropolitan areas. In the last review the EPA tightened the constraints on spatial averaging, and in this review has eliminated the option altogether, on the basis of analyses in each review that showed that such constraints may be inadequate to avoid substantially greater exposures for people living in locations around the monitors recording the highest PM 2.5 concentrations in some areas, potentially resulting in disproportionate impacts on at-risk populations of persons with lower SES levels as well as minorities. In light of these analyses, and consistent with the Administrator's decision to revise the form of the annual PM 2.5 standard by eliminating the option for spatial averaging, the EPA continues to conclude that a standard level based on consideration of long-term mean concentrations from composite monitors, and applied at each monitor within an area including the monitor measuring the highest concentration, is the appropriate approach to use in setting a standard that will protect public health, including the health of at-risk populations, with an adequate margin of safety, as required by the CAA.

The EPA acknowledges that at proposal, the Agency characterized the approach of using maximum monitor concentrations to determine compliance with the standard, while selecting the standard level based on consideration of composite monitor concentrations, as one that inherently had the potential to build in a margin of safety (77 FR 38905), and CASAC reiterated that view in supporting the EPA's approach (Samet, 2010d, p. 3). Nonetheless, in light of the discussion above, the EPA more specifically recognizes that this approach does not build in any margin of safety in the large number of areas across the country with only one monitor. Further, based on the analyses done to inform consideration of the form of the standard (Schmidt, 2011, Analysis A), the EPA concludes that this approach does not provide a margin of safety for the at-risk populations that live around the monitor measuring the highest concentration, such as in those few areas in which the maximum monitor concentration is appreciably higher than the composite monitor concentration. Rather, this approach properly treats those at-risk populations the same way it does the broader populations that live in areas with only one monitor, by providing the same degree of protection for those at-risk populations that would otherwise be disproportionately impacted as it does for the broader populations in other areas, While the EPA recognizes that this approach can result in some additional margin of safety for the subset of areas with multiple monitors in which at-risk populations may not be disproportionately represented in areas around the maximum monitor, which may be the case in areas with relatively small differences between the maximum and composite monitor concentrations, the EPA notes that this margin would be relatively small in such areas.

Based on the above considerations, the EPA does not agree that the Agency's approach of using maximum monitor concentrations to determine compliance with the standard, while selecting the standard level based on consideration of composite monitor concentrations creates an unwarranted case for lowering the standard level based on a margin of safety that would be arbitrary, not based on evidence, or lack quantification. The EPA recognizes that setting a standard to protect public health, including the health of at-risk populations, with an adequate margin of safety, depends upon selecting a standard level sufficiently below where the EPA has found the strongest evidence of health effects so as to provide such protection, and that the EPA's approach regarding consideration of composite and maximum monitor concentrations is intended to, and does, serve to address this requirement as part of and not separate from the selection of an appropriate standard level based on the health effects evidence.

In considering the second issue related to the commenter's suggested alternative approach, the EPA strongly disagrees with the commenter's view that a more “reasoned and consistent approach would be to decide on a mean composite monitor PM 2.5 level that should be achieved and then identify the maximum monitor level that would result in that composite value” (UARG, 2012, Attachment 1, p. 4). As discussed above, the EPA notes that for areas with only one monitor, or with multiple monitors that measure concentrations that are very close in magnitude, the maximum monitor level that would limit the composite monitor PM 2.5 level to be no greater than the level that should be achieved to protect public health with an adequate margin of safety, would essentially be the same as that composite monitor level. Further, as discussed above, even for areas in which the maximum monitor concentration is appreciably higher than other monitor concentrations within the same area, public health would not be protected with an adequate margin of safety if the disproportionately higher exposures of at-risk, susceptible populations around the monitor measuring the highest concentration were in essence averaged away with measurements from monitors in other locations within large urban areas. Further, the commenter's suggested approach would be based on annual average PM 2.5 concentrations that have been measured over some past time period. Such an approach would reflect the air quality that existed in the past, but it would not necessarily provide appropriate constraints on the range of concentrations that would be allowed by such a standard in the future, when relationships between maximum and composite monitor concentrations in areas across the country may be different. For these reasons, the EPA fundamentally rejects the commenter's suggested approach because in the EPA's view it would not protect public health, including providing protection for at-risk populations, with an adequate margin of safety in areas across the country.

More specifically, in further considering the commenter's analysis of design values based on maximum versus composite monitor annual mean PM 2.5 concentrations using monitoring data from 2006 to 2008 which they assert supports retaining the current standard level of 15 μg/m [3] , the EPA finds flaws with the numerical results and the scope of the analysis, as well as flaws in the commenter's translation of the analysis results into the basis for selecting an annual standard level.

In considering the commenter's analysis, the EPA notes that the analysis compared maximum versus composite monitor annual mean PM 2.5 concentrations, averaged over 3 years, for two groups of areas: (1) Areas with design values that exceed the current annual standard level (i.e., greater than 15.0 μg/m [3] ) and (2) areas with design values that are just attaining the current annual standard (i.e., between 14.5 and 15.0 μg/m [3] ). [90] The commenter indicated that they used the full body of PM 2.5 monitoring data from the EPA's AQS database (UARG, 2012, Attachment 1, p. 4), In attempting to reproduce the commenter's results, the EPA repeated the calculations using only valid air quality data (i.e., data that meet data completeness and monitor siting criteria) from the AQS database for the same time period (Frank, 2012a). [91] Based on this corrected analysis, the EPA finds that the composite monitor concentrations averaged across the areas within each group are somewhat higher than those calculated by the commenter, and the average differences between the maximum and composite monitor concentrations are somewhat smaller (Frank, 2012a, Table 3). [92] Notably, the difference between the maximum and composite monitor average concentrations for the second group of areas is substantially reduced in the corrected analysis, such that the difference (averaged across the 10 areas with valid data in the second group) is approximately 0.5 μg/m [3] , not 1.2 μg/m [3] as in the commenter's analysis. In addition, the commenter's analysis compared the average of the composite monitors to the average of the maximum monitors for each subset of areas. This comparison of averages across all the areas in each subset masks the fact that the large majority of areas across the country have only one monitor, with the composite monitor and maximum monitor values the same for such areas, and many other areas have a maximum monitor value that is close to the composite monitor value. As discussed above, these circumstances have a major impact on the protection that would be achieved by the approach suggested by the commenter.

With regard to the scope of the commenter's analysis, the EPA finds that by limiting the scope to a small subset of areas with design values above or just below the current annual standard level of 15 μg/m [3] , the analysis ignores the large number of areas across the country with lower design values that are relevant to consider in light of the epidemiological evidence of serious health effects at lower concentrations, well below the level of the current standard.

In translating the analysis results into the basis for selecting an annual standard level, the commenter's translation is premised on the view that the “natural focal point” for setting an annual PM 2.5 standard level should be somewhere within the range of the long-term mean PM 2.5 concentrations from the subset of epidemiological studies that included only long-term exposure studies of effects for which the evidence is categorized as causal or likely causal, but not for effects categorized as suggestive of causality, nor did it include short-term exposure studies (which are included in Figure 4 of the proposal notice and above). Such a view is not consistent with setting a standard that would provide sufficient protection from the serious health effects reported even in the limited subset of studies considered by the commenter, including protecting public health with an adequate margin of safety. As discussed below, the EPA does not agree with the commenter's view as to the appropriate focal point for selecting the level of an annual PM 2.5 standard, or with the limited set of studies considered by the commenter as a basis for selecting the level of the annual PM 2.5 standard.

Regarding an appropriate focal point for selecting the level of the annual standard, as discussed in the proposal and as advised by CASAC, the EPA has focused on PM 2.5 concentrations somewhat below the lowest long-term mean concentrations from each of the key studies of both long- and short-term exposures of effects for which the evidence is causal or likely causal, as considered by the EPA (i.e., the first two sets of studies shown in Figure 4). If the level of the annual standard was set just somewhere within the range of the long-term mean concentrations from the various long-term exposure studies, then one or more of the studies would have a long-term mean concentration below the selected level of the standard. Absent some reason to ignore or discount these studies, which the commenter does not provide (and of which the EPA is unaware), setting such a standard would allow that level of air quality, where the evidence of health effects is strongest, and its associated risk of PM 2.5-related mortality and/or morbidity effects to continue. Selecting such a standard level could not be considered sufficient to protect the public health with an adequate margin of safety.

Further, focusing on just the long-term mean PM 2.5 concentrations in the key epidemiological studies—even the lowest long-term mean concentration from the set of key studies—is not appropriate. Concentrations at and around the long-term mean concentrations represent the part of the air quality distribution where the data in any given study are most concentrated and, thus, where the confidence in the magnitude and significance of an association in such study is strongest. However, the evidence of an association with adverse health effects in the studies is not limited to the PM 2.5 concentrations just at and around the long-term mean, but rather extends more broadly to a lower part of the distribution, recognizing that no discernible population-level threshold for any such effects can be identified based on the available evidence. This broader region of the distribution of PM 2.5 concentrations should be considered to the extent relevant information is available, recognizing that the degree of confidence in the association identified in a study would become lower as one moves below concentrations at and around the long-term mean concentration in any given study. The commenter's approach ignores this fundamental consideration.

Regarding the set of studies that is appropriate to inform the selection of the level of the annual PM 2.5 standard, the EPA finds that limiting consideration only to the long-term exposure studies, as this commenter suggests, would be tantamount to ignoring the short-term exposure studies, [93] which provide some of the strongest evidence from the entire body of epidemiological studies. Thus, selecting an annual standard level using the limited set of studies suggested by the commenter would fail to provide a degree of protection that would be sufficient to protect public health with an adequate margin of safety.

For all the reasons discussed above, the EPA finds the commenter's concerns with the EPA's approach to considering composite and maximum monitor PM 2.5 concentrations in selecting the level of the annual PM 2.5 standard to be without merit. Further, the EPA finds no support in the commenter's analysis for their suggested alternative approach.

(2) With respect to the appropriate exposure period for mortality effects observed in long-term exposure studies, some commenters in this group generally expressed views consistent with comments from UARG that argued that these studies “are most likely detecting health risk from earlier, higher PM 2.5 levels and misattributing those risks to more recent, lower PM 2.5 levels” (UARG, 2012, Attachment 1, p 7). Further, this commenter asserted that “there is no knowledge or evidence indicating whether premature deaths are the result of PM 2.5 exposures in the most recent year; or due to physical damages incurred from PM 2.5 exposures much earlier in life (with the impact on lifespan only emerging later in life); or due to total accumulated PM 2.5 exposure over many years.”Id. In addition, the commenter asserted that the long-term exposure studies of mortality are central to the EPA's basis for proposing to set a lower annual standard level, since most of the estimated benefits associated with a lower annual PM 2.5 standard are based on reductions in mortality related to long-term exposures to PM 2.5.

As an initial matter, the EPA has recognized the challenge in distinguishing between PM 2.5-associated effects due to past and recent long-term exposures, and in identifying the relevant latency period for long-term exposure to PM and resultant health effects (U.S. EPA, 2009a, section 7.6.4; 77 FR 38941/1). While the EPA has acknowledged that there remain important uncertainties related to characterizing the most relevant exposure periods in long-term exposure studies, the assertion that there is “no knowledge or evidence” that helps to inform this issue is not correct, as discussed below.

Both in the last review and in the current review, the EPA has assessed studies that used different air quality periods for estimating long-term exposure and tested associations with mortality for the different exposure periods (U.S. EPA, 2004, section 8.2.3.5; U.S. EPA 2009a, section 7.6.4). In this review, the Integrated Science Assessment discussed studies available since the last review that have assessed the relationship between long-term exposure to PM 2.5 and mortality to explore the issue of the latency period between exposure to PM 2.5 and death (U.S. EPA, 2009a, section 7.6.4).

Notably, in a recent analysis of the extended Harvard Six Cities Study, Schwartz et al. (2008) used model averaging (i.e., multiple models were averaged and weighted by probability of accuracy) to assess exposure periods prospectively (77 FR 38907/1-2). The exposure periods were estimated across a range of unconstrained distributed lag models (i.e., same year, one year prior, two years prior to death). In comparing lags, the authors reported that the effects of changes in exposure to PM 2.5 on mortality were strongest within a 2-year period prior to death (U.S. EPA, 2009a, p. 7-92, Figure 7-9). Similarly, a large multi-city study of the elderly found that the mortality risk associated with long-term exposure to PM 10 reported cumulative effects that extended over the years that deaths were observed in the study population (i.e., the follow-up period) and for the 3-year period prior to death (Zanobetti et al., 2008).

Further, in a study of two locations that experienced an abrupt decline in PM 2.5 concentrations (i.e., Utah Steel Strike, coal ban in Ireland), Röösli et al. (2005) reported that approximately 75 percent of health benefits were observed in the first 5 years (U.S. EPA, 2009a, Table 7-9). Schwartz et al. (2008) and Puett et al. (2008) found, in a comparison of exposure periods ranging from 1 month to 48 months prior to death, that exposure to PM 10 24 months prior to death exhibited the strongest association, and the weakest association was reported for exposure in the time period of 1 month prior to death.

Overall, the EPA notes that the available evidence for determining the exposure period that is causally related to the mortality effects of long-term PM 2.5 exposures, as discussed above, cannot specifically disentangle the effects observed in long-term exposure studies associated with more recent air quality measurements from effects that may have been associated with earlier, and most likely higher, PM 2.5 exposures. While the evidence suggests that a latency period of up to five years would account for the majority of deaths, it does not provide a basis for concluding that it is solely recent PM 2.5 concentrations that account for the mortality risk observed in such studies. Nonetheless, the more recent air quality data does well at explaining the relationships observed between long-term exposures to PM 2.5 and mortality, with the strongest association observed in the two years prior to death. Further, the EPA recognizes that there is no discernible population-level threshold below which effects would not occur, such that it is reasonable to consider that health effects may occur over the full range of concentrations observed in the epidemiological studies, including the lower concentrations in the latter years.

In light of this evidence and these considerations, the EPA concludes that it is appropriate to consider air quality concentrations that are generally contemporaneous with the collection of health event data (i.e., collected over the same time period) as being causally associated with at least some proportion of the deaths assessed in a long-term exposure study. This would include long-term mean PM 2.5 concentrations from most of the key long-term exposure studies of effects with causal or likely causal evidence shown in Figure 4 above, which reported long-term mean PM 2.5 concentrations ranging from 13.6 µg/m [3] to 14.3 µg/m [3] . These studies include studies of mortality by Eftim et al. (2008), which separately analyzed the ACS and Harvard Six City sites, Zeger et al. (2008), and Lipfert et al. (2006a), as well as studies of morbidity endpoints by Goss et al. (2004), McConnell et al. (2003) and Gauderman et al. (2004), and Dockery et al. (1996) and Razienne et al. (1996). The EPA acknowledges that uncertainty in the relevant exposure period is most notable in two other long-term exposure studies of mortality. The Miller et al. (2007) reported a long-term mean PM 2.5 concentration for a 1-year exposure period that post-dated the follow-up period in which health event data were collected by two years. Also, the Krewski et al. (2009) study reported a long-term mean PM 2.5 concentration for an exposure period that included only the last two years of the 18-year follow-up period. Based on these considerations, the EPA does not now consider it appropriate to put weight on the reported long-term mean concentrations from these two studies for the purpose of translating the information from the long-term mortality studies into a basis for selecting the level of the annual PM 2.5 standard. [94]

In addition, the EPA acknowledges that exposure periods that extend at least a couple years prior to the follow-up period in which health event data were collected would likely more fully capture the PM-related deaths in such studies. To explore how much higher the long-term mean PM 2.5 concentrations would likely have been had air quality data prior to the follow-up years of the studies been included, the EPA conducted a sensitivity analysis of long-term mean PM 2.5 concentrations (Schmidt, 2012a) particularly considering studies that only included deaths from a relatively recent follow-up period. As examples of such studies, this analysis considered the Eftim et al. (2008) study of mortality in the ACS sites and the Harvard Six Cities sites, as well as sites in the eastern region in the Zeger et al. (2008) study. Using data from the EPA's AQS database, the analysis added the two years of air quality data just prior to the follow-up period in each study, which was 2000 to 2002 in Eftim et al. (2008) and 2000 to 2005 in Zeger et al. (2008). The analysis then calculated the extended long-term mean PM 2.5 concentration for each study. As discussed in Schmidt (2012a), in each case the long-term mean PM 2.5 concentration averaged over the extended exposure period was less than 0.4 µg/m [3] higher than the long-term mean PM 2.5 concentration averaged over the follow-up period. The EPA finds it reasonable to conclude that such a relatively small difference in long-term mean PM 2.5 concentrations would likely apply for other long-term exposure studies that used similarly recent follow-up periods as well (e.g., Goss et al., 2004; Lipfert et al., 2006a).

Based on the above considerations, the EPA concludes that it is appropriate to consider the available air quality information from the long-term exposure studies, while taking into account the uncertainties in the relevant long-term exposure periods in weighing the information from these studies. The EPA recognizes that considering such information in selecting an appropriate annual standard level has the potential to build in some margin of safety. The EPA further concludes that it is appropriate to consider the air quality information from the set of long-term exposure studies discussed above in the context of the broader array of epidemiological studies that inform the EPA's consideration of the level of the annual PM 2.5 standard.

The EPA also notes that while the long-term exposure studies are an important component of the epidemiological evidence that informs the Agency's consideration of the level of the annual standard, they do not provide the only relevant information, nor are they the set of studies for which the relevant long-term mean PM 2.5 concentrations are the lowest. As discussed in the proposal, the EPA also considers the long-term mean PM 2.5 concentrations from the short-term mortality and morbidity studies as providing important information in considering the level of the annual standard. As discussed above, a large proportion of the aggregate risk associated with short-term exposures results from the large number of days during which the 24-hour average concentrations are in the low- to mid-range of the concentrations observed in the studies. Thus, setting the level of the annual standard based on long-term mean concentrations, as well as the distribution of concentrations below the mean, in the short-term exposure studies is the most effective and efficient way to reduce total PM 2.5- related risk from the broad array of mortality and morbidity effects associated with short-term exposures.

Further, the EPA notes that the relevant exposure period for the short-term exposure studies is the period contemporaneous with the collection of health event data, and that this exposure period is not subject to the uncertainties discussed above related to the long-term exposure studies. Recognizing that the long-term mean PM 2.5 concentrations from several of the multi-city short-term exposure studies shown in Figure 4 are below the long-term mean PM 2.5 concentrations from the long-term exposure studies (with the exception of Miller et al., 2007). [95] It is reasonable that in selecting the level of the annual standard primary consideration should be given to the information from this set of short-term exposure studies. There is no reasonable basis to discount the long-term mean concentrations of the short-term exposure studies for purposes of setting the level of the annual standard. Thus, the commenter is incorrect in asserting that the long-term exposure studies, not the short-term exposure studies, would be central in the Administrator's decision on the level of the annual standard. The standard is ultimately intended to protect not just against the single type of effect that contributes the most to quantitative estimates of risk to public health, but rather to the broad array of effects, including mortality and morbidity effects from long- and short-term exposures across the range of at-risk populations impacted by PM 2.5-related effects.

(3) With regard to the EPA's analysis of distributions of underlying population-level data (i.e., health event and study population data) and corresponding air quality data from each study area in certain key multi-city epidemiological studies (Rajan et al., 2011), some commenters in this group raised a number of issues related to this analysis (API, 2012, Attachment 1 pp. 5 to 6; McClellan, 2012, pp.2 to 4). Some commenters noted the limited number of studies for which health event and study population data were available, and questioned whether these distributions would apply to other studies. Commenters expressed concerns that this analysis had not been formally reviewed by CASAC and was not published in the peer-review literature. Based on such concerns, some commenters asserted that the EPA should not consider this information as a basis for selecting a standard level.

As an initial matter, as discussed in section III.E.4.b above, the EPA agrees with CASAC's advice that it is appropriate to consider additional data beyond the mean PM 2.5 concentrations in key multi-city studies to help inform selection of the level of the annual PM 2.5 standard. As both the EPA and CASAC recognize, in the absence of a discernible threshold, health effects may occur over the full range of concentrations observed in the epidemiological studies. Nonetheless, the EPA recognizes that confidence in the magnitude and significance of an association is highest at and around the long-term mean PM 2.5 concentrations reported in the studies and the degree of confidence becomes lower at lower concentrations within any given study. Following CASAC's advice (Samet, 2010d, p.2), the EPA used additional population-level and air quality data made available by study authors to conduct an analysis of the distributions of such data, to help inform consideration of how the degree of confidence in the magnitude and significance of observed associations varies across the range of long-term mean PM 2.5 concentrations in study areas within key multi-city epidemiological studies. In the EPA's view, such consideration is important in selecting a level for an annual standard that will protect public health with an adequate margin of safety.

With regard to the number of multi-city studies for which an analysis of the distributions of population-level data across the study areas and the corresponding annual mean PM 2.5 concentrations was done, the EPA noted at proposal that data for such an analysis were made available from study authors for four studies, including two long-term exposure studies and two short-term exposure studies. [96] The EPA recognized that access to health event data can be restricted due to confidentiality issues, such that it is not reasonable to expect that such information could be made available from all studies. In considering the information from these four studies, the EPA has further taken into consideration uncertainties discussed in response to the above comment related to the appropriate exposure period for long-term exposure studies. Based on these considerations, as noted above, the EPA concludes that such uncertainties are an important factor in evaluating the usefulness of the air quality information from the two long-term exposure studies in this analysis (Krewski et al., 2009; Miller et al., 2007) and that it would not be appropriate to place weight on the distributional analysis of health event and air quality data from these two studies specifically for the purpose of translating the information from the long-term mortality studies into a basis for selecting the level of the annual PM 2.5 standard. Such uncertainties are not relevant to the short-term exposure studies, and thus, the Agency focuses on the two short-term exposure studies in this analysis (Bell et al., 2008; Zanobetti and Schwartz, (2009).

In focusing on these two short-term exposure studies, the EPA first notes that these studies are key multi-city studies that reported positive and statistically significant associations between mortality and cardiovascular-related hospital admissions across a large number of areas throughout the U.S. (112 U.S. cities in Zanobetti and Schwartz, 2009; 202 U.S. counties in Bell et al., 2008) using relatively recent air quality and health event data (i.e., 1999 through 2005 in both studies). The EPA considers this to be a modest but important data set to use for this distributional analysis to help inform consideration of how much below the long-term mean PM 2.5 concentrations in key multi-city long- and short-term exposure studies the annual PM 2.5 standard level should be set. While the EPA acknowledges that having such data available from more studies would have been useful, the Agency finds the information from this limited set of studies to be an important consideration in selecting an annual standard level, consistent with CASAC advice to consider such information.

In considering the results of this distributional analysis, as discussed more fully in the Response to Comments document, the EPA considers PM 2.5 concentrations between the 25th and 10th percentiles of the distribution of health events to be a reasonable range for providing a general frame of reference for that part of the distribution in which confidence in the magnitude and significance of the association may be appreciably lower than confidence at and around the long-term mean concentration. For the two short-term exposure studies included in this analysis, the EPA notes that the PM 2.5 concentrations corresponding to the 25th percentiles of the distributions of health events were 12.5 µg/m [3] and 11.5 µg/m [3] , respectively, for Zanobetti and Schwartz (2009) and for Bell et al. (2008), with the 10th percentiles being lower by approximately 2 µg/m [3] in each study (Rajan et al., 2011, Table 1). In considering this information, the EPA recognizes, however, that there is no clear dividing line or single percentile within a given distribution (including both above and below the 25th percentile) provided by the scientific evidence that is most appropriate or `correct' to use to characterize where the degree of confidence in the associations warrants setting the annual standard level. The decision as to the appropriate standard level below the long-term mean concentrations of the key studies is largely a public health policy judgment to be made by the Administrator, taking into account all of the evidence and its related uncertainties, as discussed in section III.E.4.d below.

In response to concerns that this analysis was not reviewed by CASAC nor published in the peer-reviewed literature, the EPA notes that this analysis was conducted to directly respond to advice from CASAC, as discussed in section III.E.4.b.i above, in conjunction with their review of the Policy Assessment. The EPA notes that the same type of distributional analysis was presented in the second draft Policy Assessment based on air quality data, as well as population-weighted air quality data, rather than health event or study population data. In considering that distributional information, CASAC urged that the EPA redo the analysis using health event or study population data, which is exactly what the EPA did and presented in the final Policy Assessment. The EPA provided CASAC with the final Policy Assessment and communicated how the final staff conclusions reflected consideration of its advice and that those staff conclusions were based in part on the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 2011, Attachment p. 2). CASAC did not choose to provide any additional comments or advice after receiving the final Policy Assessment. The EPA considers this distributional analysis to be the product of the peer review conducted by CASAC of the Policy Assessment, and thus does not agree with commenters' characterization that the analysis lacked appropriate peer review. The EPA's final analysis was based on the comments provided by CASAC, the peer review committee established pursuant to the CAA, on the draft analysis, such that the final analysis stems directly from CASAC's advice and the EPA's response to its comments.

Based on the above considerations, the EPA continues to conclude that its analysis of distributions of health event and air quality data from two key multi-city epidemiological studies provides important information related to understanding the associations between health events observed in each city (e.g., deaths, hospitalizations) and the corresponding long-term mean PM 2.5 concentrations observed in the studies. While recognizing that this is a relatively modest data set, the EPA further concludes that such information can appropriately help to inform the selection of the level of an annual standard that will protect public health with an adequate margin of safety from these types of health effects which are causally related to long- and short-term exposures to PM 2.5.

(4) Some commenters in this group asserted there were limitations in the long-term exposure studies of morbidity, including studies evaluating respiratory effects in children. For example, one commenter (UARG, 2012, p. 12, Attachment 1, pp. 14 to 16) asserted there were serious limitations in the long-term exposure studies of respiratory morbidity in each of the studies considered by the EPA (including McConnell et al., 2003; Gauderman et al., 2004; Dockery et al., 1996; Raizenne et al., 1996; and Goss et al., 2004) and argued that this evidence provides only a “weak association” with PM 2.5 exposures. This commenter asserted that many of these long-term exposure studies evaluating respiratory effects were considered at the time the EPA reaffirmed the current annual standard level of 15 µg/m [3] in 2006, that the Administrator in the last review determined that the information they provided “was too limited to serve as the basis for setting a level of a national standard,” and that they should be given little weight in setting the level of the annual standard in this review (UARG, 2012, Attachment 1, p. 14).

More specifically, this commenter asserted that the McConnell et al. (2003) and Gauderman et al. (2004) studies reported mixed results for associations with PM 2.5 and stronger associations with NO 2 (API, 2012, Attachment 1, pp. 14 to 15). Similarly, this commenter argued that the Dockery et al. (1996) and Raizenne et al. (1996) studies showed stronger associations with acidity than with fine particles (measured as PM 2.1). Id. pp. 15 to 16. With regard to the cystic fibrosis study, this commenter noted that the association between pulmonary exacerbations and PM 2.5 in this study was no longer statistically significant when the model adjusted for each individual's baseline lung function. The commenters referred to the data on lung function as an “important explanatory variable,” and suggested that the EPA should rely on results from the model that included individual baseline lung function information. Id. p. 16. For the reasons discussed below and in more detail in the Response to Comments document, the EPA disagrees with the commenters' interpretation of these studies.

As an initial matter, the EPA notes that three of these studies (McConnell et al., 2003; Dockery et al., 1996; Raizenne et al., 1996) as well as the initial studies from the Southern California Children's Health Study (Peters et al., 1999; McConnell et al., 1999; Gauderman et al., 2000, 2002; Avol et al., 2001) were discussed and considered in the 2004 Air Quality Criteria Document (U.S. EPA, 2004) and, thus, considered within the air quality criteria supporting the EPA's final decisions in the review completed in 2006. Two additional studies (Gauderman et al., 2004; Goss et al., 2004) were discussed and considered in the provisional science assessment conducted for the last review (U.S. EPA, 2006a). The EPA concluded that “new” studies considered in the provisional assessment completed in 2006 did not materially change any of the broad scientific conclusions regarding the health effects of PM exposure made in the Criteria Document (71 FR 61148 to 61149, October 17, 2006). All of these studies were considered in the Integrated Science Assessment that informs the current review (U.S. EPA, 2009a).

With regard to the Southern California Children's Health Study, extended analyses considered in the Integrated Science Assessment provided evidence that clinically important deficits in lung function [97] associated with long-term exposure to PM 2.5 persist into early adulthood (U.S. EPA, 2009a, p. 7-27; Gauderman et al., 2004). These effects remained positive in copollutant models. [98] Additional analyses of the Southern California Children's Health Study cohort reported an association between long-term PM 2.5 exposure and bronchitic symptoms (U.S. EPA, 2009a, p. 7-23 to 7-24; McConnell et al., 2003, long-term mean concentration of 13.8 µg/m [3] ) that remained positive in co-pollutant models, with the PM 2.5 effect estimates increasing in magnitude in some models and decreasing in others, and a strong modifying effect of PM 2.5 on the association between lung function and asthma incidence (U.S. EPA, 2009a, 7-24; Islam et al., 2007). The outcomes observed in the more recent reports from the Southern California Children's Health Study, including evaluation of a broader range of endpoints and longer follow-up periods, were larger in magnitude and more precise than reported in the initial version of the study. Supporting these results were new longitudinal cohort studies conducted by other researchers in varying locations using different methods (U.S. EPA, 2009a, section 7.3.9.1). The EPA, therefore, disagrees with the commenters that the studies by McConnell et al. (2003) and Gauderman et al. (2004) are flawed and should not be used in the PM NAAQS review process.

The 24-City study [99] by Dockery et al. (1996) (long-term mean concentration of 14.5 µg/m [3] ) was considered in the current as well as two previous reviews (U.S. EPA, 2009a; U.S. EPA, 2004; U.S. EPA, 1996). This study observed that PM, specifically “particle strong acidity” and sulfate particles (indicators of fine particles), were associated with reports of bronchitis in the previous year. Similarly, the magnitude of the associations between bronchitis and PM 10 and PM 2.1 were similar to those for acidic aerosols and sulfate particles, though the confidence intervals for the PM 10 and PM 2.1 associations were slightly wider and the associations were not statistically significant. Acid aerosols, sulfate, and fine particles are formed in secondary reactions of the emissions from incomplete combustion and these pollutants have similar regional and temporal distributions. As noted by the study authors, “the strong correlations of several pollutants in this study, especially particle strong acidity with sulfate (r=0.90) and PM 2.1 (r=0.82), make it difficult to distinguish the agent of interest” (Dockery et al., 1996, p. 505). Overall, Dockery et al. (1996) (and, similarly, Raizenne et al., 1996) observed similar associations between respiratory health effects and acid aerosols, sulfate, PM 10 and PM 2.1 concentrations. The commenters noted that the associations with particle acidity were sensitive to the inclusion of the six Canadian sites. The EPA notes that none of these Canadian cities were in the “sulfate belt” where particle strong acidity was highest. Thus, the change in the effect estimate when the six Canadian cities were excluded from the analysis is likely due to the lower prevalence of bronchitis and the lower concentrations of acid aerosols in these cities, and not due to some difference in susceptibility to bronchitis between the U.S. and Canadian populations that is not due to air pollution, as suggested by the commenters (UARG, 2012, Attachment 1, p. 15). In fact, contrary to the statements made by the commenters, the authors did not observe any subgroups that appeared to be markedly more susceptible to the risk of bronchitis.

The Goss et al. (2004) study considered a U.S. cohort of cystic fibrosis patients and provided evidence of association between long-term PM 2.5 exposures and exacerbations of respiratory symptoms resulting in hospital admissions or use of home intravenous antibiotics (U.S. EPA, 2009a, p. 7-25; long-term mean concentration of 13.7 µg/m [3] ). The commenters noted that the association between pulmonary exacerbations and PM 2.5 in this study was no longer statistically significant when the model adjusted for each individual's baseline lung function. The commenters referred to the data on lung function as an “important explanatory variable,” and suggested that the EPA should rely on results from the model that included individual baseline lung function information. The EPA disagrees with the commenters' interpretation of this study. The Agency concludes it is unlikely that lung function is a potential confounder or an important explanatory variable in this study. In fact, the authors noted that “it is more likely that lung function decline may be intimately associated with chronic exposure to air pollutants and may be part of the causal pathway in worsening prognosis in CF [cystic fibrosis]; in support of this explanation, we found both cross-sectional and longitudinal strong inverse relationships between FEV 1 and PM levels” (Goss et al., 2004, p. 819). The EPA notes that adjusting for a variable that is on the causal pathway can lead to overadjustment bias, which is likely to attenuate the association (Schisterman et al. 2009); this is likely what was observed by the authors. Thus, the EPA continues to believe it is appropriate to focus on the results reported in Goss et al. (2004) that did not include individual baseline lung function in the model.

In addition, the EPA disagrees with commenters' reliance solely on statistical significance when interpreting the study results from individual study results and the collective evidence across studies. As discussed in section III.D.2 above, statistical significance of individual study findings has played an important role in the EPA's evaluation of the study's results and the EPA has placed greater emphasis on studies reporting statistically significant results. However, in the broader evaluation of the evidence from many epidemiological studies, and subsequently during the process of forming causality determinations in the Integrated Science Assessment by integrating evidence from across epidemiological, controlled human exposure, and toxicological studies, the EPA has emphasized the pattern of results across epidemiological studies and whether the effects observed were coherent across the scientific disciplines for drawing conclusions on the relationship between PM 2.5 and different health outcomes.

As noted in section III.B.1.a of the proposal, with regard to respiratory effects, the Integrated Science Assessment concluded that extended analyses of studies available in the last review as well as new epidemiological studies conducted in the U.S. and abroad provided stronger evidence of respiratory-related morbidity associated with long-term PM 2.5 exposure (77 FR 38918). The strongest evidence for respiratory-related effects available in this review was from epidemiological studies that evaluated decrements in lung function growth in children and increased respiratory symptoms and disease incidence in adults (U.S. EPA, 2009a, sections 2.3.1.2, 7.3.1.1, and 7.3.2.1).

In considering the collective evidence from epidemiological, toxicological, and controlled human exposure studies, including the studies discussed above, the EPA recognizes that the Integrated Science Assessment concluded that a causal relationship is likely to exist between long-term PM 2.5 exposures and respiratory effects (U.S. EPA, 2009a, p. 2-12, pp. 7-42 to 7-43). CASAC concurred with this causality determination (Samet, 2009f, p.9).

The commenter's assertion that the EPA should adhere to its assessment of these studies as it did in the review completed in 2006 is significantly mistaken. Most obviously, the EPA's final decision in the last review was held to be deficient by the DC Circuit in remanding the 2006 primary annual PM 2.5 standard. As discussed in section III.A.2 above, the DC Circuit specifically held that the EPA did not provide a reasonable explanation of why certain morbidity studies, including an earlier study from the Southern California Children's Health Study (Gauderman et al., 2000, long-term mean PM 2.5 concentration approximately 15 μg/m [3] ) and the 24-Cities Study (Raizenne et al., 1996, long-term mean concentrations approximately 14.5 µg/m [3] ) did not warrant a more stringent annual PM 2.5 standard when the long-term mean PM 2.5 concentrations reported in those studies were at or lower than the level of the annual standard. American Farm Bureau Federation v. EPA. 559 F. 3d at 525. Indeed, the court found that, viewed together, the Gauderman et al. (2000) and Raizenne et al., (1996) studies “are related and together indicate a significant public health risk * * * On this record, therefore, it appears the EPA too hastily discounted the Gauderman and 24-Cities studies as lacking in significance.”Id.

In this review, the EPA recognizes a significant amount of evidence beyond these two studies that expands our understanding of respiratory effects associated with long-term PM 2.5 exposures. This body of scientific evidence includes an extended and new analyses from the Southern California Children's Health Study (Gauderman et al., 2004; Islam et al., 2007; Stanojevic et al., 2008) as well as additional studies that examined these health effects (Kim et al., 2004; Goss et al., 2004). Thus, even more so than in the last review, the evidence indicates a “significant public health risk” to children from long-term PM 2.5 exposures at concentrations below the level of the current annual standard. A standard that does not reflect appropriate consideration of this evidence would not be requisite to protect public health with an adequate margin of safety.

(5) With regard to the use of studies of health effects for which the EPA finds the evidence to be “suggestive” of a causal relationship, some commenters argued that such studies “do not merit any weight in the setting of the annual NAAQS” (e.g., UARG, 2012, Appendix 1, p. 3).

The EPA disagrees with the commenter's view that studies of health effects for which the evidence is suggestive of a causal relationship, rather than studies of health effects for which the evidence supports a causal or likely causal relationship, merit no weight at all in setting the NAAQS. To place no weight at all on such evidence would in essence treat such evidence as though it had been categorized as “not likely to be a causal relationship.” To do so would ignore the important distinctions in the nature of the evidence supporting these different causality determinations in the Integrated Science Assessment. It would also ignore the CAA requirement that primary standards are to be set to provide protection with an adequate margin of safety, including providing protection for at-risk populations. Thus, ignoring this information in making decisions on the appropriate standard level would not be appropriate. [100] Nonetheless, in considering studies of health effects for which the evidence is suggestive of a causal relationship, the EPA does believe that it is appropriate to place less weight on such studies than on studies of health effects for which there is evidence of a causal or likely causal relationship.

A second group of commenters supported revising the suite of primary PM 2.5 standards to provide increased public health protection. These commenters found the available scientific information and technical analyses to be stronger and more compelling than in the last review. These commenters generally placed substantial weight on CASAC advice and on the EPA staff analyses presented in the final Policy Assessment, which concluded that the evidence most strongly supported an annual standard level within a range of 11 to 12 μg/m [3] (U.S. EPA, 2011a, p. 2-206). While some of these commenters felt that the level should be set within the proposed range (12 to 13 μg/m [3] ), most of these commenters advocated a level of 11 μg/m [3] . [101] For example, ALA et al., asserted:

The EPA's proposed PM 2.5 standards, while a step in the right direction are insufficient to protect public health, including the health of susceptible populations, with an adequate margin of safety as required by the Clean Air Act * * *we will discuss the enormous gap in public health protection afforded by an annual standard of 13 µg/m [3] , at the upper end of the proposed range, compared to the more protective 11 µg/m [3] , as advocated by our organizations (ALA et al., 2012, p. 6).

In general, these commenters expressed the view that given the strength of the available scientific evidence, the serious nature of the health effects associated with PM 2.5 exposures, the large size of the at-risk populations, the risks associated with long- and short-term PM 2.5 exposures, and the important uncertainties inherently present in the evidence, the EPA should follow a highly precautionary policy response by selecting an annual standard level that incorporates a large margin of safety.

More specifically, these commenters offered a range of comments related to the general approach used by the EPA to select standard levels, including: (1) The EPA's approach for setting a generally controlling annual standard; (2) the importance of the greatly expanded and stronger overall scientific data base; (3) consideration of the distributional statistical analysis conducted by the EPA and other approaches for translating the air quality information from specific epidemiological studies into standard levels; and (4) the significance of the PM 2.5-related public health impacts, especially potential impacts on at-risk populations, including children, in reaching judgments on setting standards that provide protection with an adequate margin of safety. These comments are discussed in turn below.

(1) Some of these commenters disagreed with the EPA's approach for setting a “generally controlling” annual standard in conjunction with a 24-hour standard providing supplemental protection particularly for areas with high peak-to-mean ratios. These commenters argued this approach would lead to “regional inequities” as demonstrated in the EPA's analyses contained in Appendix C of the Policy Assessment (ALA et al., pp. 26 to 27). Specifically, these commenters argued:

There is no basis in the Clean Air Act for such a determination. The Clean Air Act requires only that the NAAQS achieve public health protection with an adequate margin of safety. It is well-documented that both long- and short-term exposures to PM 2.5 have serious and sometimes irreversible health impacts. There is no health protection reason to argue that one standard should be “controlling” as a matter of policy without regard to the health consequences of such a policy. To adopt such a policy ignores the obligation to provide equal protection under the law to all Americans because it would result in uneven protection from air pollution in different localities and regions of the country (ALA et al., 2012, p. 26).

The EPA believes these commenters misunderstood the basis for the EPA's policy goal of setting a “generally controlling” annual standard. This approach relates exclusively to setting standards that will provide requisite protection against effects associated with both long- and short-term PM 2.5 exposures. It does so by lowering the overall air quality distributions across an area, recognizing that changes in PM 2.5 air quality designed to meet an annual standard would likely result not only in lower annual mean PM 2.5 concentrations but also in fewer and lower peak 24-hour PM 2.5 concentrations. As discussed in section III.A.3 in the proposal and above, the EPA recognizes that there are various ways to combine the two primary PM 2.5 standards to achieve an appropriate degree of public health protection. Furthermore, the extent to which these two standards are interrelated in any given area depends in large part on the relative levels of the standards, the peak-to-mean ratios that characterize air quality patterns in an area, and whether changes in air quality designed to meet a given suite of standards are likely to be of a more regional or more localized nature.

In focusing on an approach of setting a generally controlling annual standard, the EPA's intent is in fact to avoid the potential “regional inequities” that are of concern to the commenters. The EPA judges that the most appropriate way to set standards that provide more consistent public health protection is by using the approach of setting a generally controlling annual standard. This judgment builds upon information presented in the Policy Assessment as discussed in section III.A.3 above. More specifically, the Policy Assessment recognized that the short-term exposure studies primarily evaluated daily variations in health effects with monitor(s) that measured the variation in daily PM 2.5 concentrations over the course of several years. The strength of the associations observed in these epidemiological studies was demonstrably in the numerous “typical” days within the air quality distribution, not in the peak days (U.S. EPA, 2011a, p. 2-9). In addition, the quantitative risk assessments conducted for this and previous reviews demonstrated the same point, that is, much, if not most, of the aggregate risk associated with short-term PM 2.5 exposures results from the large number of days during which the 24-hour average concentrations are in the low-to mid-range, below the peak 24-hour concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, section 3.1.2.2). In addition, there was no evidence suggesting that risks associated with long-term exposures were likely to be disproportionately driven by peak 24-hour concentrations. [102]

For these reasons, the Policy Assessment concluded that strategies that focused primarily on reducing peak days were less likely to achieve reductions in the PM 2.5 concentrations that were most strongly associated with the observed health effects. Furthermore, the Policy Assessment concluded that an approach that focused on reducing peak exposures would most likely result in more uneven public health protection across the U.S. by either providing inadequate protection in some areas or overprotecting in other areas (U.S. EPA, 2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). This is because reductions based on control of peak days are less likely to control the bulk of the air quality distribution.

As a result, the EPA believes an approach that focuses on a generally controlling annual standard would likely reduce aggregate risks associated with both long- and short-term exposures more consistently than a generally controlling 24-hour standard and, therefore, would be the most effective and efficient way to reduce total PM 2.5-related population risk. The CASAC agreed with this approach and considered it was “appropriate to return to the strategy used in 1997 that considers the annual and the short-term standards together, with the annual standard as the controlling standard, and the short-term standard supplementing the protection afforded by the annual standard” (Samet, 2010d, p. 1). For the reasons discussed above, the EPA disagrees with the comments that this approach will result in the concerns raised by the commenters; rather the EPA concludes that this approach will help to address these concerns.

(2) Many of these commenters asserted that the currently available scientific information is greatly expanded and stronger compared to the last review. Some of these commenters highlighted the availability of multiple, multi-city long- and short-term exposure studies providing “repeated, consistent evidence of effects below the current annual standard level” (ALA et al., 2012, pp. 39 to 49) and, more specifically, “significant evidence of harm with strong confidence well below EPA's proposed annual standard range of 12-13 μg/m [3] ” (AHA et al., 2012, pp. 10 to 12).

The EPA recognizes that in setting standards that are requisite to protect public health with an adequate margin of safety, the Administrator must weigh the various types of available scientific information in reaching public health policy judgments that neither overstate nor understate the strength and limitations of this information or the appropriate inferences to be drawn from the available science.

In general, the EPA agrees with these commenters' views that the currently available scientific evidence is stronger “because of its breadth and the substantiation of previously observed health effects” (77 FR 38906/2) and provides “greater confidence in the reported associations than in the last review” (77 FR 38940/1). The EPA also agrees with the commenters' position that it is appropriate to consider the regions within the broader air quality distributions where we have the strongest confidence in the associations reported in epidemiological studies in setting the level of the annual standard. However, as discussed in section III.E.4.d below, in weighing the available evidence and technical analyses, as well as the associated uncertainties and limitations in that information, the EPA disagrees with the commenters' views regarding the extent to which the available scientific information provides support for considering an annual standard level below the proposed range (i.e., below 12 to 13 μg/m [3] ). In particular, the EPA disagrees with the degree to which these commenters place more weight on the relatively more uncertain evidence that is suggestive of a causal relationship (e.g., low birth weight). Consistent with CASAC advice (Samet, 2010d, p. 1), the Agency concludes it is appropriate and reasonable to place the greatest emphasis on health effects for which the Integrated Science Assessment concluded there is evidence of a causal or likely causal relationship and to place less weight on the health effects that provide evidence that is only suggestive of a causal relationship.

(3) With regard to using the air quality information from epidemiological studies to inform decisions on standard levels, commenters in this group generally supported the EPA's efforts to explore different statistical metrics from epidemiological studies to inform the Administrator's decisions. These commenters argued that by considering different analytic measures—either concentrations one standard deviation below the long-term means reported in the epidemiological studies or the EPA's distributional statistical analysis of population-level data that extends the approach used in previous PM NAAQS reviews to consider information beyond a single statistical metric—“the annual standard must be significantly lower than EPA has proposed” (ALA et al., 2012, pp. 50 to 61). Furthermore, with regard to characterizing the PM 2.5 air quality at which associations have been observed, some of these commenters highlighted CASAC's recommendation that “[f]urther consideration should be given to using the 10th percentile as a level for assessing various scenarios of levels for the PM NAAQS” (Samet, 2010c, p. 11) (ALA et al., 2012, p. 55). Other commenters urged that the EPA extend the distributional analysis to include additional studies. For example, CHPAC urged the EPA to also conduct distributional analysis for children's health studies to better inform standards that would protect both children and adults from adverse health outcomes (CHPAC, 2012, p. 3).

The EPA agrees with these commenters' views that it is appropriate to take into account different statistical metrics from epidemiological studies to inform the decisions on standard levels that are appropriate to consider in setting a standard that will protect public health with an adequate margin of safety. In the development of the Policy Assessment, the EPA staff explored various approaches for using information from epidemiological studies in setting the standards. The general approach used in the final Policy Assessment, discussed in sections III.A.3 and III.E.4.a above, reflects consideration of CASAC advice (Samet, 2010c, d) and public comments on multiple drafts of the Policy Assessment.

With regard to using the distributional statistical analysis to characterize the confidence in the associations, the EPA emphasizes that there is no clear dividing line provided by the scientific evidence, and that choosing how far below the long-term mean concentrations from the epidemiological studies is appropriate to identify a standard level that will provide protection for the public health with an adequate margin of safety is largely a public health policy judgment. The EPA considers the region from approximately the 25th to 10th percentiles to be a reasonable range for providing a general frame of reference as to the part of the distribution over which our confidence in the magnitude and significance of the associations observed in epidemiological studies is appreciably lower. Based on these considerations, the EPA concludes that it is not appropriate to place as much confidence in the magnitude and significance of the associations over the lower percentiles of the distributions in each study as at and around the long-term mean concentrations. Thus, the EPA disagrees with the commenters' views that this analysis compels placing more emphasis on the lower part of this range in selecting a level for an annual standard that will protect public health with an adequate margin of safety. The EPA recognizes that this information comes primarily from two short-term exposure studies, a relatively modest data set. In light of the limited nature of this information, and in recognition of more general uncertainties inherent in the epidemiological evidence, the Administrator deems it reasonable not to place more emphasis on concentrations in the lower part of this range, as discussed below in section III.E.4.d.

With regard to the scope of the distributional statistical analysis, the EPA requested additional population-level data from the study authors for a group of six multi-city studies for which previous air quality analyses had been conducted (Hassett-Sipple et al., 2010; Schmidt et al., 2010, Analysis 2). These six studies were originally selected because they considered multiple locations representing varying geographic regions across multiple years. Thus, these studies provided evidence on the influence of different particle mixtures on health effects associated with long- and short-term PM 2.5 exposures. In addition, these multi-city studies considered relatively more recent health events and air quality conditions (1999 to 2005). As discussed in section III.E.4.b.i above, the EPA received and analyzed population-level data for four of the six studies (Rajan et al., 2011). Three of these four studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009) served as the basis for the concentration-response functions used to develop the core risk estimates (U.S. EPA, 2010a, section 3.3.3). While, the EPA agrees that it would be useful to have such data from more studies, the Agency believes that the additional data that was requested and received from study authors provide useful information to help inform the Administrator's selection of the annual standard level.

(4) Many commenters in this group highlighted PM 2.5-related impacts on at-risk populations, including potential impacts on children, older adults, persons with pre-existing heart and lung disease, and low-income populations, to support their views that the annual standard should be revised to a level of 11 μg/m [3] or lower (CHPAC, 2012; AHA et al., 2012; ALA, 2012, pp. 29 to 38; Rom et al., 2012; Air Alliance Houston, et al., 2012). These commenters urged the EPA to adopt a policy approach that placed less weight on the remaining uncertainties and limitations in the evidence and placed more emphasis on margin of safety considerations, including providing protection against effects for which there is more limited scientific evidence. For example, CHPAC urged the EPA “to place the same weight on studies examining impacts on children's health as that of adult studies. * * * The fact that there may be stronger evidence from adult studies does not mean that standards based on adult studies will be protective for children and consequently will meet the standard requisite to protect public health with an adequate margin of safety” (CHPAC, 2012 p. 3). Furthermore, with regard to the EPA's approach for weighing uncertainties, some of these commenters stated that “we find no justification in the preamble for an annual standard level as high as 13 μg/m [3] , other than the vague assertion that uncertainties increase at lower concentrations. Further, the final proposal completely failed to address the Policy Assessment recommendations that if 13 μg/m [3] was proposed, the 24-hour standard should be strengthened as well” (ALA et al., p. 7).

The EPA has carefully evaluated and considered evidence of effects in at-risk populations. With regard to effects classified as having evidence of a causal or likely causal relationship with long- or short-term PM 2.5 exposures (i.e., premature mortality, cardiovascular effects, and respiratory effects), the Agency takes note that it considered the full range of studies evaluating these effects, including studies of at-risk populations, to inform its review of the primary PM 2.5 standards. Specific multi-city studies summarized in Figures 1, 2, and 3 above highlight evidence of effects observed in two different lifestages—children and older adults—that have been identified as at-risk populations. Thus, the EPA places as much weight on studies that explored effects in children for which the evidence is causal or likely causal in nature as on studies of such effects in adults, including older adults. As discussed above in responses to commenters supporting the retention of the current standards, in setting the standard, the EPA has focused on considering PM 2.5 concentrations somewhat below the lowest long-term mean concentrations from each of the key studies of both long- and short-term exposures of effects for which the evidence supports a causal or likely causal relationship (i.e., the first two sets of studies shown in Figure 4). Absent some reason to ignore or discount these studies, which the commenter does not provide (and of which the EPA is unaware), the EPA considers the available evidence of effects in children as well as other at-risk populations.

With respect to the EPA's consideration of more limited studies providing evidence suggestive of a causal relationship (e.g., developmental and reproductive effects), as noted above in responding to comments from the first group of commenters, the Agency agrees that it is important to place some weight on this body of evidence in setting standards that provide protection for at-risk populations, as required by the CAA. However, the Agency does not agree that the same weight must be placed on this information as on the body of scientific information for which there is evidence of a causal or likely causal relationship. To do so would ignore the difference in the breadth and strength of the evidence supporting the different causality determinations reached in the Integrated Science Assessment.

With regard to weighing the uncertainties and limitations remaining in the evidence and technical analyses, as discussed in section II.A above, the EPA recognizes that in setting a primary NAAQS that provides an adequate margin of safety, the Administrator must consider a number of factors including the nature and severity of the health effects involved, the size of sensitive population(s) at risk, and the kind and degree of the uncertainties that remain. As discussed in section III.E.4.d below, the Agency agrees with these commenters that, in weighing the available evidence and technical analyses including the uncertainties and limitations in this scientific information, there is no justification for setting a primary PM 2.5 annual standard level as high as 13 μg/m [3] .

Finally, some commenters in both groups also identified “new” studies that were not included in the Integrated Science Assessment as providing further support for their views on the level of the annual standard. As discussed in section II.B.3 above, the EPA completed a provisional review and assessment of “new” studies published since the close of the Integrated Science Assessment, including “new” studies submitted by commenters (U.S. EPA, 2012b). The provisional assessment found that the “new” studies expand the scientific information considered in the Integrated Science Assessment and provide important insights on the relationship between PM 2.5 exposure and health effects of PM (U.S. EPA, 2012b). However, the EPA notes that the provisional assessment found that the “new” science did not materially change the conclusions reached in the Integrated Science Assessment. The EPA notes that, as in past NAAQS reviews, the Agency is basing the final decisions in this review on the studies and related information included in the Integrated Science Assessment that have undergone CASAC and public review, and will consider newly published studies for purposes of decision making in the next PM NAAQS review.

ii. 24-Hour Standard Level

With respect to the level of the 24-hour standard, the EPA received comments on the proposal from two distinct groups of commenters. One group that included virtually all commenters representing industry associations, businesses, and many States agreed with the Agency's proposed decision to retain the level of the 24-hour PM 2.5 standard. The other group of commenters included many medical groups, numerous physicians and academic researchers, many public health organizations, some State and local agencies, five State Attorneys General, and a large number of individual commenters. These commenters disagreed with the Agency's proposed decision and argued that EPA should lower the level of the 24-hour standard to 30 or 25 μg/m [3] . Comments from these groups on the level of the 24-hour PM 2.5 standard are addressed below and in the Response to Comments Document.

As noted above, of the public commenters who addressed the level of the 24-hour PM 2.5 standard, all industry commenters and most State and local commenters supported the proposed decision to retain the current level of 35 μg/m [3] . In many cases, these groups agreed with the rationale supporting the Administrator's proposed decision to retain the current 24-hour PM 2.5 standard, including her emphasis on the annual standard as the generally controlling standard with the 24-hour standard providing supplementary protection, and her conclusion that multi-city, short-term exposure studies provide the strongest data set for informing decisions on the appropriate 24-hour standard level. Many of these commenters agreed with the Administrator's view that the single-city, short-term studies provided a much more limited data set (e.g., limited statistical power, limited exposure data) and more equivocal results (e.g., mixed results within the same study area), making them an unsuitable basis for setting the level of the 24-hour standard.

While these commenters agreed with the EPA's proposed decision to retain the current 24-hour PM 2.5 standard, some did not agree with the EPA's approach to considering the evidence from short-term multi-city studies. For example, a commenter representing UARG pointed out that the 98th percentile concentrations reported in the proposal for multi-city studies reflect the averages of 98th percentile concentrations across the cities included in those studies (UARG, 2012; Attachment 1;p. 25). This commenter contended that such averaged 98th percentile PM 2.5 concentrations do not provide information that can appropriately inform a decision on the adequacy of the public health protection provided by the current or alternative 24-hour standards.

While the EPA agrees that there is uncertainty in linking effects reported in multi-city studies to specific air quality concentrations (U.S. EPA, 2011a, section 2.3.4.1), the EPA disagrees with this commenter's view that such uncertainty precludes the use of averaged 98th percentile PM 2.5 concentrations to inform a decision on the appropriateness of the protection provided by the 24-hour PM 2.5 standard. In particular, the EPA notes that averaged 98th percentile concentrations do provide information on the extent to which study cities contributing to reported associations would likely have met or violated the current 24-hour PM 2.5 standard during the study period. As evidence of this, the EPA notes the three multi-city studies specifically highlighted by this commenter as having averaged 98th percentile 24-hour PM 2.5 concentrations below 35 μg/m [3] (Dominici et al., 2006a; Bell et al., 2008; Zanobetti and Schwartz, 2009). Based on the 98th percentiles of 24-hour PM 2.5 concentrations in the individual cities evaluated in these studies, the EPA notes that the majority of these study cities would likely have met the current standard during the study periods (Hassett-Sipple et al., 2010). Therefore, regardless of whether the averaged 98th percentile concentrations or the 98th percentile concentrations in each city are considered, these studies provide evidence for associations between short-term PM 2.5 and mortality or morbidity across a large number of U.S. cities, the majority of which would likely have met the current 24-hour PM 2.5 standard during study periods. In their review of the PM Policy Assessment, CASAC endorsed the conclusions drawn from analyses of averaged 98th percentile 24-hour PM 2.5 concentrations, and the EPA continues to conclude that this type of information can appropriately inform the Administrator's decision on the current 24-hour PM 2.5 standard. [103]

Another group of commenters argued that the 24-hour standard level should be lowered. Many of these commenters supported setting the level of the 24-hour PM 2.5 standard at either 25 or 30 μg/m [3] . In support of their position, the ALA et. al., AHA et al., five state Attorneys General, and a number of additional groups pointed to 98th percentile PM 2.5 concentrations in locations of multi-city and single-city epidemiological studies. For example, the ALA and others pointed to multi-city studies by Dominici et al. (2006a), Zanobetti and Schwartz (2009), Burnett et al. (2000), and Bell et al. (2008) as providing evidence for associations with mortality and morbidity in study locations with averaged (i.e., averaged across cities) 98th percentile 24-hour PM 2.5 concentrations below 35 μg/m [3] . These commenters also pointed to several single-city and panel studies reporting associations between short-term PM 2.5 and mortality or morbidity in locations with relatively low 24-hour PM 2.5 concentrations. Because some of these multi- and single-city studies have reported associations with health effects in locations with 98th percentile PM 2.5 concentrations below 35 μg/m [3] , commenters maintained that the current 24-hour PM 2.5 standard (i.e., with its level of 35 μg/m [3] ) does not provide an appropriate degree of protection in all areas.

In further support of their position that the level of the current 24-hour standard should be lowered, these commenters pointed out the variability across the U.S. in ratios of 24-hour to annual PM 2.5 concentrations. They noted that some locations, including parts of the northwestern U.S., experience relatively low annual PM 2.5 concentrations but can experience relatively high 24-hour concentrations at certain times of the year. In order to provide protection against effects associated with short-term PM 2.5 exposures, especially in locations with high ratios of 24-hour to annual PM 2.5 concentrations, these commenters advocated setting a lower level for the 24-hour standard.

The EPA agrees with these commenters that it is appropriate to maintain a 24-hour PM 2.5 standard in order to supplement the protection provided by the revised annual standard, particularly in locations with relatively high ratios of 24-hour to annual PM 2.5 concentrations. However, in highlighting 98th percentile PM 2.5 concentrations in study locations without also considering the impact of a revised annual standard on short-term concentrations, these commenters ignore the fact that many areas would be expected to experience decreasing short- and long-term PM 2.5 concentrations in response to a revised annual standard.

In considering the specific multi-city studies highlighted by public commenters who advocated a more stringent 24-hour standard, the EPA notes that such studies have reported consistently positive and statistically significant associations with short-term PM 2.5 exposures in locations with averaged 98th percentile PM 2.5 concentrations ranging from 45.8 to 34.2 μg/m [3] and long-term mean PM 2.5 concentrations ranging from 13.4 to 12.9 (Burnett and Goldberg, 2003; Burnett et al., 2004; Dominici et al., 2006a; Bell et al., 2008; Franklin et al., 2008; Zanobetti and Schwartz, 2009). [104] The EPA notes that to the extent air quality distributions are reduced to meet the current 24-hour standard with its level of 35 μg/m [3] and/or the revised annual standard with its level of 12 μg/m [3] , additional protection would be anticipated against the effects reported in these short-term, multi-city studies. Put another way, to attain an annual standard with a level below the long-term means in the locations of these short-term studies (as EPA is adopting here), the overall air quality distributions in the majority of study cities will necessarily be reduced, resulting in lower daily PM 2.5 ambient concentrations. We therefore expect that the revised annual standard will result in 98th percentile PM 2.5 concentrations in these cities that are lower than those measured in the studies, and that the overall distributions of PM 2.5 concentrations will be lower than those reported to be associated with health effects. Thus, even for effects reported in multi-city studies with averaged 98th percentile concentrations below 35 μg/m [3] , additional protection from the risks associated with short-term exposures is anticipated from the revised annual standard, without revising the 24-hour standard, because long-term average PM 2.5 concentrations in multi-city study locations were above the level of the revised annual standard (i.e., 12 μg/m [3] ). [105] As discussed above, reducing the annual standard is the most efficient way to reduce the risks from short-term exposures identified in these studies, as the bulk of the risk comes from the large number of days across the bulk of the air quality distribution, not the relatively small number of days with peak concentrations.

In considering the single-city studies highlighted by public commenters who advocated a more stringent 24-hour standard, the EPA first notes that, overall, these single-city studies reported mixed results. Specifically, some studies reported positive and statistically significant associations with PM 2.5, some studies reported positive but non-significant associations, and several studies reported negative associations or a mix of positive and negative associations with PM 2.5. In light of these inconsistent results, the proposal noted that the overall body of evidence from single-city studies is mixed, particularly in locations with 98th percentiles of 24-hour concentrations below 35 μg/m [3] . Therefore, although some single-city studies reported effects at appreciably lower PM 2.5 concentrations than short-term multi-city studies, the uncertainties and limitations associated with the single-city studies were noted to be greater. In light of these greater uncertainties and limitations, the Administrator concluded in the proposal that she had less confidence in using these studies as a basis for setting the level of the standard (77 FR 38943).

Given the considerations and conclusions noted above, in the proposal the Administrator concluded that the short-term multi-city studies provide the strongest evidence to inform decisions on the level of the 24-hour standard. Further, she viewed single-city, short-term exposure studies as a much more limited data set providing mixed results, and she had less confidence in using these studies as a basis for setting the level of a 24-hour standard (77 FR 38942). In highlighting specific single-city studies, public health, environmental, and State and local commenters appear to have selectively focused on studies reporting associations with PM 2.5 and to have overlooked studies that reported more equivocal results (e.g., Ostro et al., 2003; Rabinovitch et al., 2004; Slaughter et al., 2005; Villeneuve et al., 2006) (U.S. EPA, 2011, Figure 2-9). As such, these commenters have not presented new information that causes the EPA to reconsider its decision to emphasize multi-city studies over single-city studies when identifying the appropriate level of the 24-hour PM 2.5 standard.

In further considering the single-city studies highlighted by public commenters, the EPA notes that some commenters advocating for a lower level for the 24-hour PM 2.5 standard also discussed short-term studies that have been published since the close of the Integrated Science Assessment. These recent studies were conducted in single cities or in small panels of volunteers. As in prior NAAQS reviews and as discussed above in more detail (section II.B.3), the EPA is basing its decisions in this review on studies and related information assessed in the Integrated Science Assessment. The studies assessed in the Integrated Science Assessment, and the conclusions based on those studies, have undergone extensive critical review by the EPA, CASAC, and the public. The rigor of that review makes the studies assessed in the Integrated Science Assessment, and the conclusions based on those studies, the most reliable source of scientific information on which to base decisions on the NAAQS.

However, as discussed above (section II.B.3), the EPA recognizes that “new studies” may sometimes be of such significance that it is appropriate to delay a decision on revision of a NAAQS and to supplement the pertinent air quality criteria so the studies can be taken into account. In the present case, the EPA's provisional consideration of “new studies” concludes that, taken in context, the “new” information and findings do not materially change any of the broad scientific conclusions made in the air quality criteria regarding the health effects of PM 2.5 (U.S. EPA, 2012b).

For this reason, reopening the air quality criteria review would not be warranted, even if there were time to do so under the court order governing the schedule for completing this review. Accordingly, the EPA is basing its final decisions in this review on the studies and related information included in the PM Integrated Science Assessment (i.e., the air quality criteria) that has undergone CASAC and public review. The EPA will consider the “new studies” in the next periodic review of the PM NAAQS, which will provide an opportunity to fully assess these studies through a more rigorous review process involving the EPA, CASAC, and the public.

Some public health, medical, and environmental commenters also criticized the EPA's interpretation of PM 2.5 risk results. These commenters presented risk estimates for combinations of annual and 24-hour standards using more recent air quality data than that used in the EPA's Risk Assessment (U.S. EPA, 2010a). Based on these additional risk analyses, the ALA and other commenters contended that public health benefits could continue to increase as annual and 24-hour standard levels decrease below 13 μg/m [3] and 35 μg/m [3] , respectively.

The EPA agrees with commenters that important public health benefits are expected as a result of revising the level of the annual standard to 12 μg/m [3] , as is done in this rule, rather than 13 μg/m [3] . The Agency also acknowledges that estimated PM 2.5-associated health risks continue to decrease with annual standard levels below 12 μg/m [3] and/or with 24-hour standard levels below 35 μg/m [3] . However, the EPA disagrees with the commenters' views regarding the extent to which risk estimates support setting standard levels below 12 μg/m [3] (annual standard) and 35 μg/m [3] (24-hour standard). [106]

The CAA charges the Administrator with setting NAAQS that are “requisite” (i.e., neither more nor less stringent than necessary) to protect public health with an adequate margin of safety. In setting such standards the Administrator must weigh the available scientific evidence and information, including associated uncertainties and limitations. As described above, in reaching her proposed decisions on the PM 2.5 standards that would provide “requisite” protection, the Administrator carefully considered the available scientific evidence and risk information, making public health policy judgments that, in her view, neither overstated nor understated the strengths and limitations of that evidence and information. In contrast, as discussed more fully above, public health, medical, and environmental commenters who recommended levels below 35 μg/m [3] for the 24-hour PM 2.5 standard have not provided new information or analyses to suggest that such standard levels are appropriate, given the uncertainties and limitations in the available health evidence, particularly uncertainties in studies conducted in locations with 98th percentile 24-hour PM 2.5 concentrations below 35 μg/m [3] and long-term average concentrations below 12 μg/m [3] .

d. Administrator's Final Conclusions on the Primary PM 2.5 Standard Levels

In reaching her conclusions regarding appropriate standard levels, the Administrator has considered the epidemiological and other scientific evidence, estimates of risk reductions associated with just meeting alternative annual and/or 24-hour standards, air quality analyses, related limitations and uncertainties, the advice of CASAC, and extensive public comments on the proposal. After careful consideration of all of these, the Administrator has decided to revise the level of the primary annual PM 2.5 standard from 15.0 μg/m [3] to 12.0 μg/m [3] and to retain the level of the primary 24-hour standard at 35 μg/m [3] .

As an initial matter, the Administrator agrees with the approach supported by CASAC and discussed in the Policy Assessment as summarized in sections III.A.3 and III.E.4.a above, of considering the annual and 24-hour standards together in determining the protection afforded against mortality and morbidity effects associated with both long- and short-term exposures to PM 2.5. This approach is consistent with the approach taken in the review completed in 1997, in contrast to the approach used in the review completed in 2006 where each standard was considered independently of the other (i.e., only data from long-term exposure studies were used to inform the level of the annual standard and only data from short-term exposure studies were used to inform the level of the 24-hour standard). [107]

Based on the evidence and quantitative risk assessment, the Administrator concludes that it is appropriate to set an annual standard that is generally controlling, which will lower the broad distribution of 24-hour average concentrations in an area as well as the annual average concentration, so as to provide protection from both long- and short-term PM 2.5 exposures. In conjunction with this, it is appropriate to set a 24-hour standard focused on providing supplemental protection, particularly for areas with high peak-to-mean ratios of 24-hour concentrations, possibly associated with strong local or seasonal sources, and for PM 2.5-related effects that may be associated with shorter-than daily exposure periods. The Administrator concludes this approach will reduce aggregate risks associated with both long- and short-term exposures more consistently than a generally controlling 24-hour standard and is the most effective and efficient way to reduce total PM 2.5-related population risk and to protect public health with an adequate margin of safety.

In selecting the level of the annual PM 2.5 standard, based on the characterization and assessment of the epidemiological and other studies presented and assessed in the Integrated Science Assessment and the Policy Assessment, the Administrator recognizes the substantial increase in the number and diversity of studies available in this review. This expanded body of evidence includes extended analyses of the seminal studies of long-term PM 2.5 exposures (i.e., ACS and Harvard Six Cities studies) as well as important new long-term exposure studies (as summarized in Figures 1 and 2). Collectively, the Administrator notes that these studies, along with evidence available in the last review, provide consistent and stronger evidence than previously observed of an association between long-term PM 2.5 exposures and premature mortality in areas with lower long-term ambient concentrations than previously observed, with the strongest evidence related to cardiovascular-related mortality. The Administrator also recognizes the availability of stronger evidence of morbidity effects associated with long-term PM 2.5 exposures, including evidence of respiratory effects such as decreased lung function growth, from the extended analyses for the Southern California Children's Health Study and evidence of cardiovascular effects from the WHI study. Furthermore, the Administrator recognizes new U.S. multi-city studies that greatly expand and reinforce our understanding of mortality and morbidity effects associated with short-term PM 2.5 exposures, providing stronger evidence of associations in areas with ambient concentrations similar to those previously observed in short-term exposure studies considered in the previous review (as summarized in Figure 3).

The Administrator recognizes the strength of the scientific evidence for evaluating health effects associated with fine particles, noting that the newly available scientific evidence builds upon the previous scientific data base to provide evidence of generally robust associations and a basis for greater confidence in the reported associations than in the last review. She notes the conclusion of the Integrated Science Assessment that this body of evidence supports a causal relationship between long- and short-term PM 2.5 exposures and mortality and cardiovascular effects and a likely causal relationship between long- and short-term PM 2.5 exposures and respiratory effects. In addition, the Administrator notes additional, but more limited evidence, for a broader range of health endpoints including evidence suggestive of a causal relationship for developmental and reproductive effects as well as for carcinogenic effects.

Based on information discussed and presented in the Integrated Science Assessment, the Administrator recognizes that health effects may occur over the full range of concentrations observed in the epidemiological studies of both long-term and short-term exposures, since no discernible population-level threshold for any such effects can be identified based on the currently available evidence (U.S. EPA, 2009a, section 2.4.3). To inform her decisions on an appropriate level for the annual standard that will protect public health with an adequate margin of safety, in the absence of any discernible population-level thresholds, the Administrator judges that it is appropriate to consider the relative degree of confidence in the magnitude and significance of the associations observed in epidemiological studies across the range of long-term PM 2.5 concentrations in such studies. Further, she recognizes, in taking note of CASAC advice and the distributional statistics analysis discussed in the Policy Assessment and in section III.E.4.a above, that there is significantly greater confidence in the magnitude and significance of observed associations for the part of the air quality distribution corresponding to where the bulk of the health events evaluated in each study have been observed, generally at and around the long-term mean concentrations. Conversely, she also recognizes that there is significantly diminished confidence in the magnitude and significance of observed associations in the lower part of the air quality distribution corresponding to where a relatively small proportion of the health events were observed. Further, the Administrator recognizes that the long-term mean concentrations, or any other specific point in the air quality distribution of each study, do not represent a “bright line” at and above which effects have been observed and below which effects have not been observed.

In considering the long-term mean concentrations reported in epidemiological studies, the Administrator recognizes that in selecting a level of the annual standard that will protect public health with an adequate margin of safety, it is not sufficient to focus on a concentration generally somewhere within the range of long-term mean concentrations from the key long-term and short-term exposure studies that reported lower concentrations than had been observed in earlier reviews. These key studies provide information for various types of serious health endpoints (including mortality and morbidity effects), different study populations (which may include at-risk populations such as children and older adults), and different air quality distributions that are specific to each study. A level somewhere within the range of long-term mean concentrations of the full set of key studies would be higher than the long-term mean of at least one of the studies being considered and therefore would not provide a sufficient degree of protection against the health effects observed in that study. Absent some reasoned basis to place less weight on the evidence in the epidemiological study with the lowest long-term mean concentration among these key studies, this approach would not be consistent with the requirement to set a standard that will protect public health with an adequate margin of safety. [108] Thus, the Administrator recognizes it is important to protect against the serious effects observed in each of these studies so as to protect public health with an adequate margin of safety. In so doing, she looks to identify the study with the lowest long-term mean concentration within the full set of key studies to help inform her decision of the appropriate standard level which will provide protection for the broad array of health outcomes observed in all of the studies, including effects observed in at-risk populations.

Further, consistent with the general approach summarized in section III.E.4.a above and supported by CASAC as discussed in section III.E.4.b.ii above, the Administrator recognizes that it is appropriate to consider a level for an annual standard that is not just at but rather is somewhat below the long-term mean PM 2.5 concentrations reported in each of the key long- and short-term exposure studies. In so doing, she focuses especially on multi-city studies that evaluated health endpoints for which the associations are causal or likely causal (i.e., mortality and cardiovascular and respiratory effects associated with both long- and short-term PM 2.5 exposures). As discussed above, the importance of considering a level somewhat below the lowest long-term mean concentrations in this set of key studies is to establish a standard that would be protective against the observed effects in all of the studies, and that takes into account the relative degree of confidence in the magnitude and significance of observed associations across the air quality distributions in these studies.

The Administrator recognizes that there is no clear way to identify how much below the long-term mean concentrations of key studies to set a standard that would provide requisite protection with an adequate margin of safety. She therefore must use her judgment to weigh the available scientific and technical information, and associated uncertainties, to reach a final decision on the appropriate standard level. In considering the information in Figures 1-4 for effects classified as having evidence of a causal or likely causal relationship with long- or short-term PM 2.5 exposures, she observes a cluster of short-term exposure studies with long-term mean concentrations within a range of 13.4 μg/m [3] down to 12.8 μg/m [3] (Dominici et al., 2006a; Burnett and Goldberg, 2003; Zanobetti and Schwartz, 2009; Bell et al., 2008; Burnett et al., 2004). She also observes a cluster of long-term exposure studies with long-term mean concentrations within a range of 14.5 μg/m [3] to 13.6 μg/m [3] (Dockery et al., 1996; Lipfert et al., 2006a; Zeger et al., 2008; McConnell et al., 2003; Goss et al., 2004; Eftim et al., 2008). For the reasons discussed in response to public comments in section III.E.4.c above, the Administrator is less influenced by the long-term mean PM 2.5 concentrations from the Miller et al. (2007) and Krewski et al. (2009) studies with reported long-term mean PM 2.5 concentrations of 12.9 and 14.0 μg/m [3] , respectively. In each case, the most relevant exposure periods would likely have had higher mean PM 2.5 concentrations than those reported in the studies. [109] Thus, the Administrator considers the long-term mean PM 2.5 concentrations from these two studies to be a highly uncertain basis for informing her selection of the annual standard level. [110]

To help guide her judgment of the appropriate level below the long-term mean concentrations in the epidemiological studies at which to set the standard, the Administrator considered additional information from epidemiological studies concerning the broader distribution of PM 2.5 concentrations which correspond to the health events observed in these studies (e.g., deaths, hospitalizations). The Administrator observes that the development and use of this information in considering standard levels is consistent with CASAC's advice, as discussed in section III.E.4.b.ii above, to focus on understanding the concentrations that were most influential in generating the health effect estimates in individual studies (Samet, 2010d, p. 2).

In considering this additional population-level information, the Administrator recognizes that, in general, the confidence in the magnitude and significance of an association identified in a study is strongest at and around the long-term mean concentration for the air quality distribution, as this represents the part of the distribution in which the data in any given study are generally most concentrated. She also recognizes that the degree of confidence decreases as one moves towards the lower part of the distribution. Consistent with the approach used in the Policy Assessment, the Administrator believes that the range from approximately the 25th to 10th percentiles is a reasonable range for providing a general frame of reference as to the part of the distribution in which her confidence in the associations observed in epidemiological studies is appreciably lower. However, as noted above, it is important to emphasize that there is no clear dividing line or single percentile within a given distribution provided by the scientific evidence that is most appropriate or `correct' to use to characterize where the degree of confidence in the associations warrants setting the annual standard level. The decision of the appropriate standard level below the long-term mean concentrations of the key studies, which in conjunction with the other elements of the standard would protect public health with an adequate margin of safety, is largely a public health policy judgment, taking into account all of the evidence and its related uncertainties.

As discussed in section III.E.4.b, the Administrator takes note of additional population-level data that were made available to the EPA by study authors. [111] In considering this information, the Administrator particularly focuses on the analysis of the distributions of the health event data for each area within these studies and the corresponding air quality data for the two short-term exposure studies (Zanobetti and Schwartz, 2009; Bell et al., 2008). These short-term exposure studies evaluate the relationship between daily changes (one or more days) in PM 2.5 concentrations and daily changes in health events (e.g., deaths, hospitalizations), such that the air quality concentrations that comprise the most relevant exposure periods in these studies are contemporaneous with the health event data. In addition, these studies considered more recent air quality data, representing generally lower PM 2.5 concentrations, in a large number of study areas across the U.S. Thus, such studies provide the most useful evidence for an analysis evaluating the distribution of health event data and the corresponding long-term mean PM 2.5 concentrations across the areas included in each multi-city study.

The Administrator also considered the additional population-level data that were made available to EPA for two long-term exposure studies (Krewski et al., 2009; Miller et al., 2007). She recognizes that in long-term exposure studies investigators follow a specific group of study participants (i.e., cohort) over time and across urban study areas, and evaluate how PM 2.5 concentrations averaged over a period of years are associated with specific health endpoints (e.g., deaths) across cities. As discussed in response to public comments in section III.E.4.c, disentangling the effects observed in long-term exposure studies associated with more recent air quality measurements from effects that may have been associated with earlier, and most likely higher, PM 2.5 exposures introduces some uncertainty with regard to understanding the appropriate exposure window associated with the observed effects. This is in contrast to the short-term exposure studies where the relevant exposure period is contemporaneous to the period for which the health data were collected. In light of these considerations, as noted above, the Administrator considers the analysis of air quality concentrations that correspond to the distribution of population-level data in these two studies to be a highly uncertain basis for informing her selection of the annual standard level.

Based on the above considerations, the Administrator views the additional population-level data for the two short-term exposure studies as appropriate to help inform her judgment of how much below the long-term mean concentrations to set the level of the annual standard. The Administrator notes that the long-term mean PM 2.5 concentrations corresponding with study areas contributing to the 25th percentiles of the distribution of deaths and cardiovascular-related hospitalizations in these two short-term exposure studies were 12.5 µg/m [3] and 11.5 µg/m [3] , respectively, for Zanobetti and Schwartz (2009) and for Bell et al. (2008), with the 10th percentiles being lower by approximately 2 µg/m [3] in each study.

The Administrator recognizes, as summarized in section III.B above and discussed more fully in section III.B.2 of the proposal, that important uncertainties remain in the evidence and information considered in this review of the primary fine particle standards. These uncertainties are generally related to understanding the relative toxicity of the different components in the fine particle mixture, the role of PM 2.5 in the complex ambient mixture, exposure measurement errors, and the nature and magnitude of estimated risks related to increasingly lower ambient PM 2.5 concentrations. Furthermore, the Administrator notes that epidemiological studies have reported heterogeneity in responses both within and between cities and geographic regions across the U.S. She recognizes that this heterogeneity may be attributed, in part, to differences in fine particle composition in different regions and cities. [112]

With regard to evidence for reproductive and developmental effects identified as being suggestive of a causal relationship with long-term PM 2.5 exposures, the Administrator recognizes that there are a number of limitations associated with this body of evidence including: the limited number of studies evaluating such effects; uncertainties related to identifying the relevant exposure time periods of concern; and limited toxicological evidence providing little information on the mode of action(s) or biological plausibility for an association between long-term PM 2.5 exposures and adverse birth outcomes. Nonetheless, the Administrator believes that this more limited body of evidence provides some support for considering that serious effects may be occurring in a susceptible population at concentrations lower than those associated with effects classified as having a causal or likely causal relationship with long-term PM 2.5 exposures (i.e., mortality, cardiovascular, and respiratory effects).

Overall, the Administrator believes that the available evidence interpreted in light of the remaining uncertainties, as summarized above and discussed more fully in the Integrated Science Assessment and the Policy Assessment, provides increased confidence relative to information available in the last review and provides a strong basis for informing her final decisions in the current review. The Administrator is mindful that considering what standards are requisite to protect public health with an adequate margin of safety requires public health policy judgments that neither overstate nor understate the strength and limitations of the evidence or the appropriate inferences to be drawn from the evidence. In considering how to translate the available information into appropriate standard levels, the Administrator weighs the available scientific information and associated uncertainties and limitations. For the purpose of determining what annual standard level is appropriate the Administrator recognizes that there is no single factor or criterion that comprises the “correct” approach to weighing the various types of available evidence and information.

In considering this information, the Administrator notes the advice of CASAC that “there are significant public health consequences at the current levels of the standards that justify consideration of lowering the PM 2.5 NAAQS further” (Samet, 2010c, p. 12). In addition, she recognizes that CASAC concluded, “although there is increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a level below which there is no risk for adverse effects)” (Samet, 2010d, p.ii) and that the final decisions on standard levels must reflect a judgment of the available scientific information with respect to her interpretation of the CAA's requirement to set primary standards that provide requisite protection to public health with an adequate margin of safety (Samet, 2010d, p. 4). The Administrator recognizes CASAC's advice that the currently available scientific information provided support for considering an annual standard level within a range of 13 to 11 μg/m [3] and a 24-hour standard level within a range of 35 to 30 μg/m [3] . In considering how the annual and 24-hour standards work together to provide appropriate public health protection, the Administrator observes that CASAC did not express support for any specific levels or combinations of standards within these ranges. She also notes that CASAC encouraged the EPA staff to consider additional data from epidemiological studies to help quantify the characterization of the PM 2.5 concentrations that were most influential in generating the health effect estimates in these studies (Samet, 2010d, p. 2).

In response to CASAC's advice, the Administrator recognizes that the EPA staff acquired additional data from authors of key epidemiological studies and analyzed these data to characterize the distribution of PM 2.5 concentrations in relation to health events data to better understand the degree of confidence in the associations observed in the studies as discussed above. The Administrator recognizes that the final Policy Assessment included consideration of these additional analyses in reaching final staff conclusions with regard to the broadest range of alternative standard levels supported by the science. She takes note that the final Policy Assessment concluded that while alternative standard levels within the range of 13 to 11 μg/m [3] were appropriate to consider, the evidence most strongly supported consideration of an annual standard level in the range of 12 to 11 μg/m [3] . The Administrator is aware that, in transmitting the final Policy Assessment to CASAC, the Agency notified CASAC that the final staff conclusions reflected consideration of CASAC's advice and that those staff conclusions were based, in part, on the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 2011). Thus, CASAC had an opportunity to comment on the final Policy Assessment, but chose not to provide any additional comments or advice after receiving it.

In selecting the annual standard level, the Administrator has considered many factors including the nature and severity of the health effects involved, the strength of the overall body of scientific evidence as considered in reaching causality determinations, the size of the at-risk populations, and the estimated public health impacts. She has also considered the kind and degree of the uncertainties that remain in the available scientific information. She recognizes that the association between PM 2.5 and serious health effects is well established, including at concentrations below those allowed by the current standard. Further, she recognizes the CAA requirement that requires primary standards to provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information as well as to provide a reasonable degree of protection against hazards that research has not yet identified. In considering the currently available evidence, as summarized and discussed more broadly above, the information on risk, CASAC advice, the conclusions of the Policy Assessment, and public comments on the proposal, the Administrator strongly believes that a lower annual standard level is needed to protect public health with an adequate margin of safety.

In reaching her final decision on the appropriate annual standard level to set, the Administrator is mindful that the CAA does not require that primary standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to protect public health, including the health of at-risk populations, with an adequate margin of safety. On balance, the Administrator concludes that an annual standard level of 12 μg/m [3] would be requisite to protect the public health with an adequate margin of safety from effects associated with long- and short-term PM 2.5 exposures, while still recognizing that uncertainties remain in the scientific information.

In the Administrator's judgment, an annual standard of 12 μg/m [3] appropriately reflects placing greatest weight on evidence of effects for which the Integrated Science Assessment determined there is a causal or likely causal relationship with long- and short-term PM 2.5 exposures. An annual standard level of 12 μg/m [3] is below the long-term mean PM 2.5 concentrations reported in each of the key multi-city, long- and short-term exposures studies providing evidence of an array of serious health effects (e.g., premature mortality, increased hospitalization for cardiovascular and respiratory effects). As noted above, the importance of considering a level somewhat below the lowest long-term mean concentration in the full set of studies considered is to set a standard that would provide appropriate protection against the observed effects in all such studies.

In reaching her decision, the Administrator has taken into account that at and around the mean PM 2.5 concentration in any given study represents a part of the air quality distribution in which the health event data in that study are generally most concentrated. Furthermore, in identifying an appropriate annual standard level below the long-term mean concentrations, she recognizes that there is no evidence to support the existence of any discernible threshold, and, therefore, she has a high degree of confidence that the observed effects are associated with concentrations not just at but extending somewhat below the long-term mean concentration. To further inform her judgment in setting the annual standard level so as to protect public health with an adequate margin of safety, the Administrator has placed weight on additional population-level information available from a subset of these epidemiological studies, consistent with CASAC advice. In particular, she has drawn from two short-term exposure studies, which provide the most relevant information for evaluating the distribution of health events and corresponding long-term PM 2.5 concentrations. As explained above, this helps inform her judgment as to the degree of confidence in the observed associations in the epidemiological studies. In this regard, the Administrator generally judges the region around the 25th percentile as a reasonable part of the distribution to help guide her decision on the appropriate standard level. Since this evidence comes primarily from two studies, a relatively modest data set, the Administrator deems it reasonable not to draw further inferences from air quality and health event data in the lower part of the distribution for the purpose of setting a standard level. The Administrator notes that the long-term mean PM 2.5 concentrations around the 25th percentile of the distributions of deaths and cardiovascular-related hospitalizations were approximately around 12 μg/m [3] in these two studies. The Administrator views this information as helpful in guiding her determination as to where her confidence in the magnitude and significance of the associations is reduced to such a degree that a standard set at a lower level would not be warranted to provide requisite protection that is neither more nor less than needed to provide an adequate margin of safety.

The Administrator also recognizes that a level of 12 μg/m [3] places some weight on studies which provide evidence of reproductive and developmental effects (e.g., infant mortality, low birth weight). These studies were identified in the Integrated Science Assessment as having evidence suggestive of a causal relationship with long-term PM 2.5 concentrations. A level of 12 μg/m [3] is approximately the same level as the lowest long-term mean concentration reported in such studies (Figures 2 and 4; 11.9 μg/m [3] for Bell et al., 2007). [113] While the Administrator acknowledges that this evidence is limited, she believes it is appropriate to place some weight on these studies in order to set a standard that provides protection with an adequate margin of safety, including providing protection for at-risk populations, as required by the CAA. Due to the limited nature of this evidence, she has determined it is not necessary to set a standard below the lowest long-term mean concentration in these studies.

In reflecting on extensive public comments received on the proposal as discussed in section III.E.4.c above, the Administrator recognizes that some commenters have offered different evaluations of the evidence and other information available in this review and would make different judgments about the weight to place on the relative strengths and limitations of the scientific information and about how such information could be used in making public health policy decisions on the annual standard level. One group of such commenters who supported a higher annual standard level (e.g., above 13 μg/m [3] ) would place greater weight on the remaining uncertainties in the evidence as a basis for supporting a higher standard level than the Administrator judges to be appropriate. Such an approach is based on these commenters' judgment that the uncertainties remaining in the evidence are too great to warrant setting an annual standard below the current level. The Administrator does not agree.

As an initial matter, an annual standard level of 13 μg/m [3] or higher would be above the long-term mean concentrations reported in two well-conducted, multi-city short-term exposure studies reporting positive and statistically significant associations of serious effects (Burnett et al., 2004 and Bell et al., 2008). These important studies are fully consistent with the pattern of evidence presented by the large body of evidence in this review. As the Administrator recognized in the proposal, and as advised by CASAC, the appropriate focus for selecting the level of the annual PM 2.5 standard is on concentrations somewhat below the lowest long-term mean concentrations from the set of key studies of both long-term and short-term PM 2.5 exposures considered by the EPA (i.e., as shown in Figure 4). Thus, a standard level set at 13 μg/m [3] or higher would clearly not provide protection for the effects observed in the full set epidemiological studies and, therefore, this standard level could not be judged to be requisite with an adequate margin of safety. [114]

In addition, as noted above, in recognizing that there is no evidence to support the existence of a discernible threshold below which an effect would not occur, the Administrator is mindful that effects occur around and below the long-term mean concentrations reported in both the short-term and long-term the epidemiological studies. A standard level of 13 μg/m [3] or higher would not appropriately take into account evidence from the two well-conducted, multi-city, short-term exposure studies reporting serious effects with long-term mean concentrations below 13 μg/m [3] noted above (Burnett et al, 2004; Bell et al., 2008). Such a standard level would also not appropriately take into account additional population-level data from a limited number of epidemiological studies. This approach would ignore CASAC's advice to consider such information in order to better understand the concentrations over which there is a high degree of confidence regarding the magnitude and significance of the associations observed in individual epidemiological studies and where there is appreciably less confidence.

Furthermore, a standard level of 13 μg/m [3] or higher would not appropriately take into account the more limited evidence of effects in some at-risk populations (e.g., low birth weight). In the Administrator's view, a standard set at this level would not provide protection with an adequate margin of safety, including providing protection for at-risk populations. The Administrator is mindful that the CAA requirement that primary standards provide an adequate margin of safety, discussed in section II.A above, was intended to address uncertainties associated with inconclusive scientific and technical information available at the time of standard setting as well as to provide a reasonable degree of protection against hazards that research has not yet identified.

In light of the entire body of evidence as discussed above, the Administrator judges that an annual standard level set above 12 μg/m [3] would not be sufficient to protect public health with an adequate margin of safety from the serious health effects associated with long- and short-term exposure to PM 2.5.

The Administrator also recognizes that a second group of commenters supported a lower annual standard level (e.g., no higher than 11 μg/m [3] ). Such a standard level would reflect placing essentially as much weight on the relatively more limited data providing evidence suggestive of a causal relationship for effects observed in some at-risk populations (e.g., low birth weight) as on more certain evidence of effects classified as having a causal or likely causal relationship with PM 2.5 exposures. In the Administrator's view, while it is important to place some weight on such suggestive evidence, it would not be appropriate to place as much weight on it as the commenters would do.

An annual standard level of 11 μg/m [3] would also reflect these commenters' judgment that it is appropriate to focus on a lower part of the distributions of health event data from the small number of epidemiological studies for which this information was made available than the Administrator believes is warranted. In the Administrator's view, using this type of information to set a standard level of 11 μg/m [3] or below would assume too high a degree of confidence in the magnitude and significance of the associations observed in the lower part of the distributions of health events observed in these studies. Given the uncertainties in the evidence and the limited set of studies for which the EPA has information on the distribution of health event data and corresponding air quality data, the Administrator believes it is not appropriate to focus on the lower part of the distributions of health events data.

On balance, the Administrator finds that the available evidence interpreted in light of the remaining uncertainties does not justify a standard level set below 12 μg/m [3] as necessary to protect public health with an adequate margin of safety.

After carefully considering the above considerations and the public comments summarized in section III.E.4.c above, the Administrator has decided to set the level of the primary annual PM 2.5 standard at 12 μg/m [3] . In her judgment, a standard set at this level provides the requisite degree of public health protection, including the health of at-risk populations, with an adequate margin of safety and is neither more nor less stringent than necessary for this purpose.

As discussed above, the Administrator concludes that an approach that focuses on setting a generally controlling annual standard is the most effective and efficient way to reduce total population risk associated with both long- and short-term PM 2.5 exposures. Such an approach would result in more uniform protection across the U.S. than the alternative of setting the levels of the 24-hour and annual standard such that the 24-hour standard would generally be the controlling standard in areas across the country (see section III.A.3).

The Administrator recognizes that potential air quality changes associated with meeting an annual standard level of 12.μg/m [3] will result in lowering risks associated with both long- and short-term PM 2.5 exposures by lowering the overall air quality distribution. However, the Administrator recognizes that such an annual standard alone would not be expected to offer sufficient protection with an adequate margin of safety against the effects of short-term PM 2.5 exposures in all parts of the country. As a result, in conjunction with an annual standard level of 12 μg/m [3] , the Administrator concludes that it is appropriate to continue to provide supplemental protection by means of a 24-hour standard set at the appropriate level, particularly for areas with high peak-to-mean ratios possibly associated with strong local or seasonal sources and for areas with PM 2.5-related effects that may be associated with shorter-than-daily exposure periods.

In selecting the level of a 24-hour standard meant to provide such supplemental protection, the Administrator relies upon evidence and air quality information from key short-term exposure studies. In considering these studies, the Administrator notes that to the extent air quality distributions in the study areas considered are reduced to meet the current 24-hour standard (at a level of 35 μg/m [3] ) or to meet the revised annual standard discussed above (at a level of 12 μg/m [3] ), additional protection would be anticipated against the effects observed in these studies. In light of this, when selecting the appropriate level for the 24-hour standard, the Administrator considers both the 98th percentiles of 24-hour PM 2.5 concentrations and the long-term mean PM 2.5 concentrations in the locations of the short-term exposure studies. She notes that such consideration of both short- and long-term PM 2.5 concentrations can inform her decision on the extent to which a given 24-hour standard, in combination with the revised annual standard established in this rule, would provide protection against the health effects reported in short-term studies.

As discussed in section III.E.4.a above, the Administrator concludes that multi-city short-term exposure studies provide the strongest data set for informing her decisions on appropriate 24-hour standard levels. With regard to the limited number of single-city studies that reported positive and statistically significant associations for a range of health endpoints related to short-term PM 2.5 concentrations in areas that would likely have met the current suite of PM 2.5 standards, the Administrator recognizes that many of these studies had significant limitations (e.g., limited statistical power, limited exposure data) or equivocal results (mixed results within the same study area) that make them unsuitable to form the basis for setting the level of a 24-hour standard.

With regard to multi-city studies that evaluated effects associated with short-term PM 2.5 exposures, the Administrator observes an overall pattern of positive and statistically significant associations in studies with 98th percentile 24-hour values averaged across study areas within the range of 45.8 to 34.2 μg/m [3] (Burnett et al., 2004; Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and Goldberg, 2003; Franklin et al., 2008). The Administrator notes that, to the extent air quality distributions are reduced to reflect just meeting the current 24-hour standard, additional protection would be provided for the effects observed in the three multi-city studies with 98th percentile values greater than 35 μg/m [3] (Burnett et al., 2004; Burnett and Goldberg, 2003; Franklin et al., 2008). In the three additional multi-city studies with 98th percentile values below 35 μg/m [3] , specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 μg/m [3] , the Administrator notes that these studies reported long-term mean PM 2.5 concentrations of 12.9, 13.2, and 13.4 μg/m [3] , respectively (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a). In revising the level of the annual standard to 12 μg/m [3] , as discussed above, the Administrator recognizes that additional protection would be provided for the short-term effects observed in these multi-city studies such that revision to the 24-hour standard would not be warranted. That is, by lowering the level of the annual standard to 12 μg/m [3] , the 98th percentile of the distribution would be lowered as well such that additional protection from effects associated with short-term exposures would be afforded. Therefore, the epidemiological evidence supports a conclusion that it is appropriate to retain the level of the 24-hour standard at 35 μg/m [3] , in conjunction with a revised annual standard level of 12 μg/m [3] .

In addition to considering the epidemiological evidence, the Administrator also has taken into account air quality information based on county-level 24-hour and annual design values to understand the implications of revising the annual standard level from 15 to 12 μg/m [3] in conjunction with retaining the 24-hour standard level at 35 μg/m [3] . She has considered this information to evaluate the public health protection provided by the two standards in combination and to evaluate the most appropriate means of developing a suite of standards providing requisite public health protection with an adequate margin of safety.

In considering the air quality information, the Administrator observes that a suite of PM 2.5 standards that includes an annual standard level of 12 μg/m [3] and a 24-hour standard level of 35 μg/m [3] would result in the annual standard as the generally controlling standard in most regions across the country, except for certain areas in the Northwest, where the annual mean PM 2.5 concentrations have historically been low but where relatively high 24-hour concentrations occur, often related to seasonal wood smoke emissions (U.S. EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). In fact, these are the type of areas for which the supplemental protection afforded by the 24-hour standard is intended, such that the two standards together provide the requisite degree of protection. The Administrator concludes the current 24-hour standard at a level of 35 μg/m [3] , in conjunction with a revised annual standard level of 12 μg/m [3] , will provide appropriate protection from effects observed in studies in such areas in which the long-term mean concentrations were below 12 μg/m [3] and the 98th percentile 24-hour concentrations were above 35 μg/m [3] (e.g., areas in the Northwest U.S.).

After carefully taking the public comments and above considerations into account, the Administrator has decided to retain the current level of the primary PM 2.5 24-hour standard at 35 μg/m [3] in conjunction with revising the annual standard level from 15.0 μg/m [3] to 12.0 μg/m [3] . [115] In the Administrator's judgment, this suite of primary PM 2.5 standards and the rationale supporting these levels appropriately reflects consideration of the strength of the available evidence and other information and its associated uncertainties as well as the advice of CASAC and consideration of public comments. In the Administrator's judgment, this suite of primary PM 2.5 standards is sufficient but not more protective than necessary to protect the public health, including at-risk populations, with an adequate margin of safety from effects associated with long- and short-term exposures to fine particles. This suite of standards will provide significant protection from serious health effects including premature mortality and cardiovascular and respiratory morbidity effects that are causally or likely causally related to long- and short-term PM 2.5 exposures. These standards will also provide an appropriate degree of protection against other health effects for which there is more limited evidence of effects and causality, such as reproductive and developmental effects. This judgment by the Administrator appropriately considers the requirement for a standard that is requisite to protect public health but is neither more nor less stringent than necessary. [116]

D. Administrator's Final Decisions on Primary PM 2.5 Standards

For the reasons discussed above, and taking into account the information and assessments presented in the Integrated Science Assessment, Risk Assessment, and Policy Assessment, the advice and recommendations of CASAC, and public comments to date, the Administrator revises the current suite of primary PM 2.5 standards. Specifically, the Administrator revises: (1) The level of the primary annual PM 2.5 standard to 12.0 μg/m [3] and (2) the form of the primary annual PM 2.5 standard to one based on the highest appropriate area-wide monitor in an area, with no option for spatial averaging. In conjunction with revising the primary annual PM 2.5 standard to provide protection from effects associated with long- and short-term PM 2.5 exposures, the Administrator retains the level of 35 μg/m [3] and the 98th percentile form of the primary 24-hour PM 2.5 standard to continue to provide supplemental protection for areas with high peak PM 2.5 concentrations. The Administrator is not revising the current PM 2.5 indicator or the annual and 24-hour averaging times for the primary PM 2.5 standards. The Administrator concludes that this suite of standards would be requisite to protect public health with an adequate margin of safety against health effects potentially associated with long- and short-term PM 2.5 exposures.

IV. Rationale for Final Decision on Primary PM 10 Standard Back to Top

This section presents the rationale for the Administrator's final decision to retain the current 24-hour primary PM 10 standard in order to continue to provide public health protection against short-term exposures to inhalable particles in the size range of 2.5 to 10 μm (i.e., PM 10-2.5 or thoracic coarse particles). These are particles capable of reaching the most sensitive areas of the lung, including the trachea, bronchi, and deep lungs. The current standard uses PM 10 as the indicator for thoracic coarse particles, and thus is referred to as a PM 10 standard. [117]

As discussed more fully in the proposal and below, this rationale is based on a thorough review of the latest scientific evidence, published through mid-2009 and assessed in the Integrated Science Assessment (U.S. EPA, 2009a), evaluating human health effects associated with long- and short-term exposures to thoracic coarse particles. The Administrator's final decision also takes into account: (1) The EPA staff analyses of air quality information and health evidence and staff conclusions regarding the current and potential alternative standards, as presented in the Policy Assessment for the PM NAAQS (U.S. EPA, 2011a); (2) CASAC advice and recommendations, as reflected in discussions at public meetings of drafts of the Integrated Science Assessment and Policy Assessment, and in CASAC's letters to the Administrator; (3) the multiple rounds of public comments received during the development of the Integrated Science Assessment and Policy Assessment, both in connection with CASAC meetings and separately; and (4) public comments (including testimony at the public hearings) received on the proposal.

In presenting the rationale for the final decision to retain the current primary PM 10 standard, this section discusses the EPA's past reviews of the PM NAAQS and the general approach taken to review the current standard (section IV.A), the health effects associated with exposures to ambient PM 10-2.5 (section IV.B), the consideration of the current and potential alternative standards in the Policy Assessment (section IV.C), CASAC recommendations regarding the current and potential alternative standards (section IV.D), the Administrator's proposed decision to retain the current primary PM 10 standard (section IV.E), public comments received in response to the Administrator's proposed decision (section IV.F), and the Administrator's final decision to retain the current primary PM 10 standard (section IV.G).

A. Background

The following sections discuss previous reviews of the PM NAAQS (section IV.A.1), the litigation of the EPA's 2006 decision on the PM 10 standards (section IV.A.2), and the general approach taken to review the primary PM 10 standard in the current review (section IV.A.3).

1. Previous Reviews of the PM NAAQS

a. Reviews Completed in 1987 and 1997

The PM NAAQS have always included some type of a primary standard to protect against effects associated with exposures to thoracic coarse particles. In 1987, when the EPA first revised the PM NAAQS, the EPA changed the indicator for PM from TSP to focus on inhalable particles, those which can penetrate into the trachea, bronchi, and deep lungs (52 FR 24634, July 1, 1987). In that review, the EPA changed the PM indicator to PM 10 based on evidence that the risk of adverse health effects associated with particles with a nominal mean aerodynamic diameter less than or equal to 10 μm was significantly greater than risks associated with larger particles (52 FR 24639, July 1, 1987).

In the 1997 review, in conjunction with establishing new fine particle (i.e., PM 2.5) standards (discussed above in sections II.B.1 and III.A.1), the EPA concluded that continued protection was warranted against potential effects associated with thoracic coarse particles in the size range of 2.5 to 10 μm. This conclusion was based on particle dosimetry, toxicological information, and on limited epidemiological evidence from studies that measured PM 10 in areas where the coarse fraction was likely to dominate PM 10 mass (62 FR 38677, July 18, 1997). The EPA concluded there that a PM 10 standard could provide requisite protection against effects associated with particles in the size range of 2.5 to 10 μm. [118] Although the EPA considered a more narrowly defined indicator for thoracic coarse particles in that review (i.e., PM 10-2.5), the EPA concluded that it was more appropriate, based on existing evidence, to continue to use PM 10 as the indicator. This decision was based, in part, on the recognition that the only studies of clear quantitative relevance to health effects most likely associated with thoracic coarse particles used PM 10. These were two studies conducted in areas where the coarse fraction was the dominant fraction of PM 10, and which substantially exceeded the 24-hour PM 10 standard (62 FR 38679). In addition, there were only very limited ambient air quality data then available specifically for PM 10-2.5, in contrast to the extensive monitoring network already in place for PM 10. Therefore, the EPA considered it more administratively feasible to use PM 10 as an indicator. The EPA also stated that the PM 10 standards would work in conjunction with the PM 2.5 standards by regulating the portion of particulate pollution not regulated by the then newly adopted PM 2.5 standards.

In May 1998, a three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit found “ample support” for the EPA's decision to regulate coarse particle pollution, but vacated the 1997 PM 10 standards, concluding that the EPA had failed to adequately explain its choice of PM 10 as the indicator for thoracic coarse particles American Trucking Associations v. EPA, 175 F. 3d 1027, 1054-56 (D.C. Cir. 1999). In particular, the court held that the EPA had not explained the use of an indicator under which the allowable level of coarse particles varied according to the amount of PM 2.5 present, and which, moreover, potentially double regulated PM 2.5. The court also rejected considerations of administrative feasibility as justification for use of PM 10 as the indicator for thoracic coarse PM, since NAAQS (and their elements) are to be based exclusively on health and welfare considerations. Id. at 1054. Pursuant to the court's decision, the EPA removed the vacated 1997 PM 10 standards from the CFR (69 FR 45592, July 30, 2004) and deleted the regulatory provision (at 40 CFR 50.6(d)) that controlled the transition from the pre-existing 1987 PM 10 standards to the 1997 PM 10 standards (65 FR 80776, December 22, 2000). The pre-existing 1987 PM 10 standards thus remained in place. Id. at 80777.

b. Review Completed in 2006

In the review of the PM NAAQS that concluded in 2006, the EPA considered the growing, but still limited, body of evidence supporting associations between health effects and thoracic coarse particles measured as PM 10-2.5. [119] The new studies available in the 2006 review included epidemiological studies that reported associations with health effects using direct measurements of PM 10-2.5, as well as dosimetric and toxicological studies. In considering this growing body of PM 10-2.5 evidence, as well as evidence from studies that measured PM 10 in locations where the majority of PM 10 was in the PM 10-2.5 fraction (U.S. EPA, 2005, section 5.4.1), staff concluded that the level of protection afforded by the existing 1987 PM 10 standard remained appropriate (U.S. EPA, 2005, p. 5-67) but recommended that the indicator for the standard be revised. Specifically, staff recommended replacing the PM 10 indicator with an indicator of urban thoracic coarse particles in the size range of 10-2.5 μm (U.S. EPA, 2005, pp. 5-70 to 5-71). The agency proposed to retain a standard for a subset of thoracic coarse particles, proposing a qualified PM 10-2.5 indicator to focus on the mix of thoracic coarse particles generally present in urban environments. More specifically, the proposed revised thoracic coarse particle standard would have applied only to an ambient mix of PM 10-2.5 dominated by resuspended dust from high-density traffic on paved roads and/or by industrial and construction sources. The proposed revised standard would not have applied to any ambient mix of PM 10-2.5 dominated by rural windblown dust and soils. In addition, agricultural sources, mining sources, and other similar sources of crustal material would not have been subject to control in meeting the standard (71 FR 2667 to 2668, January 17, 2006).

The Agency received a large number of comments overwhelmingly and persuasively opposed to the proposed qualified PM 10-2.5 indicator (71 FR 61188 to 61197, October 17, 2006). After careful consideration of the scientific evidence and the recommendations contained in the 2005 Staff Paper, the advice and recommendations from CASAC, and the public comments received regarding the appropriate indicator for coarse particles, and after extensive evaluation of the alternatives available to the Agency, the Administrator decided it would not be appropriate to adopt the proposed qualified PM 10-2.5 indicator, or any qualified indicator. Underlying this determination was the Administrator's decision that it was requisite to provide protection from exposure to all thoracic coarse PM, regardless of its origin. The Administrator thus rejected arguments that there are no health effects from community-level exposures to coarse PM in non-urban areas (71 FR 61189). The EPA concluded that dosimetric, toxicological, occupational and epidemiological evidence supported retention of a primary standard for short-term exposures that included all thoracic coarse particles (i.e., particles of both urban and non-urban origin), consistent with the Act's requirement that primary NAAQS must be requisite to protect the public health and provide an adequate margin of safety. At the same time, the Agency concluded that the standard should target protection toward urban areas, where the evidence of health effects from exposure to PM 10-2.5 was strongest (71 FR at 61193, 61197). The proposed indicator was not suitable for that purpose. Not only did it inappropriately provide no protection at all to many areas, but it failed to identify many areas where the ambient particle mix was dominated by coarse particles contaminated with urban/industrial types of coarse particles for which evidence of health effects was strongest (71 FR 61193).

The Agency ultimately concluded that the existing indicator, PM 10, was most consistent with the evidence. Although PM 10 includes both coarse and fine PM, the Agency concluded that it remained an appropriate indicator for thoracic coarse particles because, as discussed in the PM Staff Paper (U.S. EPA, 2005, p. 2-54, Figures 2-23 and 2-24), fine particle levels are generally higher in urban areas and, therefore, a PM 10 standard set at a single unvarying level will generally result in lower allowable concentrations of thoracic coarse particles in urban areas than in non-urban areas (71 FR 61195-96). The EPA considered this to be an appropriate targeting of protection given that the strongest evidence for effects associated with thoracic coarse particles came from epidemiological studies conducted in urban areas and that elevated fine particle concentrations in urban areas could result in increased contamination of coarse fraction particles by PM 2.5, potentially increasing the toxicity of thoracic coarse particles in urban areas (id.). Given the evidence that the existing (i.e., 1987) PM 10 standard was established at a level and form which afforded requisite protection with an adequate margin of safety, the Agency retained the level and form of the 24-hour PM 10 standard. [120]

The Agency also revoked the annual PM 10 standard, in light of the conclusion in the PM Criteria Document (U.S. EPA, 2004, p. 9-79) that the available evidence does not suggest an association with long-term exposure to PM 10-2.5 and the conclusion in the Staff Paper (U.S. EPA, 2005, p. 5-61) that there is no quantitative evidence that directly supports retention of an annual standard. This decision was consistent with CASAC advice and recommendations (Henderson, 2005a,b).

In the same rulemaking, the EPA also included a new FRM for the measurement of PM 10-2.5 in the ambient air (71 FR 61212 to 61213, October 17, 2006). Although the standard for thoracic coarse particles does not use a PM 10-2.5 indicator, the new FRM for PM 10-2.5 was established to provide a basis for approving FEMs and to promote the gathering of scientific data to support future reviews of the PM NAAQS (71 FR 61202/3, October 17, 2006). [121]

2. Litigation Related to the 2006 Primary PM 10 Standards

A number of groups filed suit in response to the final decisions made in the 2006 review. See American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). Among the petitions for review were challenges from industry groups on the decision to retain the PM 10 indicator and the level of the PM 10 standard and from environmental and public health groups on the decision to revoke the annual PM 10 standard. The court upheld both the decision to retain the 24-hour PM 10 standard and the decision to revoke the annual standard.

First, the court upheld the EPA's decision for a standard to encompass all thoracic coarse PM, both of urban and non-urban origin. The court rejected arguments that the evidence showed there are no risks from exposure to non-urban coarse PM. The court further found that the EPA had a reasonable basis not to set separate standards for urban and non-urban coarse PM, namely the inability to reasonably define what ambient mixes would be included under either `urban' or `non-urban;' and the evidence in the record that supported the EPA's appropriately cautious decision to provide “some protection from exposure to thoracic coarse particles * * * in all areas.” 559 F. 3d at 532-33. Specifically, the court stated,

Although the evidence of danger from coarse PM is, as EPA recognizes, “inconclusive,” (71 FR 61193, October 17, 2006), the agency need not wait for conclusive findings before regulating a pollutant it reasonably believes may pose a significant risk to public health. The evidence in the record supports the EPA's cautious decision that “some protection from exposure to thoracic coarse particles is warranted in all areas.”Id. As the court has consistently reaffirmed, the CAA permits the Administrator to “err on the side of caution” in setting NAAQS. 559 F. 3d at 533.

The court also upheld the EPA's decision to retain the level of the standard at 150 μg/m [3] and to use PM 10 as the indicator for thoracic coarse particles. In upholding the level of the standard, the court referred to the conclusion in the Staff Paper that there is “little basis for concluding that the degree of protection afforded by the current PM 10 standards in urban areas is greater than warranted, since potential mortality effects have been associated with air quality levels not allowed by the current 24-hour standard, but have not been associated with air quality levels that would generally meet that standard, and morbidity effects have been associated with air quality levels that exceeded the current 24-hour standard only a few times.” 559 F. 3d at 534. The court also rejected arguments that a PM 10 standard established at an unvarying level will result in arbitrarily varying levels of protection given that the level of coarse PM would vary based on the amount of fine PM present. The court agreed that the variation in allowable coarse PM was in accord with the strength of the evidence: Typically less coarse PM would be allowed in urban areas (where levels of fine PM are typically higher), in accord with the strongest evidence of health effects from coarse particles. 559 F. 3d at 535-36. In addition, such regulation would not impermissibly double regulate fine particles, since any additional control of fine particles (beyond that afforded by the primary PM 2.5 standard) would be for a different purpose: To prevent contamination of coarse particles by fine particles. 559 F. 3d at 535, 536. These same explanations justified the choice of PM 10 as an indicator and provided the reasoned explanation for that choice lacking in the record for the 1997 standard. 559 F. 3d at 536.

With regard to the challenge from environmental and public health groups, the court upheld the EPA's decision to revoke the annual PM 10 standard. The court rejected the argument that the EPA is required by law to have an annual PM 10 standard, holding that section 109(d)(1) of the Act allows the EPA to revoke a standard no longer warranted by the current scientific understanding. 559 F. 3d at 538. The court further held that the EPA's decision to revoke the annual standard was supported by the science:

The EPA reasonably decided that an annual coarse PM standard is not necessary because, as the Criteria Document and the Staff Paper make clear, the latest scientific data do not indicate that long-term exposure to coarse particles poses a health risk. The CASAC also agreed that an annual coarse PM standard is unnecessary. 559 F. 3d at 538-39.

3. General Approach Used in the Current Review

The approach taken to considering the existing and potential alternative primary PM 10 standards in the current review builds upon the approaches used in previous PM NAAQS reviews. This approach is based most fundamentally on using information from epidemiological studies and air quality analyses to inform the identification of a range of policy options for consideration by the Administrator. The Administrator considers the appropriateness of the current and potential alternative standards, taking into account the four elements of the NAAQS: Indicator, averaging time, form, and level.

Evidence-based approaches to using information from epidemiological studies to inform decisions on PM standards are complicated by the recognition that no population threshold, below which it can be concluded with confidence that PM-related effects do not occur, can be discerned from the available evidence (U.S. EPA, 2009a, sections 2.4.3 and 6.5.2.7). [122] As a result, any approach to reaching decisions on what standards are appropriate requires judgments about how to translate the information available from the epidemiological studies into a basis for appropriate standards, which includes consideration of how to weigh the uncertainties in reported associations across the distributions of PM concentrations in the studies. The approach taken to informing these decisions in the current review recognizes that the available health effects evidence reflects a continuum consisting of ambient levels at which scientists generally agree that health effects are likely to occur through lower levels at which the likelihood and magnitude of the response become increasingly uncertain. Such an approach is consistent with setting standards that are neither more nor less stringent than necessary, recognizing that a zero-risk standard is not required by the CAA.

Because the purpose of the PM 10 standard is to protect against exposures to PM 10-2.5, it is most appropriate to focus on PM 10-2.5 health studies when considering the degree of public health protection provided by the current PM 10 standard. Compared to health studies of PM 10, studies that evaluate associations with PM 10-2.5 provide clearer evidence for health effects following exposures to thoracic coarse particles. In contrast, it is difficult to interpret PM 10 studies within the context of a standard meant to protect against exposures to PM 10-2.5 because PM 10 is comprised of both fine and coarse particles, even in locations with the highest concentrations of PM 10-2.5 (U.S. EPA, 2011a, Figure 3-4). Therefore, the extent to which PM 10 effect estimates reflect associations with PM 10-2.5 versus PM 2.5 can be highly uncertain. In light of this uncertainty, it is preferable to consider PM 10-2.5 studies when such studies are available. Given the availability in this review of a number of studies that evaluated associations with PM 10-2.5, and given that the Integrated Science Assessment weight-of-evidence conclusions for thoracic coarse particles were based on studies of PM 10-2.5, in this review the EPA focuses primarily on studies that have specifically evaluated PM 10-2.5. [123]

As discussed in more detail in the Risk Assessment (U.S. EPA, 2010a, Appendix H), the EPA did not conduct a quantitative assessment of health risks associated with PM 10-2.5. The Risk Assessment concluded that limitations in the monitoring network and in the health studies that rely on that monitoring network, which would be the basis for estimating PM 10-2.5 health risks, would introduce significant uncertainty into a PM 10-2.5 risk assessment such that the risk estimates generated would be of limited value in informing review of the standard. Therefore, it was judged that a quantitative assessment of PM 10-2.5 risks is not supportable at this time (U.S. EPA, 2010a, p. 2-6). This decision does not indicate that health effects are not associated with exposure to thoracic coarse particles. Rather, as noted above, it reflects the conclusion that limitations in the available health studies and air quality information would introduce significant uncertainty into a quantitative assessment of PM 10-2.5 risks such that the risk estimates generated would be of limited value in informing review of the standard.

B. Health Effects Related to Exposure to Thoracic Coarse Particles

This section briefly outlines the key information presented in section IV.B of the proposal (77 FR 38947 to 38951, June 29, 2012), and discussed more fully in the Integrated Science Assessment (U.S. EPA, 2009a, Chapters 2, 4, 5, 6, 7, and 8) and the Policy Assessment (U.S. EPA, 2011a, Chapter 3), related to health effects associated with thoracic coarse particle exposures. In looking across the new scientific evidence available in this review, our overall understanding of health effects associated with thoracic coarse particle exposures has been expanded, though important uncertainties remain. Some highlights of the key policy-relevant scientific evidence available in this review include the following:

(1) A number of multi-city and single-city epidemiological studies have evaluated associations between short-term PM 10-2.5 and mortality, cardiovascular effects (e.g., including hospital admissions and emergency department visits), and/or respiratory effects. Despite differences in the approaches used to estimate ambient PM 10-2.5 concentrations, the majority of these studies have reported positive, though often not statistically significant, associations with short-term PM 10-2.5 concentrations. Most PM 10-2.5 effect estimates remained positive in co-pollutant models that included either gaseous or particulate co-pollutants. In U.S. study locations likely to have met the current PM 10 standard during the study period, a few PM 10-2.5 effect estimates were statistically significant and remained so in co-pollutant models. [124]

(2) A small number of controlled human exposure studies have reported alterations in heart rate variability or increased pulmonary inflammation following short-term exposure to PM 10-2.5, providing some support for the associations reported in epidemiological studies. Toxicological studies that have examined the effects of PM 10-2.5 have used intratracheal instillation and, because these studies do not directly mirror any real-world mode of exposure, they provide only limited evidence for the biological plausibility of PM 10-2.5-induced effects.

(3) Using a more formal framework for reaching causal determinations than used in previous reviews, the Integrated Science Assessment concluded that the existing evidence is “suggestive” of a causal relationship between short-term PM 10-2.5 exposures and mortality, cardiovascular effects, and respiratory effects (U.S. EPA, 2009a, section 2.3.3). [125] In contrast, the Integrated Science Assessment concluded that available evidence is “inadequate” to infer a causal relationship between long-term PM 10-2.5 exposures and various health effects.

(4) There are several at-risk populations that may be especially susceptible or vulnerable to PM-related effects, including effects associated with exposures to coarse particles. These groups include those with preexisting heart and lung diseases, specific genetic differences, and lower socioeconomic status as well as the lifestages of childhood and older adulthood. Evidence for PM-related effects in these at-risk populations has expanded and is stronger than previously observed. There is emerging, though still limited, evidence for additional potentially at-risk populations, such as those with diabetes, people who are obese, pregnant women, and the developing fetus.

(5) The Integrated Science Assessment concludes that currently available evidence is insufficient to draw distinctions in particle toxicity based on composition and notes that recent studies have reported that PM (both PM 2.5 and PM 10-2.5) from a variety of sources, including sources likely to be present in urban and non-urban locations, is associated with adverse health effects.

Although new PM 10-2.5 scientific studies have become available since the last review and have expanded our understanding of the association between PM 10-2.5 and adverse health effects (see above and U.S. EPA, 2009a, Chapter 6), important uncertainties remain. These uncertainties, and their implications for interpreting the scientific evidence, include the following:

(1) The potential for confounding by co-occurring pollutants, especially PM 2.5, has been addressed with co-pollutant models in only a relatively small number of PM 10-2.5 epidemiological studies (U.S. EPA, 2009a, section 2.3.3). This is a particularly important limitation given the relatively small body of experimental evidence (i.e., controlled human exposure and animal toxicological studies) available to support the associations between PM 10-2.5 and adverse health effects. The net impact of such limitations is to increase uncertainty in characterizations of the extent to which PM 10-2.5 itself, rather than one or more co-occurring pollutants, is responsible for the mortality and morbidity effects reported in epidemiological studies.

(2) There is greater spatial variability in PM 10-2.5 concentrations than PM 2.5 concentrations, resulting in increased exposure error for PM 10-2.5 (U.S. EPA, 2009a, p. 2-8). Available measurements do not provide sufficient information to adequately characterize the spatial distribution of PM 10-2.5 concentrations (U.S. EPA, 2009a, section 3.5.1.1). The net effect of these uncertainties on PM 10-2.5 epidemiological studies is to bias the results of such studies toward the null hypothesis. That is, as noted in the Integrated Science Assessment, these limitations in estimates of ambient PM 10-2.5 concentrations “would tend to increase uncertainty and make it more difficult to detect effects of PM 10-2.5 in epidemiologic studies” (U.S. EPA, 2009a, p. 2-21).

(3) Only a relatively small number of PM 10-2.5 monitoring sites are currently operating and such sites have been in operation for a relatively short period of time, limiting the spatial and temporal coverage for routine measurement of PM 10-2.5 concentrations. Given these limitations in routine monitoring, epidemiological studies have employed different approaches for estimating PM 10-2.5 concentrations. Given the relatively small number of PM 10-2.5 monitoring sites, the relatively large spatial variability in ambient PM 10-2.5 concentrations (see above), the use of different approaches to estimating ambient PM 10-2.5 concentrations across epidemiological studies, and the limitations inherent in such estimates, the distributions of thoracic coarse particle concentrations over which reported health outcomes occur remain highly uncertain (U.S. EPA, 2009a, sections 2.2.3, 2.3.3, 2.3.4, and 3.5.1.1).

(4) There is relatively little information on the chemical and biological composition of PM 10-2.5 and the effects associated with the various components (U.S. EPA, 2009a, section 2.3.4). Without more information on the chemical speciation of PM 10-2.5, the apparent variability in associations with health effects across locations is difficult to characterize (U.S. EPA, 2009a, section 6.5.2.3).

(5) One of the implications of the uncertainties and limitations discussed above is that the Risk Assessment concluded it would not be appropriate to conduct a quantitative assessment of health risks associated with PM 10-2.5. The lack of a quantitative PM 10-2.5 risk assessment in the current review adds to the uncertainty in any conclusions about the extent to which revision of the current PM 10 standard would be expected to improve the protection of public health, beyond the protection provided by the current standard. [126]

C. Consideration of the Current and Potential Alternative Standards in the Policy Assessment

The following sections discuss the Policy Assessment's consideration of the current and potential alternative standards to protect against exposures to thoracic coarse particles (U.S. EPA, 2011a, chapter 3). Section IV.C.1 discusses the consideration of the current standard while section IV.C.2 discusses the consideration of potential alternative standards in terms of the basic elements of a standard: Indicator, averaging time, form, and level.

1. Consideration of the Current Standard in the Policy Assessment

As discussed above the 24-hour PM 10 standard is meant to protect the public health against exposures to thoracic coarse particles (i.e., PM 10-2.5). In considering the adequacy of the current PM 10 standard, the Policy Assessment considered the health effects evidence linking short-term PM 10-2.5 exposures with mortality and morbidity (U.S. EPA, 2009a, chapters 2 and 6), the ambient PM 10 concentrations in PM 10-2.5 study locations (U.S. EPA, 2011a, section 3.2.1), the uncertainties and limitations associated with this health evidence (U.S. EPA, 2011a, section 3.2.1), and the consideration of these uncertainties and limitations as part of the weight of evidence conclusions in the Integrated Science Assessment (U.S. EPA, 2009a).

In considering the health evidence, air quality information, and associated uncertainties as they relate to the current PM 10 standard, the Policy Assessment noted that a decision on the adequacy of the public health protection provided by that standard is a public health policy judgment in which the Administrator weighs the evidence and information, as well as its uncertainties. Therefore, depending on the emphasis placed on different aspects of the evidence, information, and uncertainties, consideration of different conclusions on the adequacy of the current standard could be supported. For example, the Policy Assessment noted that one approach to considering the evidence, information, and its associated uncertainties would be to place emphasis on the following (U.S. EPA, 2011a, section 3.2.3):

(1) While most of PM 10-2.5 effect estimates reported for mortality and morbidity were positive, many were not statistically significant, even in single-pollutant models. This includes effect estimates reported in study locations with PM 10 concentrations above those allowed by the current 24-hour PM 10 standard.

(2) The number of epidemiological studies that have employed co-pollutant models to address the potential for confounding, particularly by PM 2.5, remains limited. Therefore, the extent to which PM 10-2.5 itself, rather than one or more co-pollutants, contributes to reported health effects remains uncertain.

(3) Only a limited number of experimental studies provide support for the associations reported in epidemiological studies, resulting in further uncertainty regarding the plausibility of a causal link between PM 10-2.5 and mortality and morbidity.

(4) Limitations in PM 10-2.5 monitoring and the different approaches used to estimate PM 10-2.5 concentrations across epidemiological studies result in uncertainty as to the ambient PM 10-2.5 concentrations at which the reported effects occur.

(5) The chemical and biological composition of PM 10-2.5, and the effects associated with the various components, remains uncertain. Without more information on the chemical speciation of PM 10-2.5, the apparent variability in associations across locations is difficult to interpret.

(6) In considering the available evidence and its associated uncertainties, the Integrated Science Assessment concluded that the evidence is “suggestive” of a causal relationship between short-term PM 10-2.5 exposures and mortality, cardiovascular effects, and respiratory effects. These weight-of-evidence conclusions contrast with those for the relationships between PM 2.5 exposures and adverse health effects, which were judged in the Integrated Science Assessment to be either “causal” or “likely causal” for mortality, cardiovascular effects, and respiratory effects.

The Policy Assessment concluded that, to the extent a decision on the adequacy of the current 24-hour PM 10 standard were to place emphasis on the considerations noted above, it could be judged that, although it remains appropriate to maintain a standard to protect against short-term exposures to thoracic coarse particles, the available evidence suggests that the current 24-hour PM 10 standard appropriately protects public health and provides an adequate margin of safety against effects that have been associated with PM 10-2.5 exposures. Although such an approach to considering the adequacy of the current standard would recognize the positive, and in some cases statistically significant, associations between all types of PM 10-2.5 and mortality and morbidity, it would place relatively greater emphasis on the limitations and uncertainties noted above, which tend to complicate the interpretation of that evidence.

In addition, the Policy Assessment noted the judgment that, given the uncertainties and limitations in the PM 10-2.5 health evidence and air quality information, it would not have been appropriate to conduct a quantitative assessment of health risks associated with PM 10-2.5 (U.S. EPA, 2011a, p. 3-6; U.S. EPA, 2010a, pp. 2-6 to 2-7, Appendix H). As discussed above, the lack of a quantitative PM 10-2.5 risk assessment adds to the uncertainty associated with any characterization of potential public health improvements that would be realized with a revised standard.

The Policy Assessment also noted an alternative approach to considering the evidence and its uncertainties would place emphasis on the following (U.S. EPA, 2011a, section 3.2.3):

(1) Several multi-city epidemiological studies conducted in the U.S., Canada, and Europe, as well as a number of single-city studies, have reported generally positive, and in some cases statistically significant, associations between short-term PM 10-2.5 concentrations and adverse health endpoints including mortality and cardiovascular-related and respiratory-related hospital admissions and emergency department visits.

(2) Both single-city and multi-city analyses, using different approaches to estimate ambient PM 10-2.5 concentrations, have reported positive PM 10-2.5 effect estimates in locations that would likely have met the current 24-hour PM 10 standard. In a few cases, these PM 10-2.5 effect estimates were statistically significant.

(3) While limited in number, studies that have evaluated co-pollutant models have generally reported that PM 10-2.5 effect estimates remain positive, and in a few cases statistically significant, when these models include gaseous pollutants or fine particles.

(4) Support for the plausibility of the associations reported in epidemiological studies is provided by a small number of controlled human exposure studies reporting that short-term (i.e., 2-hour) exposures to PM 10-2.5 decrease heart rate variability and increase markers of pulmonary inflammation.

This approach to considering the health evidence, air quality information, and the associated uncertainties would place substantial weight on the generally positive PM 10-2.5 effect estimates that have been reported for mortality and morbidity, even those effect estimates that are not statistically significant. The Policy Assessment concluded that this could be judged appropriate given that consistent results have been reported across multiple studies using different approaches to estimate ambient PM 10-2.5 concentrations and that exposure measurement error, which is likely to be larger for PM 10-2.5 than for PM 2.5, tends to bias the results of epidemiological studies toward the null hypothesis, making it less likely that associations will be detected. Such an approach would place less weight on the uncertainties and limitations in the evidence that resulted in the Integrated Science Assessment conclusions that the evidence is only suggestive of a causal relationship.

Given all of the above, the Policy Assessment concluded that it would be appropriate to consider either retaining or revising the current 24-hour PM 10 standard, depending on the approach taken to considering the available evidence, air quality information, and the uncertainties and limitations associated with that evidence and information (U.S. EPA, 2011a, section 3.2.3).

2. Consideration of Potential Alternative Standards in the Policy Assessment

Given the conclusion that it would be appropriate to consider either retaining or revising the current PM 10 standard, the Policy Assessment also considered what potential alternative standards, if any, could be supported by the available scientific evidence in order to increase public health protection against exposures to PM 10-2.5. The Policy Assessment considered such potential alternative standards defined in terms of the elements of a standard (i.e., indicator, averaging time, form, and level). Key conclusions from the Policy Assessment regarding indicator, averaging time, and form included the following:

(1) A PM 10 indicator would continue to appropriately target protection against thoracic coarse particle exposures to those locations where the evidence is strongest for associations with adverse health effects (i.e., urban areas).

(2) The available evidence supports the importance of maintaining a standard that protects against short-term exposures to all thoracic coarse particles. Given that the majority of this evidence is based on 24-hour average thoracic coarse particle concentrations, consideration of a 24-hour averaging time remains appropriate.

(3) Given the limited body of evidence supporting PM 10-2.5-related effects following long-term exposures, which resulted in the Integrated Science Assessment conclusion that the available evidence is “inadequate” to infer a causal relationship between long-term PM 10-2.5 exposures and a variety of health effects, consideration of an annual thoracic coarse particle standard is not supported at this time.

(4) To the extent it is judged appropriate to revise the current 24-hour PM 10 standard, it would be appropriate to consider revising the form to the 3-year average of the 98th percentile of the annual distribution of 24-hour PM 10 concentrations.

In considering the available evidence and air quality information within the context of identifying potential alternative standard levels for consideration (assuming a decision were made that it is appropriate to amend the standard), the Policy Assessment first noted that a standard level as high as about 85 μg/m [3] , for a 24-hour PM 10 standard with a 98th percentile form, could be supported. Based on considering air quality concentrations in study locations, the Policy Assessment noted that such a standard level would be expected to maintain PM 10 and PM 10-2.5 concentrations below those present in U.S. locations of single-city studies where PM 10-2.5 effect estimates have been reported to be positive and statistically significant and below those present in some locations where single-city studies reported PM 10-2.5 effect estimates that were positive, but not statistically significant. These include some locations likely to have met the current PM 10 standard during the study periods (U.S. EPA, 2011a, section 3.3.4).

The Policy Assessment also noted that, based on analysis of the number of people living in counties that could violate the current and potential alternative PM 10 standards, a 24-hour PM 10 standard with a 98th percentile form and a level between 75 and 80 μg/m [3] would provide a level of public health protection that is generally equivalent, across the U.S., to that provided by the current standard. Given this, the Policy Assessment concluded that it would be appropriate to consider standard levels in the range of approximately 75 to 80 μg/m [3] (with a 98th percentile form), to the extent population counts were emphasized in comparing the public health protection provided by the current and potential alternative standards and to the extent it was judged appropriate to set a revised standard providing at least the level of public health protection that is provided by the current standard, based on such population counts (U.S. EPA, 2011a, section 3.3.4).

The Policy Assessment also concluded that alternative approaches to considering the evidence could lead to consideration of standard levels below 75 μg/m [3] for a standard with a 98th percentile form. For example, a number of single-city epidemiological studies have reported positive, though not statistically significant, PM 10-2.5 effect estimates in locations with 98th percentile PM 10 concentrations below 75 μg/m [3] . Given that exposure error is particularly important for PM 10-2.5 epidemiological studies and can bias the results of these studies toward the null hypothesis (see section IV.B above), the Policy Assessment noted that it could be judged appropriate to place more weight on positive associations reported in these epidemiological studies, even when those associations are not statistically significant. In addition, the Policy Assessment noted that multi-city averages of 98th percentile PM 10 concentrations in the locations evaluated by U.S. multi-city studies of thoracic coarse particles (Zanobetti and Schwartz, 2009; Peng et al., 2008) were near or below 75 ppb. Despite uncertainties in the extent to which effects reported in multi-city studies are associated with the short-term air quality in any particular location, the Policy Assessment noted that emphasis could be placed on these multi-city averaged concentrations. The Policy Assessment concluded that, to the extent more weight is placed on single-city studies reporting positive, but not statistically significant, PM 10-2.5 effect estimates and on multi-city studies, it could be appropriate to consider standard levels as low as 65 μg/m [3] with a 98th percentile form (U.S. EPA, 2011a, section 3.3.4).

In considering potential alternative standard levels below 65 μg/m [3] , the Policy Assessment noted that the overall body of PM 10-2.5 health evidence is relatively uncertain, with somewhat stronger support in U.S. studies for associations with PM 10-2.5 in locations with 98th percentile PM 10 concentrations above 85 μg/m [3] than in locations with 98th percentile PM 10 concentrations below 65 μg/m [3] . In light of the limitations in the evidence for a relationship between PM 10-2.5 and adverse health effects in locations with relatively low PM 10 concentrations, along with the overall uncertainties in the body of PM 10-2.5 health evidence as described above and in the Integrated Science Assessment, the Policy Assessment concluded that consideration of standard levels below 65 μg/m [3] was not appropriate (U.S. EPA, 2011a, section 3.3.4).

D. CASAC Advice

Following their review of the first and second draft Policy Assessments, CASAC provided advice and recommendations regarding the current and potential alternative standards for thoracic coarse particles (Samet, 2010c,d). With regard to the existing PM 10 standard, CASAC concluded that “the current data, while limited, is sufficient to call into question the level of protection afforded the American people by the current standard” (Samet, 2010d, p. 7). In drawing this conclusion, CASAC noted the positive associations in multi-city and single-city studies, including in locations with PM 10 concentrations below those allowed by the current standard. In addition, CASAC gave “significant weight to studies that have generally reported that PM 10-2.5 effect estimates remain positive when evaluated in co-pollutant models” and concluded that “controlled human exposure PM 10-2.5 studies showing decreases in heart rate variability and increases in markers of pulmonary inflammation are deemed adequate to support the plausibility of the associations reported in epidemiologic studies” (Samet, 2010d, p. 7). [127] Given all of the above conclusions CASAC recommended that “the primary standard for PM 10 should be revised” (Samet, 2010d, p. ii and p. 7). In discussing potential revisions, while CASAC noted that the scientific evidence supports adoption of a standard at least as stringent as the current standard, they recommended revising the current standard in order to increase public health protection. In considering potential alternative standards, CASAC drew conclusions and made recommendations in terms of the major elements of a standard: indicator, averaging time, form, and level.

The CASAC agreed with the EPA staff's conclusions that the available evidence supports consideration in the current review of retaining the current PM 10 indicator and the current 24-hour averaging time (Samet, 2010c, Samet, 2010d). Specifically, with regard to indicator, CASAC concluded that “[w]hile it would be preferable to use an indicator that reflects the coarse PM directly linked to health risks (PM 10-2.5), CASAC recognizes that there is not yet sufficient data to permit a change in the indicator from PM 10 to one that directly measures thoracic coarse particles” (Samet, 2010d, p. ii). In addition, CASAC “vigorously recommends the implementation of plans for the deployment of a network of PM 10-2.5 sampling systems so that future epidemiological studies will be able to more thoroughly explore the use of PM 10-2.5 as a more appropriate indicator for thoracic coarse particles” (Samet, 2010d, p. 7).

The CASAC also agreed that the evidence supports consideration of a potential alternative form. Specifically, CASAC “felt strongly that it is appropriate to change the statistical form of the PM 10 standard to a 98th percentile” (Samet, 2010d, p.7). In reaching this conclusion, CASAC noted that “[p]ublished work has shown that the percentile form has greater power to identify non-attainment and a smaller probability of misclassification relative to the expected exceedance form of the standard” (Samet, 2010d. p. 7).

With regard to standard level, in conjunction with a 98th percentile form, CASAC concluded that “alternative standard levels of 85 and 65 μg/m [3] (based on consideration of 98th percentile PM 10 concentration) could be justified” (Samet, 2010d, p.8). However, in considering the evidence and uncertainties, CASAC recommended a standard level from the lower part of the range discussed in the Policy Assessment, recommending a level “somewhere in the range of 75 to 65 μg/m [3] ” (Samet, 2010d, p. ii).

In making this recommendation, CASAC noted that the number of people living in counties with air quality not meeting the current standard is approximately equal to the number living in counties that would not meet a 98th percentile standard with a level between 75 and 80 μg/m [3] . CASAC used this information as the basis for their conclusion that a 98th percentile standard between 75 and 80 μg/m [3] would be “comparable to the degree of protection afforded to the current PM 10 standard” (Samet, 2010d, p. ii). Given this conclusion regarding the comparability of the current and potential alternative standards, as well as their conclusion on the public health protection provided by the current standard (i.e., that available evidence is sufficient to call it into question), CASAC recommended a level within a range of 75 to 65 μg/m [3] in order to increase public health protection, relative to that provided by the current standard (Samet 2010d, p. ii).

E. Administrator's Proposed Conclusions Concerning the Adequacy of the Current Primary PM 10 Standard

In considering the evidence and information as they relate to the adequacy of the current 24-hour PM 10 standard, the Administrator first noted in the proposal that this standard is meant to protect the public health against effects associated with short-term exposures to PM 10-2.5. In the last review, it was judged appropriate to maintain such a standard given the “growing body of evidence suggesting causal associations between short-term exposure to thoracic coarse particles and morbidity effects, such as respiratory symptoms and hospital admissions for respiratory diseases, and possibly mortality” (71 FR 61185, October 17, 2006). Given the continued expansion in the body of scientific evidence linking short-term PM 10-2.5 to health outcomes such as premature death and hospital visits, discussed in detail in the Integrated Science Assessment (U.S. EPA, 2009a, Chapter 6) and summarized in the proposal, the Administrator provisionally concluded that the available evidence continued to support the appropriateness of maintaining a standard to protect the public health against effects associated with short-term (e.g., 24-hour) exposures to all PM 10-2.5. In drawing provisional conclusions in the proposal as to whether the current PM 10 standard remains requisite (i.e., neither more nor less stringent than necessary) to protect public health with an adequate margin of safety against such exposures, the Administrator considered the following:

(1) The extent to which it is appropriate to maintain a standard that provides some measure of protection against all PM 10-2.5, regardless of composition or source of origin;

(2) The extent to which it is appropriate to retain a PM 10 indicator for a standard meant to protect against exposures to ambient PM 10-2.5; and

(3) The extent to which the current PM 10 standard provides an appropriate degree of public health protection.

With regard to the first point, the proposal noted the conclusion from the last review that dosimetric, toxicological, occupational, and epidemiological evidence supported retention of a primary standard to provide some measure of protection against short-term exposures to all thoracic coarse particles, regardless of their source of origin or location, consistent with the Act's requirement that primary NAAQS provide requisite protection with an adequate margin of safety (71 FR 61197). In that review, the EPA concluded that PM from a number of source types, including motor vehicle emissions, coal combustion, oil burning, and vegetative burning, are associated with health effects (U.S. EPA, 2004). This information formed part of the basis for the D.C. Circuit's holding that it was appropriate for the thoracic coarse particle standard to provide “some protection from exposure to thoracic coarse particles * * * in all areas” (American Farm Bureau Federation v. EPA, 559 F. 3d at 532-33).

In considering this issue in the proposal, the Administrator judged that the expanded body of scientific evidence in this review provides even more support for a standard that protects against exposures to all thoracic coarse particles, regardless of their location or source of origin. Specifically, the Administrator noted that epidemiological studies have reported positive associations between PM 10-2.5 and mortality or morbidity in a large number of cities across North America, Europe, and Asia, encompassing a variety of environments where PM 10-2.5 sources and composition are expected to vary widely. See 77 FR 38959. In considering this evidence, the Integrated Science Assessment concluded that “many constituents of PM can be linked with differing health effects” (U.S. EPA, 2009a, p. 2-26). While PM 10-2.5 in most of these study areas is of largely urban origin, the Administrator noted that some recent studies have also linked mortality and morbidity with relatively high ambient concentrations of thoracic coarse particles of non-urban crustal origin. In considering these studies, she noted the Integrated Science Assessment's conclusion that “PM (both PM 2.5 and PM 10-2.5) from crustal, soil or road dust sources or PM tracers linked to these sources are associated with cardiovascular effects” (U.S. EPA, 2009a, p. 2-26).

In light of this body of available evidence reporting PM 10-2.5-associated health effects across different locations with a variety of sources, as well as the Integrated Science Assessment's conclusions regarding the links between adverse health effects and PM sources and composition, the Administrator provisionally concluded in the proposal that it is appropriate to maintain a standard that provides some measure of protection against exposures to all thoracic coarse particles, regardless of their location, source of origin, or composition (77 FR 38959-60).

With regard to the second point, in considering the appropriateness of a PM 10 indicator for a standard meant to provide such public health protection, the Administrator noted that the rationale used in the last review to support the unqualified PM 10 indicator (see above) remains relevant in the current review. Specifically, as an initial consideration, she noted that PM 10 mass includes both coarse PM (PM 10-2.5) and fine PM (PM 2.5). As a result, the concentration of PM 10-2.5 allowed by a PM 10 standard set at a single level declines as the concentration of PM 2.5 increases. At the same time, the Administrator noted that PM 2.5 concentrations tend to be higher in urban areas than in rural areas (U.S. EPA, 2005, p. 2-54, and Figures 2-23 and 2-24) and, therefore, a PM 10 standard will generally allow lower PM 10-2.5 concentrations in urban areas than in rural areas. 77 FR 38960.

In considering the appropriateness of this variation in allowable PM 10-2.5 concentrations, the Administrator considered the relative strength of the evidence for health effects associated with PM 10-2.5 of urban origin versus non-urban origin. She specifically noted that, as described above and similar to the scientific evidence available in the last review, the large majority of the available evidence for thoracic coarse particle health effects comes from studies conducted in locations with sources more typical of urban and industrial areas than of rural areas. Although as just noted, associations with adverse health effects have been reported in some study locations where PM 10-2.5 is largely non-urban in origin (i.e., in dust storm studies), particle concentrations in these study areas are typically much higher than reported in study locations where the PM 10-2.5 is of urban origin. Therefore, the Administrator noted that the strongest evidence for a link between PM 10-2.5 and adverse health impacts, particularly for such a link at relatively low particle concentrations, comes from studies where exposure is to PM 10-2.5 of urban or industrial origin. 77 FR 38960.

The Administrator also noted that chemical constituents present at higher levels in urban or industrial areas, including byproducts of incomplete combustion (e.g. polycyclic aromatic hydrocarbons) emitted as PM 2.5 from motor vehicles as well as metals and other contaminants emitted from anthropogenic sources, can contaminate PM 10-2.5 (U.S. EPA, 2004, p. 8-344; 71 FR 2665). While the Administrator acknowledged the uncertainty expressed in the Integrated Science Assessment regarding the extent to which, based on available evidence, particle composition can be linked to health outcomes, she also considered the possibility that PM 10-2.5 contaminants typical of urban or industrial areas could increase the toxicity of thoracic coarse particles in urban locations (77 FR 38960).

Given that the large majority of the evidence for PM 10-2.5 toxicity, particularly at relatively low particle concentrations, comes from study locations where thoracic coarse particles are of urban origin, and given the possibility that PM 10-2.5 contaminants in urban areas could increase particle toxicity, the Administrator provisionally concluded in the proposal that it remains appropriate to maintain a standard that targets public health protection to urban locations. Specifically, she concluded at proposal that it is appropriate to maintain a standard that allows lower ambient concentrations of PM 10-2.5 in urban areas, where the evidence is strongest that thoracic coarse particles are linked to mortality and morbidity, and higher concentrations in non-urban areas, where the public health concerns are less certain. Id.

Given all of the above considerations and conclusions, the Administrator judged that the available evidence supported retaining a PM 10 indicator for a standard that is meant to protect against exposure to thoracic coarse particles. In reaching this initial judgment, she noted that, to the extent a PM 10 indicator results in lower allowable concentrations of thoracic coarse particles in some areas compared to others, lower concentrations will be allowed in those locations (i.e., urban or industrial areas) where the science has shown the strongest evidence of adverse health effects associated with exposure to thoracic coarse particles and where we have the most concern regarding PM 10-2.5 toxicity. Therefore, the Administrator provisionally concluded that the varying amounts of coarse particles that are allowed in urban vs. non-urban areas under the 24-hour PM 10 standard, based on the varying levels of PM 2.5 present, appropriately reflect the differences in the strength of evidence regarding coarse particle effects in urban and non-urban areas (77 FR 38960).

In reaching this provisional conclusion, the Administrator also noted that, in their review of the second draft Policy Assessment, CASAC concluded that “[w]hile it would be preferable to use an indicator that reflects the coarse PM directly linked to health risks (PM 10-2.5), CASAC recognizes that there is not yet sufficient data to permit a change in the indicator from PM 10 to one that directly measures thoracic coarse particles” (Samet, 2010d, p. ii). In addition, CASAC “vigorously recommends the implementation of plans for the deployment of a network of PM 10-2.5 sampling systems so that future epidemiological studies will be able to more thoroughly explore the use of PM 10-2.5 as a more appropriate indicator for thoracic coarse particles” (Samet, 2010d, p. 7). Given this recommendation, the Administrator further judged that, although current evidence is not sufficient to identify a standard based on an alternative indicator that would be requisite to protect public health with an adequate margin of safety across the United States, consideration of alternative indicators (e.g., PM 10-2.5) in future reviews is desirable and could be informed by additional research, as described in the Policy Assessment (U.S. EPA, 2011a, section 3.5).

With regard to the third point, in evaluating the degree of public health protection provided by the current PM 10 standard, the Administrator noted that the Policy Assessment discussed two different approaches to considering the scientific evidence and air quality information (U.S. EPA, 2011a, section 3.2.3). These different approaches, which are described above (section IV.C.1), lead to different conclusions regarding the appropriateness of the degree of public health protection provided by the current PM 10 standard. The Administrator further noted that the primary difference between the two approaches lies in the extent to which weight is placed on the following (U.S. EPA, 2011a, section 3.2.3):

(1) The PM 10-2.5 weight-of-evidence classifications presented in the Integrated Science Assessment concluding that the existing evidence is suggestive of a causal relationship between short-term PM 10-2.5 exposures and mortality, cardiovascular effects, and respiratory effects (a classification supported by CASAC);

(2) Individual PM 10-2.5 epidemiological studies reporting associations in locations that meet the current PM 10 standard, including associations that are not statistically significant;

(3) The limited number of PM 10-2.5 epidemiological studies that have evaluated co-pollutant models;

(4) The limited number of PM 10-2.5 controlled human exposure studies;

(5) Uncertainties in the PM 10-2.5 air quality concentrations reported in epidemiological studies, given limitations in PM 10-2.5 monitoring data and the different approaches used across studies to estimate ambient PM 10-2.5 concentrations; and

(6) Uncertainties and limitations in the evidence that tend to call into question the presence of a causal relationship between PM 10-2.5 exposures and mortality/morbidity.

In evaluating the different possible approaches to considering the public health protection provided by the current PM 10 standard, the Administrator first noted that when the available PM 10-2.5 scientific evidence and its associated uncertainties are considered, the Integrated Science Assessment concluded that the evidence is suggestive of a causal relationship between short-term PM 10-2.5 exposures and mortality, cardiovascular effects, and respiratory effects. As discussed in section IV.B.1 above and in more detail in the Integrated Science Assessment (U.S. EPA, 2009a, section 1.5), a suggestive determination is made when the “[e]vidence is suggestive of a causal relationship with relevant pollutant exposures, but is limited because chance, bias and confounding cannot be ruled out.” In contrast, the Administrator noted that she proposed to strengthen the annual fine particle standard based on a body of scientific evidence judged sufficient to conclude that a causal relationship exists (i.e., mortality, cardiovascular effects) or is likely to exist (i.e., respiratory effects) (section III.B). 77 FR 38961. The suggestive judgment for PM 10-2.5 reflects the greater degree of uncertainty associated with this body of evidence, as discussed above (sections IV.B and IV.C) and summarized below.

In the proposal (77 FR 38961), the Administrator noted that the important uncertainties and limitations associated with the scientific evidence and air quality information raise questions as to whether public health benefits would be achieved by revising the existing PM 10 standard. Such uncertainties and limitations include the following:

(1) While PM 10-2.5 effect estimates reported for mortality and morbidity were generally positive, most were not statistically significant, even in single-pollutant models. This includes effect estimates reported in some study locations with PM 10 concentrations above those allowed by the current 24-hour PM 10 standard.

(2) The number of epidemiological studies that have employed co-pollutant models to address the potential for confounding, particularly by PM 2.5, remains limited. Therefore, the extent to which PM 10-2.5 itself, rather than one or more co-pollutants, contributes to reported health effects is less certain.

(3) Only a limited number of experimental studies (i.e., controlled human exposure and animal toxicological) provide support for the associations reported in epidemiological studies, resulting in further uncertainty regarding the plausibility of the associations between PM 10-2.5 and mortality and morbidity reported in epidemiological studies.

(4) Limitations in PM 10-2.5 monitoring data and the different approaches used by epidemiological study researchers to estimate PM 10-2.5 concentrations across epidemiological studies result in uncertainty in the ambient PM 10-2.5 concentrations at which the reported effects occur, increasing uncertainty in estimates of the extent to which changes in ambient PM 10-2.5 concentrations would likely impact public health.

(5) The lack of a quantitative PM 10-2.5 risk assessment further contributes to uncertainty regarding the extent to which any revisions to the current PM 10 standard would be expected to improve the protection of public health, beyond the protection provided by the current standard (see section III.B.5 above).

(6) The chemical and biological composition of PM 10-2.5, and the effects associated with the various components, remains uncertain. Without more information on the chemical speciation of PM 10-2.5, the apparent variability in associations across locations is difficult to interpret.

In considering these uncertainties and limitations, the Administrator noted in particular the considerable degree of uncertainty in the extent to which health effects reported in epidemiological studies are due to PM 10-2.5 itself, as opposed to one or more co-occurring pollutants. As discussed above, this uncertainty reflects the fact that there are a relatively small number of PM 10-2.5 studies that have utilized co-pollutant models, particularly co-pollutant models that have included PM 2.5, and a very limited body of controlled human exposure evidence supporting the biological plausibility of a causal relationship between PM 10-2.5 and mortality and morbidity at ambient concentrations. The Administrator noted that these important limitations in the overall body of health evidence introduce uncertainty into the interpretation of individual epidemiological studies, particularly those studies reporting associations with PM 10-2.5 that are not statistically significant. Given this, the Administrator reached the provisional conclusion in the proposal that it is appropriate to place relatively little weight on epidemiological studies reporting associations with PM 10-2.5 that are not statistically significant in single-pollutant and/or co-pollutant models. Id.

With regard to this provisional conclusion, the Administrator noted that, for single-city mortality studies conducted in the United States where ambient PM 10 concentration data were available for comparison to the current standard, positive and statistically significant PM 10-2.5 effect estimates were only reported in study locations that would likely have violated the current PM 10 standard during the study period (U.S. EPA, 2011a, Figure 3-2). In U.S. study locations that would likely have met the current standard, PM 10-2.5 effect estimates for mortality were positive, but not statistically significant (U.S. EPA, 2011a, Figure 3-2). In considering U.S. study loc`ations where single-city morbidity studies were conducted, and which would likely have met the current PM 10 standard during the study period, the Administrator noted that PM 10-2.5 effect estimates were both positive and negative, with most not statistically significant (U.S. EPA, 2011a, Figure 3-3).

In addition, in considering single-city analyses for the locations evaluated in a large U.S. multi-city mortality study (Zanobetti and Schwartz, 2009), the Administrator noted that associations in most of the study locations were not statistically significant and that this was the only study to estimate ambient PM 10-2.5 concentrations as the difference between county-wide PM 10 and PM 2.5 mass. As discussed in the Policy Assessment and in the proposal, it is not clear how computed PM 10-2.5 measurements, such as those used by Zanobetti and Schwartz (2009), compare with the PM 10-2.5 concentrations obtained in other studies either by direct measurement or by calculating the difference using co-located samplers (U.S. EPA, 2009a, section 6.5.2.3). For these reasons, in the proposal the Administrator noted that “there is considerable uncertainty in interpreting the associations in these single-city analyses” (77 FR 38961-62).

The Administrator acknowledged that an approach to considering the available scientific evidence and air quality information that emphasizes the above considerations differs from the approach taken by CASAC. Specifically, in its review of the draft Policy Assessment CASAC placed a substantial amount of weight on individual studies, particularly those reporting positive health effects associations for PM 10-2.5 in locations that met the current PM 10 standard during the study period. In emphasizing these studies, as well as the limited number of supporting studies that have evaluated co-pollutant models and the small number of supporting experimental studies, CASAC concluded that “the current data, while limited, is sufficient to call into question the level of protection afforded the American people by the current standard” (Samet, 2010d, p. 7) and recommended revising the current PM 10 standard (Samet, 2010d).

The Administrator carefully considered CASAC's advice and recommendations. She noted that in making its recommendation on the current PM 10 standard, CASAC did not discuss its approach to considering the important uncertainties and limitations in the health evidence, and did not discuss how these uncertainties and limitations were reflected in its recommendation. Nor did CASAC discuss uncertainties in the reported concentrations of PM 10-2.5 in the epidemiological studies, or how reported concentrations in the various studies relate to one another when differing measurement methodologies are used. As discussed above, such uncertainties and limitations contributed to the conclusions in the Integrated Science Assessment that the PM 10-2.5 evidence is only suggestive of a causal relationship, a conclusion that CASAC endorsed (Samet, 2009e,f). Given the importance of these uncertainties and limitations to the interpretation of the evidence, as reflected in the weight of evidence conclusions in the Integrated Science Assessment and as discussed above, the Administrator judged it appropriate to consider and account for them when drawing conclusions about the potential implications of individual PM 10-2.5 health studies for the current standard.

In light of the above approach to considering the scientific evidence, air quality information, and associated uncertainties, the Administrator reached the following provisional conclusions in the proposal:

(1) When viewed as a whole the available evidence and information suggests that the degree of public health protection provided against short-term exposures to PM 10-2.5 does not need to be increased beyond that provided by the current PM 10 standard. This provisional conclusion noted the important uncertainties and limitations associated with the overall body of health evidence and air quality information for PM 10-2.5, as discussed above and as reflected in the Integrated Science Assessment weight-of-evidence conclusions; that PM 10-2.5 effect estimates for the most serious health effect, mortality, were not statistically significant in U.S. locations that met the current PM 10 standard and where coarse particle concentrations were either directly measured or estimated based on co-located samplers; and that PM 10-2.5 effect estimates for morbidity endpoints were both positive and negative in locations that met the current standard, with most not statistically significant.

(2) The degree of public health protection provided by the current standard is not greater than warranted. This provisional conclusion noted that positive and statistically significant associations with mortality were reported in single-city U.S. study locations likely to have violated the current PM 10 standard. [128]

In reaching these provisional conclusions, the Administrator noted that the Policy Assessment also discussed the potential for a revised PM 10 standard (i.e., with a revised form and level) to be “generally equivalent” to the current standard, but to better target public health protection to locations where there is greater concern regarding PM 10-2.5-associated health effects (U.S. EPA, 2011a, sections 3.3.3 and 3.3.4). In considering such a potential revised standard, the Policy Assessment discussed the large amount of variability in PM 10 air quality correlations across monitoring locations and over time (U.S. EPA, 2011a, Figure 3-7) and the regional variability in the relative degree of public health protection that could be provided by the current and potential alternative standards (U.S. EPA, 2011a, Table 3-2). In light of this variability, the Administrator noted the Policy Assessment conclusion that no single revised PM 10 standard (i.e., with a revised form and level) would provide public health protection equivalent to that provided by the current standard, consistently over time and across locations (U.S. EPA, 2011a, section 3.3.4). That is, a revised standard, even one that is meant to be “generally equivalent” to the current PM 10 standard, could increase protection in some locations while decreasing protection in others (77 FR 38962).

In considering the appropriateness of revising the current PM 10 standard in this way, the Administrator noted the following:

(1) Positive PM 10-2.5 effect estimates for mortality were not statistically significant in U.S. locations that met the current PM 10 standard and where coarse particle concentrations were either directly measured or estimated based on co-located samplers, while positive and statistically significant associations with mortality were reported in locations likely to have violated the current PM 10 standard.

(2) Effect estimates for morbidity endpoints in locations that met the current standard were both positive and negative, with most not statistically significant.

(3) Important uncertainties and limitations associated with the overall body of health evidence and air quality information for PM 10-2.5, as discussed above and as reflected in the Integrated Science Assessment weight-of-evidence conclusions, call into question the extent to which the type of quantified and refined targeting of public health protection envisioned under a revised standard could be reliably accomplished.

Given all of the above considerations, the Administrator noted that there is a large amount of uncertainty in the extent to which public health would be improved by changing the locations to which the PM 10 standard targets protection. Therefore, she reached the provisional conclusion that the current PM 10 standard should not be revised in order to change that targeting of protection.

In considering all of the above, including the scientific evidence, the air quality information, the associated uncertainties, and CASAC's advice, the Administrator reached the provisional conclusion that the current 24-hour PM 10 standard is requisite (i.e., neither more protective nor less protective than necessary) to protect public health with an adequate margin of safety against effects that have been associated with PM 10-2.5. In light of this provisional conclusion, the Administrator proposed to retain the current PM 10 standard in order to protect against health effects associated with short-term exposures to PM 10-2.5 (77 FR 38963).

The Administrator recognized that her proposed conclusions and decision to retain the current PM 10 standard differed from CASAC's recommendations, stemming from the differences in how the Administrator and CASAC considered and accounted for the evidence and its limitations and uncertainties. In light of CASAC's views and recommendation to revise the current PM 10 standard, the Administrator welcomed the public's views on these different approaches to considering and accounting for the evidence and its limitations and uncertainties, as well as on the appropriateness of revising the primary PM 10 standard, including revising the form and level of the standard. In doing so, the Administrator solicited comment on all aspects of the proposed decision, including her rationale for reaching the provisional conclusion that the current PM 10 standard is requisite to protect public health with an adequate margin of safety and the provisional conclusion that it is not appropriate to revise the current PM 10 standard by setting a “generally equivalent” standard with the goal of better targeting public health protection.

F. Public Comments on the Administrator's Proposed Decision To Retain the Primary PM 10 Standard

This section discusses the major public comments received on the Administrator's proposed decision to retain the primary PM 10 standard. Additional comments are addressed in the Response to Comments Document (U.S. EPA, 2012a).

Many public commenters agreed with the Administrator's proposed decision to retain the current 24-hour primary PM 10 standard. Among those expressing a position on this proposed decision, industry groups and most State and Local commenters endorsed the Administrator's proposed rationale for retaining the current primary PM 10 standard, including her consideration of the available scientific evidence and associated uncertainties and her consideration of CASAC recommendations.

Although industry commenters generally agreed with the Administrator's proposed decision to retain the current primary PM 10 standard, some also contended that the current standard is “excessively precautionary” (NMA and NCBA, 2012, p. 4) and a few expressed support for a less stringent standard for coarse particles that are comprised largely of crustal material. For example, the Coarse Particulate Matter Coalition (CPMC) (2012) and several other industry commenters recommended that the final decision allow application of a 98th percentile form for the current standard (i.e. with its level of 150 μg/m [3] ) in cases where coarse particles consist primarily of crustal material. Such an approach would allow more yearly exceedances of the existing standard level than are allowed with the current one-expected-exceedance form. These industry commenters contended that a 98th percentile form applied in this way would provide appropriate regulatory relief for areas where the evidence for coarse particle-related health effects is relatively uncertain.

In reaching her conclusion that the current primary PM 10 standard is requisite to protect public health with an adequate margin of safety, the Administrator considered the degree of public health protection provided by the current standard as a whole, including all elements of that standard (i.e., indicator, averaging time, form, level). As discussed above and in the following section, this conclusion reflects the Administrator's judgments that (1) the current standard appropriately provides some measure of protection against exposures to all thoracic coarse particles, regardless of their location, source of origin, or composition and (2) the current standard appropriately allows lower ambient concentrations of PM 10-2.5 in urban areas, where the evidence is strongest that thoracic coarse particles are linked to mortality and morbidity, and higher concentrations in non-urban areas, where the public health concerns are less certain.

Because the considerations that led to these judgments, and to the conclusion that the current primary PM 10 standard is requisite to protect public health, took into account the degree of public health protection provided by the standard as a whole, it would not be appropriate to consider revising one element of the standard (e.g., the form, as suggested by commenters in this case) without considering the extent to which the other elements of the standard should also be revised. The change in form requested by industry commenters, without also lowering the level of the standard, would markedly reduce the public health protection provided against exposures to thoracic coarse particles. [129] However, industry commenters have not presented new evidence or analyses to support their conclusion that an appropriate degree of public health protection could be achieved by allowing the use of an alternative form (i.e., 98th percentile) for some coarse particles, while retaining the other elements of the current standard. Nor have these commenters presented new evidence or analyses challenging the basis for the conclusion in the proposal that the varying amounts of coarse particles allowed in urban versus non-urban areas under the current 24-hour PM 10 standard, based on the varying levels of PM 2.5 present, appropriately reflect the differences in the strength of evidence regarding coarse particle effects in urban and non-urban areas. In light of this, EPA does not believe that a reduction in public health protection, such as that requested by industry commenters, is warranted.

In further considering these comments, it is to be remembered that epidemiologic studies have not demonstrated that coarse particles of non-urban origin do not cause health effects, and commenters have not provided additional evidence on this point. While there are fewer studies of non-urban coarse particles than of urban coarse particles, several studies have reported positive and statistically significant associations between coarse particles of crustal, non-urban origin and mortality or morbidity (Ostro et al., 2003; Bell et al., 2008; Chan et al., 2008; Middleton et al., 2008; Perez et al., 2008). These studies formed part of the basis for the PM Integrated Science Assessment conclusion that “recent studies have suggested that PM (both PM 2.5 and PM 10-2.5) from crustal, soil or road dust sources or PM tracers linked to these sources are associated with cardiovascular effects” (U.S. EPA, 2009a, p. 2-26). Moreover, crustal coarse particles may be contaminated with toxic trace elements and other components from previously deposited fine PM from ubiquitous sources such as mobile source engine exhaust, as well as by toxic metals from smelters or other industrial activities, animal waste, or pesticides (U.S. EPA, 2004, p. 8-344). In the proposal, the Administrator acknowledged the potential for this type of contamination to increase the toxicity of coarse particles of crustal, non-urban origin (77 FR 38960; see also 71 FR 61190).

In suggesting a change in the form of the current standard, industry commenters also did not address the manifold difficulties noted above, and in the last review, associated with developing an indicator that could reliably identify ambient mixes dominated by particular types of sources of coarse particles. See above and 71 FR 61193. Yet such an indicator would be a prerequisite of the type of standard these commenters request.

For all of the reasons discussed above, the EPA does not agree with industry commenters who recommended allowing the application of a 98th percentile form for the current standard in cases where coarse particles consist primarily of crustal material.

Some industry commenters contended that the uncertainties and limitations that precluded a quantitative risk assessment also preclude revising the PM 10 standard. Although the EPA agrees that there are important uncertainties and limitations in the extent to which the quantitative relationships between ambient PM 10-2.5 and health outcomes can be characterized in risk models, the Agency does not agree that such limitations alone preclude the option of revising a NAAQS. As noted above, the lack of a quantitative PM 10-2.5 risk assessment in the current review adds uncertainty to conclusions about the extent to which revision of the current PM 10 standard would be expected to improve the protection of public health, beyond the protection provided by the current standard. However, the EPA does not agree that such uncertainties necessarily preclude revision of a NAAQS. Indeed, with respect to thoracic coarse particles, the DC Circuit noted that “[a]lthough the evidence of danger from coarse PM is, as the EPA recognizes, `inconclusive', the agency need not wait for conclusive findings before regulating a pollutant it reasonably believes may pose a significant risk to public health.” 559 F. 3d at 533. Thus, the Administrator's conclusion that the current 24-hour PM 10 standard provides requisite protection of public health relies on her consideration of the broad body of evidence, rather than solely on the uncertainties that led to the decision not to conduct a quantitative assessment of PM 10-2.5 health risks.

Commenters representing a number of environmental groups and medical organizations disagreed with the Administrator's proposal to retain the current primary PM 10 standard. These commenters generally requested that the EPA revise the PM 10 standard to increase public health protection, consistent with the recommendations from CASAC.

As discussed above and in the proposal, in reaching provisional conclusions in the proposal regarding the current standard, the Administrator carefully considered CASAC's advice and recommendations. She specifically noted that in making its recommendation on the current PM 10 standard, CASAC did not discuss its approach to considering the important uncertainties and limitations in the health evidence, and did not discuss how these uncertainties and limitations were reflected in its recommendations. Such uncertainties and limitations contributed to the conclusions in the Integrated Science Assessment that the PM 10-2.5 evidence is only suggestive of a causal relationship, a conclusion that CASAC endorsed (Samet, 2009e,f). These commenters also did not address the important uncertainties in the epidemiologic studies on which their comments are based. Given the importance of these uncertainties and limitations to the interpretation of the evidence, as reflected in the weight of evidence conclusions in the Integrated Science Assessment and as discussed in the proposal, the Administrator judges that it is appropriate to consider and account for them when drawing conclusions about the implications of individual PM 10-2.5 health studies for the current standard. Commenters have not provided new information that would change the Administrator's views on the evidence and uncertainties.

In recommending that the PM 10 standard be revised, some commenters supported their conclusions by referencing studies that evaluated PM 10, rather than PM 10-2.5. These commenters contended that “[t]he most relevant studies to the setting of a PM 10 standard are the thousands of studies that have reported adverse effects associated with PM 10 pollution” (ALA et al., 2012).

As discussed in the Policy Assessment, the proposal, and above, since the establishment of the primary PM 2.5 standards, the purpose of the primary PM 10 standard has been to protect against health effects associated with exposures to PM 10-2.5. PM 10 is the indicator, not the target pollutant. With regard to the appropriateness of considering PM 10 health studies for the purpose of reaching conclusions on a standard meant to protect against exposures to PM 10-2.5, the proposal noted that PM 10 includes both fine and coarse particles, even in locations with the highest concentrations of PM 10-2.5. Therefore, the extent to which PM 10 effect estimates reflect associations with PM 10-2.5 versus PM 2.5 can be highly uncertain and it is often unclear how PM 10 health studies should be interpreted when considering a standard meant to protect against exposures to PM 10-2.5. Given this uncertainty and the availability of a number of PM 10-2.5 health studies in this review, the Integrated Science Assessment considered PM 10-2.5 studies, but not PM 10 studies, when drawing weight-of-evidence conclusions regarding the coarse fraction. [130] In light of the uncertainty in ascribing PM 10-related health effects to the coarse or fine fractions, indicating that the best evidence for effects associated with exposures to PM 10-2.5 comes from studies evaluating PM 10-2.5 itself, and given CASAC's support for the approach adopted in the Integrated Science Assessment, which draws weight-of-evidence conclusions for PM 2.5 and PM 10-2.5 but not for PM 10 (Samet, 2009f), the EPA continues to conclude that it is appropriate to focus on PM 10-2.5 health studies when considering the degree of public health protection provided by the current primary PM 10 standard, a standard intended exclusively to provide protection against exposures to PM 10-2.5.

G. Administrator's Final Decision on the Primary PM 10 Standard

In reaching a final decision on the primary PM 10 standard, the Administrator takes into account the available scientific evidence, and the assessment of that evidence, in the Integrated Science Assessment; the analyses and staff conclusions presented in the Policy Assessment; the advice and recommendations of CASAC; and public comments on the proposal. In particular, as in the proposal, the Administrator places emphasis on her consideration of the following issues:

(1) The extent to which it is appropriate to maintain a standard that provides some measure of protection against all PM 10-2.5, regardless of composition or source of origin;

(2) The extent to which it is appropriate to retain a PM 10 indicator for a standard meant to protect against exposures to ambient PM 10-2.5; and

(3) The extent to which the current PM 10 standard provides an appropriate degree of public health protection.

Each of these issues is discussed below.

With regard to the first issue, as in the proposal the Administrator judges that the expanded body of scientific evidence available in this review provides ample support for a standard that protects against exposures to all thoracic coarse particles, regardless of their location or source of origin. There was already ample evidence for this position in the previous review, [131] and that evidence has since increased. Specifically, the Administrator notes that epidemiological studies have reported positive associations between PM 10-2.5 and mortality or morbidity in a large number of cities across North America, Europe, and Asia, encompassing a variety of environments where PM 10-2.5 sources and composition are expected to vary widely. In considering this evidence, the Integrated Science Assessment concludes that “many constituents of PM can be linked with differing health effects” (U.S. EPA, 2009a, p. 2-26). Although PM 10-2.5 in most of these study areas is of largely urban origin, the Administrator notes that some recent studies have also linked mortality and morbidity with relatively high ambient concentrations of particles of non-urban crustal origin. In considering these studies, she notes the Integrated Science Assessment's conclusion that “PM (both PM 2.5 and PM 10-2.5) from crustal, soil or road dust sources or PM tracers linked to these sources are associated with cardiovascular effects” (U.S. EPA, 2009a, p. 2-26). The Administrator likewise notes CASAC's emphatic advice that a standard remains needed for all types of thoracic coarse PM. [132] In light of this body of available evidence reporting PM 10-2.5-associated health effects across different locations with a variety of sources, the Integrated Science Assessment's conclusions regarding the links between adverse health effects and PM sources and composition, and CASAC's advice, the Administrator concludes in the current review that it is appropriate to maintain a standard that provides some measure of protection against exposures to all thoracic coarse particles, regardless of their location, source of origin, or composition.

With regard to the second issue, in considering the appropriateness of a PM 10 indicator for a standard meant to provide such public health protection, the Administrator notes that the rationale used in the last review to support the unqualified PM 10 indicator remains relevant in the current review. Specifically, as an initial consideration, she notes that PM 10 mass includes both coarse PM (PM 10-2.5) and fine PM (PM 2.5). As a result, the concentration of PM 10-2.5 allowed by a PM 10 standard set at a single level declines as the concentration of PM 2.5 increases. At the same time, the Administrator notes that PM 2.5 concentrations tend to be higher in urban areas than rural areas (U.S. EPA, 2005, p. 2-54, and Figures 2-23 and 2-24) and, therefore, a PM 10 standard will generally allow lower PM 10-2.5 concentrations in urban areas than in rural areas.

In considering the appropriateness of this variation in allowable PM 10-2.5 concentrations, the Administrator considers the relative strength of the evidence for health effects associated with PM 10-2.5 of urban origin versus non-urban origin. She specifically notes that, as discussed in the proposal, the large majority of the available evidence for thoracic coarse particle health effects comes from studies conducted in locations with sources more typical of urban and industrial areas than rural areas. While associations with adverse health effects have been reported in some study locations where PM 10-2.5 is largely non-urban in origin (i.e., in dust storm studies), particle concentrations in these study areas are typically much higher than reported in study locations where the PM is of urban origin. Therefore, the Administrator notes that the strongest evidence for a link between PM 10-2.5 and adverse health impacts, particularly for such a link at relatively low particle concentrations, comes from studies of urban or industrial PM 10-2.5.

The Administrator also notes that chemical constituents present at higher levels in urban or industrial areas, including byproducts of incomplete combustion (e.g. polycyclic aromatic hydrocarbons) emitted as PM 2.5 from motor vehicles as well as metals and other contaminants emitted from anthropogenic sources, can contaminate PM 10-2.5 (U.S. EPA, 2004, p. 8-344; 71 FR 2665, January 17, 2006). While the Administrator acknowledges the uncertainty expressed in the Integrated Science Assessment regarding the extent to which particle composition can be linked to health outcomes based on available evidence, she also considers the possibility that PM 10-2.5 contaminants typical of urban or industrial areas could increase the toxicity of thoracic coarse particles in urban locations.

Given that the large majority of the evidence for PM 10-2.5 toxicity, particularly at relatively low particle concentrations, comes from study locations where thoracic coarse particles are of urban origin, and given the possibility that PM 10-2.5 contaminants in urban areas could increase particle toxicity, the Administrator concludes that it remains appropriate to maintain a standard that provides some protection in all areas but targets public health protection to urban locations. Specifically, she concludes that it is appropriate to maintain a standard that allows lower ambient concentrations of PM 10-2.5 in urban areas, where the evidence is strongest that thoracic coarse particles are linked to mortality and morbidity, and higher concentrations in non-urban areas, where the public health concerns are less certain.

Given all of the above considerations and conclusions, the Administrator judges that the available evidence supports retaining a PM 10 indicator for a standard that is meant to protect against exposures to thoracic coarse particles. In reaching this judgment, she notes that, to the extent a PM 10 indicator results in lower allowable concentrations of thoracic coarse particles in some areas compared to others, lower concentrations will be allowed in those locations (i.e., urban or industrial areas) where the science has shown the strongest evidence of adverse health effects associated with exposure to thoracic coarse particles and where we have the most concern regarding PM 10-2.5 toxicity. Therefore, the Administrator concludes that the varying amounts of coarse particles that are allowed in urban vs. non-urban areas under the 24-hour PM 10 standard, based on the varying levels of PM 2.5 present, appropriately reflect the differences in the strength of evidence regarding coarse particle effects in urban and non-urban areas. 133 134

In reaching this conclusion, the Administrator also notes that, in their review of the second draft Policy Assessment, CASAC concluded that “[w]hile it would be preferable to use an indicator that reflects the coarse PM directly linked to health risks (PM 10-2.5), CASAC recognizes that there is not yet sufficient data to permit a change in the indicator from PM 10 to one that directly measures thoracic coarse particles” (Samet, 2010d, p. ii). Thus, consistent the considerations presented above and with CASAC advice, the Administrator concludes that it is appropriate to retain PM 10 as the indicator for thoracic coarse particles. [135]

With regard to the third issue, in evaluating the degree of public health protection provided by the current PM 10 standard, the Administrator first notes that when the available PM 10-2.5 scientific evidence and its associated uncertainties were considered, the Integrated Science Assessment concluded that the evidence is suggestive of a causal relationship between short-term PM 10-2.5 exposures and mortality, cardiovascular effects, and respiratory effects. As discussed above and in more detail in the Integrated Science Assessment (U.S. EPA, 2009a, section 1.5), a suggestive determination is made when the “[e]vidence is suggestive of a causal relationship with relevant pollutant exposures, but is limited because chance, bias and confounding cannot be ruled out.” In contrast, the Administrator notes that she is strengthening the annual fine particle standard based on a body of scientific evidence judged sufficient to conclude that a causal relationship exists (i.e., mortality, cardiovascular effects) or is likely to exist (i.e., respiratory effects). The suggestive judgment for PM 10-2.5 reflects the greater degree of uncertainty associated with this body of evidence, as discussed above and in more detail in the proposal, and as summarized below.

The Administrator notes that the important uncertainties and limitations associated with the scientific evidence and air quality information raise questions as to whether public health benefits would be achieved by revising the existing PM 10 standard. Such uncertainties and limitations include the following:

(1) While PM 10-2.5 effect estimates reported for mortality and morbidity were generally positive, most were not statistically significant, even in single-pollutant models. This includes effect estimates reported in some study locations with PM 10 concentrations above those allowed by the current 24-hour PM 10 standard.

(2) The number of epidemiological studies that have employed co-pollutant models to address the potential for confounding, particularly by PM 2.5, remains limited. Therefore, the extent to which PM 10-2.5 itself, rather than one or more co-pollutants, contributes to reported health effects remains uncertain.

(3) Only a limited number of experimental studies provide support for the associations reported in epidemiological studies, resulting in further uncertainty regarding the plausibility of the associations between PM 10-2.5 and mortality and morbidity reported in epidemiological studies.

(4) Limitations in PM 10-2.5 monitoring data and the different approaches used to estimate PM 10-2.5 concentrations across epidemiological studies result in uncertainty in the ambient PM 10-2.5 concentrations at which the reported effects occur, increasing uncertainty in estimates of the extent to which changes in ambient PM 10-2.5 concentrations would likely impact public health.

(5) The lack of a quantitative PM 10-2.5 risk assessment further contributes to uncertainty regarding the extent to which any revisions to the current PM 10 standard would be expected to improve the protection of public health, beyond the protection provided by the current standard (see section III.B.5 above).

(6) The chemical and biological composition of PM 10-2.5, and the effects associated with the various components, remains uncertain. Without more information on the chemical speciation of PM 10-2.5, the apparent variability in associations across locations is difficult to characterize.

In considering these uncertainties and limitations, the Administrator notes in particular the considerable degree of uncertainty in the extent to which health effects reported in epidemiological studies are due to PM 10-2.5 itself, as opposed to one or more co-occurring pollutants. As discussed above, this uncertainty reflects the fact that there are a relatively small number of PM 10-2.5 studies that have evaluated co-pollutant models, particularly co-pollutant models that have included PM 2.5, and a very limited body of controlled human exposure evidence supporting the plausibility of a causal relationship between PM 10-2.5 and mortality and morbidity at ambient concentrations. The Administrator notes that these important limitations in the overall body of health evidence introduce uncertainty into the interpretation of individual epidemiological studies, particularly those studies reporting associations with PM 10-2.5 that are not statistically significant. Given this, the Administrator reaches the conclusion that it is appropriate to place relatively little weight on epidemiological studies reporting associations with PM 10-2.5 that are not statistically significant in single-pollutant and/or co-pollutant models. [136]

With regard to this conclusion, the Administrator notes that, for single-city mortality studies conducted in the United States where ambient PM 10 concentration data were available for comparison to the current standard, positive and statistically significant PM 10-2.5 effect estimates were only reported in study locations that would likely have violated the current PM 10 standard during the study period (U.S. EPA, 2011a, Figure 3-2). In U.S. study locations that would likely have met the current standard, PM 10-2.5 effect estimates for mortality were positive, but not statistically significant (U.S. EPA, 2011a, Figure 3-2). In considering U.S. study locations where single-city morbidity studies were conducted, and which would likely have met the current PM 10 standard during the study period, the Administrator notes that PM 10-2.5 effect estimates were both positive and negative, with most not statistically significant (U.S. EPA, 2011a, Figure 3-3).

In addition, in considering single-city analyses for the locations evaluated in a large U.S. multi-city mortality study (Zanobetti and Schwartz, 2009), the Administrator notes that associations in most of the study locations were not statistically significant and that this was the only study to estimate ambient PM 10-2.5 concentrations as the difference between county-wide PM 10 and PM 2.5 mass. As discussed in the proposal, the Administrator notes that it is not clear how computed PM 10-2.5 measurements, such as those used by Zanobetti and Schwartz (2009), compare with the PM 10-2.5 concentrations obtained in other studies either by direct measurement by calculating the difference using co-located samplers (U.S. EPA, 2009a, section 6.5.2.3). For these reasons, as in the proposal, the Administrator notes that there is considerable uncertainty in interpreting the associations, and especially the concentrations at which such associations may have occurred, in these single-city analyses.

The Administrator acknowledges that an approach to considering the available scientific evidence and air quality information that emphasizes the above considerations differs from the approach taken by CASAC. Specifically, CASAC placed a substantial amount of weight on individual studies, particularly those reporting positive health effects associations in locations that met the current PM 10 standard during the study period. In emphasizing these studies, as well as the limited number of supporting studies that have evaluated co-pollutant models and the small number of supporting experimental studies, CASAC concluded that “the current data, while limited, is sufficient to call into question the level of protection afforded the American people by the current standard” (Samet, 2010d, p. 7) and recommended revising the current PM 10 standard (Samet, 2010d).

The Administrator has carefully considered CASAC's advice and recommendations. She notes that in making its recommendation on the current PM 10 standard, CASAC did not discuss its approach to considering the important uncertainties and limitations in the health evidence, and did not discuss how these uncertainties and limitations are reflected in its recommendation. As discussed above, such uncertainties and limitations contributed to the conclusions in the Integrated Science Assessment that the PM 10-2.5 evidence is only suggestive of a causal relationship, a conclusion that CASAC endorsed (Samet, 2009e,f). Given the importance of these uncertainties and limitations to the interpretation of the evidence, as reflected in the weight of evidence conclusions in the Integrated Science Assessment and as discussed above, the Administrator judges that it is appropriate to consider and account for them when drawing conclusions about the potential implications of individual PM 10-2.5 health studies for the current standard.

In light of the above approach to considering the scientific evidence, air quality information, and associated uncertainties, the Administrator reaches the following conclusions:

(1) When viewed as a whole the available evidence and information suggests that the degree of public health protection provided against short-term exposures to PM 10-2.5 should be maintained but does not need to be increased beyond that provided by the current PM 10 standard. This conclusion emphasizes the important uncertainties and limitations associated with the overall body of health evidence and air quality information for PM 10-2.5, as discussed above and as reflected in the Integrated Science Assessment weight-of-evidence conclusions; that PM 10-2.5 effect estimates for the most serious health effect, mortality, were not statistically significant in U.S. locations that met the current PM 10 standard and where coarse particle concentrations were either directly measured or estimated based on co-located samplers; and that PM 10-2.5 effect estimates for morbidity endpoints were both positive and negative in locations that met the current standard, with most not statistically significant. [137]

(2) The degree of public health protection provided by the current standard is not greater than warranted. This conclusion notes that positive and statistically significant associations with mortality were reported in single-city U.S. study locations likely to have violated the current PM 10 standard. [138]

In reaching these conclusions, the Administrator notes that the Policy Assessment also discussed the potential for a revised PM 10 standard (i.e., with a revised form and level) to be “generally equivalent” to the current standard, but to better target public health protection to locations where there is greater concern regarding PM 10-2.5-associated health effects (U.S. EPA, 2011a, sections 3.3.3 and 3.3.4). [139] In considering such a potential revised standard, the Policy Assessment discusses the large amount of variability in PM 10 air quality correlations across monitoring locations and over time (U.S. EPA, 2011a, Figure 3-7) and the regional variability in the relative degree of public health protection that could be provided by the current and potential alternative standards (U.S. EPA, 2011a, Table 3-2). In light of this variability, the Administrator notes the Policy Assessment conclusion that no single revised PM 10 standard (i.e., with a revised form and level) would provide public health protection equivalent to that provided by the current standard, consistently over time and across locations (U.S. EPA, 2011a, section 3.3.4). That is, a revised standard, even one that is meant to be “generally equivalent” to the current PM 10 standard, could increase protection in some locations while decreasing protection in other locations.

In considering the appropriateness of revising the current PM 10 standard in this way, the Administrator notes the following:

(1) As discussed above, positive PM 10-2.5 effect estimates for mortality were not statistically significant in U.S. locations that met the current PM 10 standard and where coarse particle concentrations were either directly measured or estimated based on co-located samplers, while positive and statistically significant associations with mortality were reported in locations likely to have violated the current PM 10 standard.

(2) Also as discussed above, effect estimates for morbidity endpoints in locations that met the current standard were both positive and negative, with most not statistically significant.

(3) Important uncertainties and limitations associated with the overall body of health evidence and air quality information for PM 10-2.5, as discussed above and as reflected in the Integrated Science Assessment weight-of-evidence conclusions, call into question the extent to which the type of quantified and refined targeting of public health protection envisioned under a revised standard could be reliably accomplished.

Given all of the above considerations, the Administrator notes that there is a large amount of uncertainty in the extent to which public health would be improved by changing the locations to which the PM 10 standard targets protection. Therefore, she reaches the conclusion that the current PM 10 standard should not be revised in order to change that targeting of protection.

In considering all of the above, including the scientific evidence, the air quality information, the associated uncertainties, CASAC's advice, and public comments received on the proposed rule, the Administrator reaches the conclusion in the current review that the existing 24-hour PM 10 standard, with its one-expected exceedance form and a level of 150 μg/m [3] , is requisite (i.e., neither more protective nor less protective than necessary) to protect public health with an adequate margin of safety against effects that have been associated with PM 10-2.5. In light of this conclusion, with this rule the Administrator retains the current PM 10 standard.

V. Communication of Public Health Information Back to Top

Sections 319(a)(1) and (3) of the CAA require the EPA to establish a uniform air quality index for reporting of air quality. These sections specifically direct the Administrator to “promulgate regulations establishing an air quality monitoring system throughout the United States which utilizes uniform air quality monitoring criteria and methodology and measures such air quality according to a uniform air quality index” and “provides for daily analysis and reporting of air quality based upon such uniform air quality index * * *” In 1979, the EPA established requirements for index reporting (44 FR 27598, May 10, 1979). The requirement for State and local agencies to report the AQI appears in 40 CFR 58.50, and the specific requirements (e.g., what to report, how to report, reporting frequency, calculations) are in appendix G to 40 CFR part 58.

Information on the public health implications of ambient concentrations of criteria pollutants is currently made available primarily by AQI reporting through EPA's AIRNow Web site. [140] The current AQI has been in use since its inception in 1999. [141] It provides accurate, timely, and easily understandable information about daily levels of pollution (40 CFR 58.50). The AQI establishes a nationally uniform system of indexing pollution levels for ozone, carbon monoxide, nitrogen dioxide, PM, and sulfur dioxide. The AQI is also recognized internationally as a proven tool to effectively communicate air quality information to the public.

The AQI converts pollutant concentrations in a community's air to a number on a scale from 0 to 500. Reported AQI values enable the public to know whether air pollution levels in a particular location are characterized as good (0-50), moderate (51-100), unhealthy for sensitive groups (101- 150), unhealthy (151-200), very unhealthy (201-300), or hazardous (301-500). The AQI index value of 100 typically corresponds to the level of the short-term (e.g., daily or hourly standard) NAAQS for each pollutant. Below an index value of 100, an intermediate value of 50 was defined either as the level of the annual standard if an annual standard has been established (e.g., PM 2.5, nitrogen dioxide), or as a concentration equal to one-half the value of the short-term standard used to define an index value of 100 (e.g., carbon monoxide). An AQI value greater than 100 means that a pollutant is in one of the unhealthy categories (i.e., unhealthy for sensitive groups, unhealthy, very unhealthy, or hazardous) on a given day. An AQI value at or below 100 means that a pollutant concentration is in one of the satisfactory categories (i.e., moderate or good). The underlying health information that supports the NAAQS review also supports the selection of the AQI “breakpoints”—the ambient concentrations that delineate the various AQI categories for each pollutant.

Historically, state and local agencies have primarily used the AQI to provide general information to the public about air quality and its relationship to public health. For more than a decade, many states and local agencies, as well as the EPA and other Federal agencies, have been developing new and innovative programs and initiatives to provide more information to the public in a more timely way. These initiatives, including air quality forecasting, real-time data reporting through the AirNow Web site, and state and local air quality action day programs, can serve to provide useful, up-to-date, and timely information to the public about air pollution and its effects. Such information will help individuals take actions to avoid or to reduce exposures to ambient pollution at levels of concern to them. Thus, these programs have significantly broadened the ways in which state and local agencies can meet the nationally uniform AQI reporting requirements and contribute to state and local efforts to provide community health protection.

With respect to an AQI value of 50, the historical approach is to set it at the same level of the annual primary standard, if there is one. This is consistent with the previous AQI sub-index for PM 2.5, in which the AQI value of 50 was set at 15 µg/m [3] in 1999, consistent with the level of the annual PM 2.5 standard at that time. In recognition of the proposed change to the annual PM 2.5 standard summarized in section III.F of the proposal, the EPA proposed a conforming change to the PM 2.5 sub-index of the AQI to be consistent with the proposed change to the annual standard. As discussed below, no state or local agencies, or their organizations (e.g., NACAA), that commented on the proposed changes to the AQI disagreed with our proposed approach. Based on these comments, the EPA continues to see no basis for deviating from this approach in this review. Thus, the EPA is taking final action to set an AQI value of 50 at 12.0 μg/m [3] , 24-hour average, consistent with the final decision on the annual PM 2.5 standard level (section III.F).

With respect to an AQI value of 100, which is the basis for advisories to individuals in sensitive groups, in the proposal we described two general approaches that could be used to select the associated PM 2.5 level. By far the most common approach, which has been used with all of the other sub-indices, is to set an AQI value of 100 at the same level as the short-term standard. In the proposal, the EPA recognized that some state and local air quality agencies have expressed a strong preference that the Agency set an AQI value of 100 equal to any short-term standard (77 FR 38964). These agencies typically express the view that this linkage is useful for the purpose of communicating with the public about the standard, as well as providing consistent messages about the health impacts associated with daily air quality. The EPA proposed to use this approach to set the AQI value of 100 at 35 μg/m [3] , 24-hour average, consistent with the proposed decision to retain the current 24-hour PM 2.5 standard. Id.

An alternative approach discussed in the proposal (77 FR 38964), was to directly evaluate the health effects evidence to select the level for an AQI value of 100. This was the approach used in the 1999 rulemaking to set the AQI value of 100 at a level of 40 μg/m [3] , 24-hour average, [142] when the 24-hour standard level was 65 μg/m [3] . This alternative approach was used in the case of the PM 2.5 sub-index, because the annual and 24-hour PM 2.5 standards set in 1997 were designed to work together, and the intended degree of health protection against short-term risks was not defined by the 24-hour standard alone, but rather by the combination of the two standards working in concert. Indeed, at that time, the 24-hour standard was set to provide supplemental protection relative to the principal protection provided by the annual standard. In the proposal, the EPA solicited comment on this alternative approach in recognition that, as proposed, the 24-hour PM 2.5 standard is intended to continue to provide supplemental protection against effects associated with short-term exposures of PM 2.5 by working in conjunction with the annual standard to reduce 24-hour exposures to PM 2.5. The EPA recognized that in the past, some state and local air quality agencies have expressed support for this alternative approach. Using this alternative approach could have resulted in consideration of a lower level for an AQI value of 100, based on the discussion of the health information pertaining to the level of the 24-hour standard in section III.E.4 of the proposal. The EPA encouraged state and local air quality agencies to comment on both the approach and the level at which to set an AQI value of 100 together with any supporting rationale. Of the state or local agencies, or their organizations (e.g., NACAA), that commented on the proposed changes to the AQI, only one organization, NESCAUM, expressed some support for this approach. In its comments, NESCAUM expressed support for a 24-hour standard set at 30 μg/m [3] , 24-hour average. NESCAUM also expressed the view that EPA should carefully consider how to set the breakpoint for an AQI value of 100. NESCAUM expressed the view that if the EPA were to keep the 24-hour PM 2.5 standard at 35 μg/m [3] , the annual standard would be controlling, and a 24-hour breakpoint at that level (35 μg/m [3] ) would not be very effective for the purposes of public health messaging. However, other agencies, such as Georgia Department of Natural Resources (Georgia DNR), expressed the view that linkage between the short-term standard and the AQI of 100 is useful for the purpose of communicating with the public about the standard as well as providing consistent messages about the health impacts associated with the daily air quality. Based on these comments, the EPA sees no basis for deviating from the approach proposed in this review. Thus, the EPA is taking final action to set an AQI value of 100 at 35 μg/m [3] , 24-hour average, consistent with the final decision on the 24-hour PM 2.5 standard level (section III.F).

With respect to an AQI value of 150, this level is based upon the same health effects information that informs the selection of the level of the 24-hour standard and the AQI value of 100. The AQI value of 150 was set in the 1999 rulemaking at a level of 65 μg/m [3] , 24-hour average. In considering what level to propose for an AQI value of 150, we stated the view that the health effects evidence indicates that the level of 55 μg/m [3] , 24-hour average, is appropriate to use [143] in conjunction with an AQI value of 100 set at the level of 35 μg/m [3] . The Agency's approach to selecting the levels at which to set the AQI values of 100 and 150 inherently recognizes that the epidemiological evidence upon which these decisions are based provides no evidence of discernible thresholds, below which effects do not occur in either sensitive groups or in the general population, at which to set these two breakpoints. Therefore, the EPA concluded the use of a proportional adjustment would be appropriate. Commenters did not comment on this proposed approach to revising the AQI value of 150; thus, the EPA is taking final action to set an AQI value of 150 at 55 μg/m [3] , 24-hour average.

Based on the air quality and health considerations discussed in section V of the proposal, the EPA concluded that it was appropriate to propose to retain the current level of 500 μg/m [3] , 24-hour average, for the AQI value of 500. In addition, the EPA solicited comment on alternative levels and approaches to setting a level for the AQI value of 500, as well as supporting information and rationales for such alternative levels. The EPA also solicited any additional information, data, research or analyses that may be useful to inform a final decision on the appropriate level to set the AQI value of 500. Receiving no information with which to inform alternative approaches to setting an AQI value of 500, the EPA is taking final action to retain the current level of 500 μg/m [3] , 24-hour average, for the AQI value of 500.

For the intermediate breakpoints in the AQI between the values of 150 and 500, the EPA proposed PM 2.5 concentrations that generally reflected a linear relationship between increasing index values and increasing PM 2.5 values (77 FR 38965). The available scientific evidence of health effects related to population exposures to PM 2.5 concentrations between the level of the 24-hour standard and an AQI value of 500 suggested a continuum of effects in this range, with increasing PM 2.5 concentrations being associated with increasingly larger numbers of people likely to experience such effects. The generally linear relationship between AQI values and PM 2.5 concentrations in this range is consistent with the health evidence. This also is consistent with the Agency's practice of setting breakpoints in symmetrical fashion where health effects information does not suggest particular levels.

Table 2 below summarizes the finalized breakpoints for the PM 2.5 sub-index. [144] Table 2 shows the intermediate breakpoints for AQI values of 200, 300 and 400 based on a linear interpolation between the proposed levels for AQI values of 150 and 500. If a different level were to be set for an AQI value of 150 or 500, intermediate levels would be calculated based on a linear relationship between the selected levels for AQI values of 150 and 500.

Table 2—Breakpoints for PM 2.5 Sub-Index Back to Top
AQI category Index values Proposed breakpoints (μg/m3, 24-hour average)
Good 0-50 0.0-(12.0)
Moderate 51-100 (12.1)-35.4
Unhealthy for Sensitive Groups 101-150 35.5-55.4
Unhealthy 151-200 55.5-150.4
Very Unhealthy 201-300 150.5-250.4
Hazardous 301-400 401-500 250.5-350.4 350.5-500.4

In retaining the 500 level for the AQI as described above, we note that the EPA is not establishing a Significant Harm Level (SHL) for PM 2.5. The SHL is an important part of air pollution Emergency Episode Plans, which are required for certain areas by CAA section 110(a)(2)(G) and associated regulations at 40 CFR 51.150, under the Prevention of Air Pollution Emergency Episodes program. The Agency believes that air quality responses established through an Emergency Episode Plan should be developed through a collaborative process working with State and Tribal air quality, forestry and agricultural agencies, Federal land management agencies, private land managers and the public. Therefore, if in future rulemaking the EPA proposes revisions to the Prevention of Air Pollution Emergency Episodes program, the proposal will include a SHL for PM 2.5 that is developed in collaboration with these organizations. As discussed in the 1999 Air Quality Index Reporting Rule (64 FR 42530), if a future rulemaking results in a SHL that is different from the 500 value of the AQI for PM 2.5, the AQI will be revised accordingly.

The EPA also received more general comments on AQI reporting, comments that did not pertain to setting specific breakpoints. One set of commenters (e.g., API and UARG), expressed the view that changes to the AQI are not appropriate. They noted that air quality is getting better, and in fact is better than when EPA established the AQI. These commenters stated that the proposed changes to the annual standard and the AQI would mean that the public would hear less often that air quality is good, and thereby would receive apparently inconsistent or misleading messages that air quality is worse. The AQI has been revised several times in conjunction with revisions to the standards. State and local air quality agencies and organizations are proficient at communicating with the public about the reasons for changes to the AQI. Therefore, the EPA strongly disagrees with these commenters that the public will receive inconsistent or misleading messages. Recognizing the importance of the AQI as a communication tool that allows the public to take exposure reduction measures when air quality may pose health risks, the EPA agrees with state and local air quality agencies and organizations that favored revising the AQI at the same time as the primary standard.

A few state and local air quality agencies and organizations recommended against using near-roadway PM 2.5 monitors for AQI reporting. In support of this comment, they expressed the following views, that near-roadway monitors are source-oriented, represent micro-scale conditions, and the agencies don't have experience using them for AQI reporting. The EPA disagrees with the comment in that these monitors will be sited at existing near-road stations sited to be representative of area-wide PM 2.5 concentrations indicative of general population exposure. Accordingly, data from these near-road monitors should be included in the AQI since they provide information about PM 2.5 levels that millions of people, who work, live and go to school near busy roadways, are exposed to. The stations are representative of somewhat elevated concentrations in near-road environments, but since these stations represent many such locations throughout a metropolitan area, they are appropriate for characterizing exposure in typical portions of major urban areas. The EPA is committed to helping air quality agencies develop appropriate ways to report PM 2.5 levels from these monitors using the AQI.

VI. Rationale for Final Decisions on the Secondary PM Standards Back to Top

This section presents the Administrator's final decisions regarding the need to revise the current suite of secondary PM 2.5 and PM 10 standards to address visibility impairment and other welfare effects considered in this review. Specifically, this section describes the Administrator's final decision to retain the current suite of secondary PM standards to address PM-related visibility impairment as well as other PM-related welfare effects, including ecological effects, effects on materials, and climate impacts. This suite of standards includes an annual PM 2.5 standard of 15 μg/m [3] , a 24-hour PM 2.5 standard of 35 μg/m [3] , and a 24-hour PM 10 standard of 150 μg/m [3] . The Administrator is revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard. Contrary to what was proposed, the Administrator has decided not to establish a distinct standard to address PM-related visibility impairment. The rationale for this decision is presented below.

The Administrator's final decisions on the secondary standards are based on a thorough review of the latest scientific information published through mid-2009 on welfare effects associated with fine and coarse particles in the ambient air, as presented in the Integrated Science Assessment. The final decisions also take into account: (1) Staff assessments of the most policy-relevant information presented and assessed in the Integrated Science Assessment and staff analyses of air quality and visibility effects presented in the Visibility Assessment and the Policy Assessment, upon which staff conclusions regarding appropriate considerations in this review are based; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the Integrated Science Assessment, Visibility Assessment, and Policy Assessment at public meetings, in separate written comments, and in CASAC's letters to the Administrator; (3) the multiple rounds of public comments received during the development of these documents, both in connection with CASAC meetings and separately; and (4) public comments received on the proposal.

In particular, this section presents background information on the EPA's previous and current reviews of the secondary PM standards (section VI.A), a summary of the proposed decisions regarding the secondary PM standards (section VI.B), a discussion of significant public comments received on those proposed decisions (section VI.C), and the Administrator's final decisions on the secondary PM standards (section VI.D).

A. Background

The current suite of secondary PM standards is identical to the suite of primary PM standards set in 2006, including 24-hour and annual PM 2.5 standards and a 24-hour PM 10 standard. The current secondary PM 2.5 standards are intended to provide protection from PM-related visibility impairment, whereas the entire suite of secondary PM standards is intended to provide protection from other PM-related effects on public welfare, including effects on sensitive ecosystems, materials damage and soiling, and climatic and radiative processes.

The approach used for reviewing the current suite of secondary PM standards built upon and broadened the approaches used in previous PM NAAQS reviews. The following discussion focuses particularly on the current secondary PM 2.5 standards related to visibility impairment and provides a summary of the approaches used to review and establish secondary PM 2.5 standards in the last two reviews (section VI.A.1); judicial review of the 2006 standards that resulted in the remand of the secondary annual and 24-hour PM 2.5 NAAQS to the EPA (section VI.A.2); and the approach used in this review for evaluating the secondary PM 2.5 standards (section VI.A.3).

1. Approaches Used in Previous Reviews

The original secondary PM 2.5 standards were established in 1997, and a revision to the 24-hour standard was made in 2006. The approaches used in making final decisions on secondary standards in those reviews, as well as the current review, utilized different ways to consider the underlying body of scientific evidence. They also reflected an evolution in EPA's understanding of the nature of the effect on public welfare from PM-related visibility impairment, from an approach that focused only on Federal Class I area visibility impacts to a more multifaceted approach that also considered PM-related impacts on visibility in non-Federal Class I areas, such as in urban areas. This evolution occurred in conjunction with the expansion of available PM data and information from visibility-related studies of public perception, valuation, and personal comfort and well-being.

In 1997, the EPA revised the PM NAAQS in part by establishing new identical primary and secondary PM 2.5 standards. In revising the secondary standards, the EPA recognized that PM produces adverse effects on visibility and that impairment of visibility was being experienced throughout the U.S., in multi-state regions, urban areas, and remote mandatory Federal Class I areas alike. However, in considering an appropriate level for a secondary standard to address adverse effects of PM 2.5 on visibility, the EPA concluded that the determination of a single national level was complicated by important regional differences influenced by factors such as background and current levels of PM 2.5, composition of PM 2.5, and average relative humidity. Variations in these factors across regions could thus result in situations where attaining an appropriately protective concentration of fine particles in one region might or might not provide adequate protection in a different region. The EPA also determined that there was insufficient information at that time to establish a level for a national secondary standard that would represent a threshold above which visibility conditions would always be adverse and below which visibility conditions would always be acceptable.

Based on an assessment of the potential visibility improvements that would result from reaching attainment with the new primary standards for PM 2.5, the EPA concluded that attainment of the annual and 24-hour PM 2.5 primary standards would lead to visibility improvements in the eastern U.S. at both urban and regional scales, but little or no change in the western U.S., except in and near certain urban areas.

The EPA also considered the potential effectiveness of a regional haze program, required by sections 169A and 169B of the CAA [145] to address those effects of PM on visibility that would not be addressed through attainment of the primary PM 2.5 standards. The regional haze program would be designed to address the widespread, regionally uniform type of haze caused by a multitude of sources. The structure and requirements of sections 169A and 169B of the CAA provide for visibility protection programs that can be more responsive to the factors contributing to regional differences in visibility than can programs addressing the kinds of nationally applicable secondary NAAQS considered in the 1997 review. The regional haze visibility goal is more protective than a secondary NAAQS since the goal is to eliminate any anthropogenic impairment rather than to provide a level of protection from visibility impairment that is requisite to protect the public welfare. Thus, an important factor considered in the 1997 review was whether a regional haze program, in conjunction with secondary standards set identical to the suite of PM 2.5 primary standards, would provide appropriate protection for visibility in non-Federal Class I areas. The EPA concluded that the two programs and associated control strategies should provide such protection due to the regional approaches needed to manage emissions of pollutants that impair visibility in many of these areas.

For these reasons, in 1997 the EPA concluded that a national regional haze program, combined with a nationally applicable level of protection achieved through secondary PM 2.5 standards set identical to the primary PM 2.5 standards, would be more effective for addressing regional variations in the adverse effects of PM 2.5 on visibility than would be national secondary standards for PM with levels lower than the primary PM 2.5 standards. The EPA further recognized that people living in certain urban areas may place a high value on unique scenic resources in or near these areas and as a result might experience visibility problems attributable to sources that would not necessarily be addressed by the combined effects of a regional haze program and PM 2.5 secondary standards. The EPA concluded that in such cases, state or local regulatory approaches, such as past action in Colorado to establish a local visibility standard for the City of Denver, would be more appropriate and effective in addressing these special situations because of the localized and unique characteristics of the problems involved. Visibility in an urban area located near a mandatory Federal Class I area could also be improved through state implementation of the then-current visibility regulations, by which emission limitations can be imposed on a source or group of sources found to be contributing to “reasonably attributable” impairment in the mandatory Federal Class I area.

Based on these considerations, in 1997 the EPA set secondary PM 2.5 standards identical to the primary PM 2.5 standards, that would work in conjunction with the Regional Haze Program to be established under sections 169A and 169B of the CAA, as the most appropriate and effective means of addressing the public welfare effects associated with visibility impairment. Together, the two programs and associated control strategies were expected to provide appropriate protection against PM-related visibility impairment and enable all regions of the country to make reasonable progress toward the national visibility goal.

In 2006, the EPA revised the suite of secondary PM 2.5 standards to address visibility impairment by making the suite of secondary standards identical to the revised suite of primary PM 2.5 standards. The EPA's decision regarding the need to revise the suite of secondary PM 2.5 standards reflected a number of new developments that had occurred and sources of information that had become available following the 1997 review. First, the EPA promulgated a Regional Haze Program in 1999 (65 FR 35713, July 1, 1999) which required states to establish goals for improving visibility in Federal Class I areas and to adopt control strategies to achieve these goals. Second, extensive new information from visibility and fine particle monitoring networks had become available, allowing for updated characterizations of visibility trends and PM concentrations in urban areas, as well as Federal Class I areas. These new data allowed the EPA to better characterize visibility impairment in urban areas and the relationship between visibility and PM 2.5 concentrations. Finally, additional studies in the U.S. and abroad provided the basis for the establishment of standards and programs to address specific visibility concerns in a number of local areas. These studies (Denver, Phoenix, and British Columbia) utilized photographic representations of visibility impairment and produced reasonably consistent results in terms of the visual ranges found to be generally acceptable by study participants. The EPA considered the information generated by these studies useful in characterizing the nature of particle-induced haze and for informing judgments about the acceptability of various levels of visual air quality in urban areas across the U.S. Based largely on this information, the Administrator concluded that it was appropriate to revise the secondary PM 2.5 standards to provide increased protection from visibility impairment principally in urban areas, in conjunction with the regional haze program for protection of visual air quality in Federal Class I areas.

In so doing, the Administrator recognized that PM-related visibility impairment is principally related to fine particle concentrations and that perception of visibility impairment is most directly related to short-term, nearly instantaneous levels of visual air quality. Thus, in considering whether the then-current suite of secondary standards would provide the appropriate degree of protection, he concluded that it was appropriate to focus on just the 24-hour secondary PM 2.5 standard to provide requisite protection.

The Administrator then considered whether PM 2.5 mass remained the appropriate indicator for a secondary standard to protect visibility, primarily in urban areas. The Administrator noted that PM-related visibility impairment is principally related to fine particle levels. Hygroscopic components of fine particles, in particular sulfates and nitrates, contribute disproportionately to visibility impairment under high humidity conditions. Particles in the coarse mode generally contribute only marginally to visibility impairment in urban areas. With the substantial addition to the air quality and visibility data made possible by the national urban PM 2.5 monitoring networks, an analysis conducted for the 2006 review found that, in urban areas, visibility levels showed far less difference between eastern and western regions on a 24-hour or shorter time basis than implied by the largely non-urban data available in the 1997 review. In analyzing how well PM 2.5 concentrations correlated with visibility in urban locations across the U.S., the 2005 Staff Paper concluded that clear correlations existed between 24-hour average PM 2.5 concentrations and calculated (i.e., reconstructed) light extinction, which is directly related to visual range (U.S. EPA, 2005, p. 7-6). These correlations were similar in the eastern and western regions of the U.S. These correlations were less influenced by relative humidity and more consistent across regions when PM 2.5 concentrations were averaged over shorter, daylight time periods (e.g., 4 to 8 hours) when relative humidity in eastern urban areas was generally lower and thus more similar to relative humidity in western urban areas. The 2005 Staff Paper noted that a standard set at any specific PM 2.5 concentration would necessarily result in visual ranges that vary somewhat in urban areas across the country, reflecting the variability in the correlations between PM 2.5 concentrations and light extinction. The 2005 Staff Paper concluded that it was appropriate to use PM 2.5 as an indicator for standards to address visibility impairment in urban areas, especially when the indicator is defined for a relatively short period (e.g., 4 to 8 hours) of daylight hours (U.S. EPA, 2005, p. 7-6). Based on their review of the Staff Paper, most CASAC Panel members also endorsed such a PM 2.5 indicator for a secondary standard to address visibility impairment (Henderson, 2005a, p. 9). Based on the above considerations, the Administrator concluded that PM 2.5 should be retained as the indicator for fine particles as part of a secondary standard to address visibility protection, in conjunction with averaging times from 4 to 24 hours.

In considering what level of protection against PM-related visibility impairment would be appropriate, the Administrator took into account the results of the public perception and attitude surveys regarding the acceptability of various degrees of visibility impairment in the U.S. and Canada, state and local visibility standards within the U.S., and visual inspection of photographic representations of several urban areas across the U.S. In the Administrator's judgment, these sources provided useful but still quite limited information on the range of levels appropriate for consideration in setting a national visibility standard primarily for urban areas, given the generally subjective nature of the public welfare effect involved. Based on photographic representations of varying levels of visual air quality, public perception studies, and local and state visibility standards, the 2005 Staff Paper had concluded that 30 to 20 μg/m [3] PM 2.5 represented a reasonable range for a national visibility standard primarily for urban areas, based on a sub-daily averaging time (U.S. EPA, 2005, p. 7-13). The upper end of this range was below the levels at which illustrative scenic views are significantly obscured, and the lower end was around the level at which visual air quality generally appeared to be good based on observation of the illustrative views. This concentration range generally corresponded to median visual ranges in urban areas within regions across the U.S. of approximately 25 to 35 km, a range that was bounded above by the visual range targets selected in specific areas where state or local agencies placed particular emphasis on protecting visual air quality. In considering a reasonable range of forms for a PM 2.5 standard within this range of levels, the 2005 Staff Paper had concluded that a concentration-based percentile form was appropriate, and that the upper end of the range of concentration percentiles for consideration should be consistent with the 98th percentile used for the primary standard and that the lower end of the range should be the 92nd percentile, which represented the mean of the distribution of the 20 percent most impaired days, as targeted in the regional haze program (U.S. EPA, 2005 pp. 7-11 to 7-13). While recognizing that it was difficult to select any specific level and form based on then-currently available information (Henderson, 2005a, p. 9), the CASAC Panel was generally in agreement with the ranges of levels and forms presented in the 2005 Staff Paper.

The Administrator also considered the level of protection that would be afforded by the proposed suite of primary PM 2.5 standards (71 FR 2681, January 17, 2006), on the basis that although significantly more information was available than in the 1997 review concerning the relationship between fine PM levels and visibility across the country, there was still little available information for use in making the relatively subjective value judgment needed in selecting the appropriate degree of protection to be afforded by such a standard. In so doing, the Administrator compared the extent to which the proposed suite of primary standards would require areas across the country to improve visual air quality with the extent of increased protection likely to be afforded by a standard based on a sub-daily averaging time. Based on such an analysis, the Administrator observed that the predicted percent of counties with monitors not likely to meet the proposed suite of primary PM 2.5 standards was actually somewhat greater than the predicted percent of counties with monitors not likely to meet a sub-daily secondary standard with an averaging time of 4 daylight hours, a level toward the upper end of the range recommended in the 2005 Staff Paper, and a form within the recommended range. Based on this comparison, the Administrator tentatively concluded that revising the secondary 24-hour PM 2.5 standard to be identical to the proposed revised primary PM 2.5 standard (and retaining the then-current annual secondary PM 2.5 standard) was a reasonable policy approach to addressing visibility protection primarily in urban areas. In proposing this approach, the Administrator also solicited comment on a sub-daily (4- to 8-hour averaging time) secondary PM 2.5 standard (71 FR 2675 to 2781, January 17, 2006).

In commenting on the proposed decision, the CASAC requested that a sub-daily standard to protect visibility “be favorably reconsidered” (Henderson, 2006a, p.6). The CASAC noted three cautions regarding the proposed reliance on a secondary PM 2.5 standard identical to the proposed 24-hour primary PM 2.5 standard: (1) PM 2.5 mass measurement is a better indicator of visibility impairment during daylight hours, when relative humidity is generally low; the sub-daily standard more clearly matches the nature of visibility impairment, whose adverse effects are most evident during the daylight hours; using a 24-hour PM 2.5 standard as a proxy introduces error and uncertainty in protecting visibility; and sub-daily standards are used for other NAAQS and should be the focus for visibility; (2) CASAC and its monitoring subcommittees had repeatedly commended EPA's initiatives promoting the introduction of continuous and near-continuous PM monitoring and recognized that an expanded deployment of continuous PM 2.5 monitors would be consistent with setting a sub-daily standard to protect visibility; and (3) the analysis showing a similarity between percentages of counties not likely to meet what the CASAC Panel considered to be a lenient 4- to 8-hour secondary standard and a secondary standard identical to the proposed 24-hour primary standard was a numerical coincidence that was not indicative of any fundamental relationship between visibility and health. The CASAC Panel further stated that “visual air quality is substantially impaired at PM 2.5 concentrations of 35 μg/m [3] ” and that “[i]t is not reasonable to have the visibility standard tied to the health standard, which may change in ways that make it even less appropriate for visibility concerns” (Henderson, 2006a, pp. 5 to 6).

In reaching a final decision, the Administrator focused on the relative protection provided by the proposed primary standards based on the above-mentioned similarities in percentages of counties meeting alternative standards and on the limitations in the information available concerning studies of public perception and attitudes regarding the acceptability of various degrees of visibility impairment in urban areas, as well as on the subjective nature of the judgment required. In so doing, the Administrator concluded that caution was warranted in establishing a distinct secondary standard for visibility impairment and that the available information did not warrant adopting a secondary standard that would provide either more or less protection against visibility impairment in urban areas than would be provided by secondary standards set equal to the proposed primary PM 2.5 standards.

2. Remand of 2006 Secondary PM 2.5 Standards

As noted above in section II.B.2 above, several parties filed petitions for review challenging EPA's decision to set the secondary NAAQS for fine PM identical to the primary NAAQS. On judicial review, the D.C. Circuit remanded to the EPA for reconsideration the secondary NAAQS for fine PM because the Agency's decision was unreasonable and contrary to the requirements of section 109(b)(2). American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir., 2009).

The petitioners argued that the EPA's decision lacked a reasoned basis. First, they asserted that the EPA never determined what level of visibility was “requisite to protect the public welfare.” They argued that the EPA unreasonably rejected the target level of protection recommended by its staff, while failing to provide a target level of its own. The court agreed, stating that “the EPA's failure to identify such a level when deciding where to set the level of air quality required by the revised secondary fine PM NAAQS is contrary to the statute and therefore unlawful. Furthermore, the failure to set any target level of visibility protection deprived the EPA's decision-making of a reasoned basis.” 559 F. 3d at 530.

Second, the petitioners challenged EPA's method of comparing the protection expected from potential standards. They contended that the EPA relied on a meaningless numerical comparison, ignored the effect of humidity on the usefulness of a standard using a daily averaging time, and unreasonably concluded that the primary standards would achieve a level of visibility roughly equivalent to the level the EPA staff and CASAC deemed “requisite to protect the public welfare.” The court found that the EPA's equivalency analysis based on the percentages of counties exceeding alternative standards “failed on its own terms.” The same table showing the percentages of counties exceeding alternative secondary standards, used for comparison to the percentages of counties exceeding alternative primary standards to show equivalency, also included six other alternative secondary standards within the recommended CASAC range that would be more “protective” under EPA's definition than the adopted primary standards. Two-thirds of the potential secondary standards within the CASAC's recommended range would be substantially more protective than the adopted primary standards. The court found that the EPA failed to explain why it looked only at one of the few potential secondary standards that would be less protective, and only slightly less so, than the primary standards. More fundamentally, however, the court found that the EPA's equivalency analysis based on percentages of counties demonstrated nothing about the relative protection offered by the different standards, and that the tables offered no valid information about the relative visibility protection provided by the standards. 559 F. 3d at 530-31.

Finally, the Staff Paper had made clear that a visibility standard using PM 2.5 mass as the indicator in conjunction with a daily averaging time would be confounded by regional differences in humidity. The court noted that the EPA acknowledged this problem, yet did not address this issue in concluding that the primary standards would be sufficiently protective of visibility. 559 F. 3d at 530. Therefore, the court granted the petition for review and remanded for reconsideration the secondary PM 2.5 NAAQS.

3. General Approach Used in the Policy Assessment for the Current Review

The approach used in this review broadened the general approaches used in the last two PM NAAQS reviews by utilizing, to the extent available, enhanced tools, methods, and data to more comprehensively characterize visibility impacts. As such, the EPA took into account considerations based on both the scientific evidence (“evidence-based”) and a quantitative analysis of PM-related impacts on visibility (“impact-based”) to inform conclusions related to the adequacy of the current secondary PM 2.5 standards and alternative standards that were appropriate for consideration in this review. As in past reviews, the EPA also considered that the secondary NAAQS should address PM-related visibility impairment in conjunction with the Regional Haze Program, such that the secondary NAAQS would focus on protection from visibility impairment principally in urban areas in conjunction with the Regional Haze Program that is focused on improving visibility in Federal Class I areas. The EPA again recognized that such an approach remains the most appropriate and effective means of addressing the public welfare effects associated with visibility impairment in areas across the country.

The Policy Assessment drew from the qualitative evaluation of all studies discussed in the Integrated Science Assessment (U.S. EPA, 2009a). Specifically, the Policy Assessment considered the extensive new air quality and source apportionment information available from the regional planning organizations, long-standing evidence of PM effects on visibility, and limited public preference study information from four urban areas (U.S. EPA, 2009a, chapter 9), as well as the integration of evidence across disciplines (U.S. EPA, 2009a, chapter 2). In addition, limited information that had become available regarding the characterization of public preferences in urban areas provided some new perspectives on the usefulness of this information in informing the selection of target levels of urban visibility protection. On these bases, the Policy Assessment again focused assessments on visibility conditions in urban areas.

The conclusions in the Policy Assessment reflected EPA staff's understanding of both evidence-based and impact-based considerations to inform two overarching questions related to (1) the adequacy of the current suite of PM 2.5 standards and (2) what potential alternative standards, if any, should be considered in this review to provide appropriate protection from PM-related visibility impairment. In addressing these broad questions, the discussions in the Policy Assessment were organized around a series of more specific questions reflecting different aspects of each overarching question (U.S. EPA, 2011a, Figure 4-1). When evaluating the visibility protection afforded by the current or any alternative standards considered, the Policy Assessment took into account the four basic elements of the NAAQS: indicator, averaging time, level, and form.

B. Proposed Decisions on Secondary PM Standards

At the time of proposal, the Administrator proposed to revise the suite of secondary PM standards by adding a distinct standard for PM 2.5 to address PM-related visibility impairment, focused primarily on visibility in urban areas. This proposed standard was to be defined in terms of a PM 2.5 visibility index, which would use measured PM 2.5 mass concentration, in combination with speciation and relative humidity data, to calculate PM 2.5 light extinction, translated into the deciview (dv) scale; a 24-hour averaging time; a 90th percentile form, averaged over 3 years; and a level of 28-30 dv. To address other non-visibility welfare effects, the Administrator proposed to retain the current suite of secondary PM standards generally, while revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this proposed change to the primary annual PM 2.5 standard. Each of these proposed decisions is described in more detail in the proposal and below.

1. PM-Related Visibility Impairment

As discussed in Section VI.B of the proposal, the Administrator's proposed decision regarding a distinct secondary standard to provide protection from visibility impairment reflected careful consideration of the following: (1) The latest scientific information on visibility effects associated with PM as described in the Integrated Science Assessment (U.S. EPA, 2009a); (2) insights gained from assessments of correlations between ambient PM 2.5 and visibility impairment prepared by EPA staff in the Visibility Assessment (U.S. EPA, 2010b); and (3) specific conclusions regarding the need for revisions to the current standards (i.e., indicator, averaging time, form, and level) that, taken together, would be requisite to protect the public welfare from adverse effects on visual air quality. This section summarizes key information from the proposal regarding the nature of visibility impairment, including the relationship between ambient PM and visibility, temporal variations in light extinction, periods during the day of interest for assessing visibility conditions, and exposure durations of interest (section VI.B.1.a); limited public perceptions and attitudes about visibility impairment and the impacts of visibility impairment on public welfare (section VI.B.1.b); CASAC advice regarding the need for, and design of, secondary standards to protect visibility (section VI.B.1.c); and the Administrator's proposed conclusions regarding setting a distinct standard to address visibility impairment (section VI.B.1.d).

a. Nature of PM-Related Visibility Impairment

As noted at the time of proposal, the fundamental science characterizing the contribution of PM, especially fine particles, to visibility impairment is well understood. This science provides the basis for the Integrated Science Assessment designation of the relationship between PM and visibility impairment as causal. New research available in this review, discussed in chapter 9 of the Integrated Science Assessment, continues to support and refine EPA's understanding of the effect of PM on visibility and the source contributions to that effect in rural and remote locations. This research provides new insights regarding the regional source contributions to urban visibility impairment and better characterization of the increment in PM concentrations and visibility impairment that occur in many cities (i.e., the urban excess) relative to conditions in the surrounding rural areas (i.e., regional background). Ongoing urban PM 2.5 speciated and aggregated mass monitoring has produced new information that has allowed for updated characterization of current visibility levels in urban areas.

i. Relationship Between Ambient PM and Visibility

Visibility impairment is caused by the scattering and absorption of light by suspended particles and gases in the atmosphere. When PM is present in the air, its contribution to light extinction typically greatly exceeds that of gases. The combined effect of light scattering and absorption by both particles and gases is characterized as light extinction, i.e., the fraction of light that is scattered or absorbed in the atmosphere. Light extinction can be quantified by a light extinction coefficient with units of 1/distance, which is often expressed as 1/(1 million meters) or inverse megameters (abbreviated Mm -1) or in terms of an alternative scale known as the deciview scale, defined by the following equation: [146]

Deciview (dv) = 10 ln (b ext/ 10 Mm -1)

The deciview scale is frequently used in the scientific literature on visibility, as well as in the Regional Haze Program. In particular, the deciview scale is used in the public perception studies that were considered in the past and current reviews to inform judgments about an appropriate degree of protection to be provided by a secondary NAAQS.

The amount of light extinction contributed by PM depends on the particle concentration as well as on the particle size distribution and composition and also on the relative humidity. As described in detail in section VI.B.1.a of the proposal, visibility scientists have developed an algorithm, known as the IMPROVE algorithm, [147] to estimate light extinction using routinely monitored fine particle (PM 2.5) speciation and coarse particle mass (PM 10-2.5) data, as well as data on relative humidity. There is both an original and a revised version of the IMPROVE algorithm (Pitchford et al., 2007). The revised version was developed to address observed biases in the predictions using the original algorithm under very low and very high light extinction conditions. [148] These IMPROVE algorithms are routinely used to calculate light extinction levels on a 24-hour basis in Federal Class I areas under the Regional Haze Program.

In either version of the IMPROVE algorithm, the concentration of each of the major aerosol components is multiplied by a dry extinction efficiency value and, for the hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also multiplied by an additional factor to account for the water growth to estimate these components' contribution to light extinction. Summing the contribution of each component gives the estimate of total light extinction per unit distance denoted as the light extinction coefficient (b ext), as shown below for the original IMPROVE algorithm.

b ext≉ 3 ×f (RH) × [Sulfate]

+ 3 ×f (RH) x [Nitrate]

+ 4 × [Organic Mass]

+ 10 × [Elemental Carbon]

+ 1 × [Fine Soil]

+ 0.6 × [Coarse Mass]

+ 10

Light extinction (b ext) is in units of Mm -1, the mass concentrations of the components indicated in brackets are in units of μg/m [3] , and f (RH) is the unitless water growth term that depends on relative humidity. The final term of 10 Mm -1 is known as the Rayleigh scattering term and accounts for light scattering by the natural gases in unpolluted air. Despite the simplicity of this algorithm, it performs reasonably well and permits the contributions to light extinction from each of the major components (including the water associated with the sulfate and nitrate compounds) to be separately approximated. Inspection of the PM component-specific terms in the simple original IMPROVE algorithm shows that most of the PM 2.5 components contribute 5 times or more light extinction than a similar concentration of PM 10-2.5.

The f (RH ) term in the original algorithm reflects the increase in light scattering caused by particulate sulfate and nitrate under conditions of high relative humidity. Particles with hygroscopic components (e.g., particulate sulfate and nitrate) contribute more light extinction at higher relative humidity than at lower relative humidity because they change size in the atmosphere in response to ambient relative humidity conditions. For relative humidity below 40 percent the f (RH) value is 1, but it increases to 2 at approximately 66 percent, 3 at approximately 83 percent, 4 at approximately 90 percent, 5 at approximately 93 percent, and 6 at approximately 95 percent relative humidity. The result is that both particulate sulfate and nitrate are more efficient per unit mass in light extinction than any other aerosol component for relative humidity above approximately 85 percent where their total light extinction efficiency exceeds the 10 m [2] /g associated with elemental carbon (EC). PM containing elemental or black carbon (BC) absorbs light as well as scattering it, making it the component with the greatest light extinction contributions per unit of mass concentration, except for the hygroscopic components under these high relative humidity conditions. [149]

As noted above, subsequent to the development of the original IMPROVE algorithm, an alternative algorithm (variously referred to as the “revised algorithm” or the “new algorithm” in the literature) was developed. The revised IMPROVE algorithm is different from the original algorithm in several important ways. First, the revised algorithm employs a more complex split-component mass extinction efficiency to correct biases believed to be related to particle size distributions. [150] Specifically, the revised algorithm incorporates terms to account for particles representing the different dry extinction and water uptake from two size modes of sulfate, nitrate and organic mass. [151] Second, the revised algorithm uses a different multiplier for organic carbon for purposes of estimating organic carbonaceous material to better represent aged aerosol found in remote areas. [152] In addition, the revised algorithm includes a term for hygroscopic sea salt that can be important for remote coastal areas, and site-specific Rayleigh light scattering terms in place of a universal Rayleigh light scattering value. As noted in section VI.B.1.a of the proposal, the revised IMPROVE algorithm can yield higher estimates of current light extinction levels in urban areas on days with relatively poor visibility as compared to the original algorithm (Pitchford, 2010). This difference is primarily attributable to the split-component mass extinction efficiency treatment in the revised algorithm. This revised algorithm was evaluated at 21 remote locations and is generally used by RPOs and States for implementation of the Regional Haze Rule.

ii. Temporal Variations of Light Extinction

Particulate matter concentrations and light extinction in urban environments vary from hour to hour throughout the 24-hour day due to a combination of diurnal changes in meteorological conditions and systematic changes in emissions activity (e.g., rush hour traffic). Various factors combine to make early morning the most likely time for peak urban light extinction; although the net effects of the systematic urban- and larger-scale variations mean that peak daytime PM light extinction levels can occur any time of day, in many areas they occur most often in early morning hours (U.S. EPA, 2010b, sections 3.4.2 and 3.4.3; Figures 3-9, 3-10, and 3-12). This temporal pattern in urban areas contrasts with the general lack of a strong diurnal pattern in PM concentrations and light extinction in most Federal Class I areas, reflective of a relative lack of local sources as compared to urban areas. The use in the Regional Haze Program of 24-hour average concentrations in the IMPROVE algorithm is consistent with this general lack of a strong diurnal pattern in Federal Class I areas.

iii. Periods During the Day of Interest for Assessment of Visibility

As noted in sections VI.B.1.b and VI.B.1.c of the proposal, daytime visibility has dominated the attention of those who have studied the visibility effects of air pollution, particularly in urban areas. The EPA recognizes, however, that physically PM light extinction behaves the same at night as during the day and can contribute to nighttime visibility effects by enhancing the scattering of anthropogenic light, contributing to the “skyglow” within and over populated areas, adding to the total sky brightness, and contributing to the reduction in contrast of stars against the background. However, little research has been conducted on nighttime visibility, and the state of the science is not comparable to that associated with daytime visibility impairment, particularly in terms of the impact on human welfare. The Policy Assessment notes that the science is not available at this time to support adequate characterization specifically of nighttime PM light extinction conditions and the related effects on public welfare (U.S. EPA, 2011a, p. 4-18). Therefore the EPA has focused its assessments of PM visibility impacts in urban areas on daylight hours during this review.

iv. Exposure Durations of Interest

As noted in section VI.B.1.d of the proposal, the roles that exposure duration and variations in visual air quality within any given exposure period play in determining the acceptability or unacceptability of a given level of visual air quality have not been investigated via preference studies. In the preference studies available for this review, subjects were simply asked to rate the acceptability or unacceptability of each image of a haze-obscured scene, without being provided any suggestion of assumed duration or of assumed conditions before or after the occurrence of the scene presented. Preference and/or valuation studies show that atmospheric visibility conditions can be quickly assessed and preferences determined. The EPA is unaware of any studies that characterize the extent to which different frequencies and durations of exposure to visibility conditions contribute to the degree of public welfare impact that occurs.

The Policy Assessment considered a variety of circumstances that are commonly expected to occur in evaluating the potential impact of visibility impairment on the public welfare based on available information (U.S. EPA, 2011a, pp. 4-19 to 4-20). In some circumstances, such as infrequent visits to scenic vistas in natural or urban environments, people are motivated specifically to take the opportunity to view a valued scene and are likely to do so for many minutes to hours to appreciate various aspects of the vista they choose to view. However, the public has many more opportunities to notice visibility conditions on a daily basis in settings associated with performing daily routines (e.g., during commutes and while working, exercising, or recreating outdoors). As noted in the Policy Assessment, information regarding the fraction of the public that has only one or a few opportunities to experience visibility during the day, or on the role the duration of the observed visibility conditions has on wellbeing effects associated with those visibility conditions, is not available (U.S. EPA, 2011a, p. 4-20). However, it is possible that people with limited opportunities to experience visibility conditions on a daily basis would receive the entire impact of the day's visual air quality based on the visibility conditions that occur during the short time period when they can see it. Since this group could be affected on the basis of observing visual air quality conditions for periods as short as one hour or less, and because during each daylight hour there are some people outdoors, commuting, or near windows, the Policy Assessment judged that it would be appropriate to use the maximum hourly value of PM light extinction during daylight hours for each day for purposes of evaluating the adequacy of the current suite of secondary standards. Other observers may have access to visibility conditions throughout the day. For this group, it might be that an hour with poor or “unacceptable” visibility can be offset by one or more other hours with clearer conditions. Therefore, the proposal acknowledged that it might also be appropriate to consider a multi-hour daylight exposure period.

v. Periods of Fog and Rain

As discussed in section VI.C of the proposal, the EPA also recognized that it is appropriate to give special treatment to periods of fog and rain when considering whether current PM 2.5 standards adequately protect public welfare from PM-related visibility impairment. Visibility impairment occurs during periods with fog or precipitation irrespective of the presence or absence of PM. Therefore, it is logical that periods with naturally impaired visibility due to fog or precipitation should not be treated as having PM-impaired visibility. There are multiple ways to adjust visibility data to reduce the effects of fog and precipitation. In the Visibility Assessment, following the advice of CASAC, the EPA evaluated the effect of excluding daylight hours for which relative humidity was greater than 90 percent from analyses in order to avoid precipitation and fog confounding estimates of PM visibility impairment. For the 15 urban areas included in the Visibility Assessment, the EPA found that a 90 percent relative humidity cutoff criterion was effective in that on average less than 6 percent of the daylight hours were removed from consideration, yet those hours had on average ten times the likelihood of rain, six times the likelihood of snow/sleet, and 34 times the likelihood of fog compared with hours with 90 percent or lower relative humidity. In the Regional Haze program, the EPA utilizes monthly average relative humidity values based on 10 years of climatological data to reduce the effect of fog and precipitation. This approach focuses on longer-term averages for each monitoring site and thereby eliminates the effect of very high humidity conditions on visibility at those locations.

b. Public Perception of Visibility Impairment

As described in section VI.B.2 of the proposal, there are two main types of studies that evaluate the public perception of urban visibility impairment: urban visibility preference studies and urban visibility valuation studies. As noted in the Integrated Science Assessment, “[b]oth types of studies are designed to evaluate individuals' desire (or demand) for good visual air quality (VAQ) where they live, using different metrics to evaluate demand. Urban visibility preference studies examine individuals' demand by investigating what amount of visibility degradation is unacceptable while economic studies examine demand by investigating how much one would be willing to pay to improve visibility” (U.S. EPA, 2009a, p. 9-66). Because of the limited number of new studies on urban visibility valuation, the Integrated Science Assessment cites to the discussion in the 2004 Criteria Document of the various methods one can use to determine the economic valuation of changes in visibility, which include hedonic valuation, contingent valuation and contingent choice, and travel cost.

Contingent valuation studies are a type of stated preference study that measures the strength of preferences and expresses that preference in dollar values. Contingent valuation studies often include payment vehicles that require respondents to consider implementation costs and their ability to pay for visibility improvements in their responses. This study design aspect is critical because the EPA cannot consider implementations costs in setting either primary or secondary NAAQS. Therefore in considering the information available to help inform the standard-setting process, the EPA has focused on the public perception studies that do not embed consideration of implementation costs. Nonetheless, the EPA recognizes that valuation studies do provide additional evidence that the public is experiencing losses in welfare due to visibility impairment. [153] The public perception studies are described in detail below.

In order to identify levels of visibility impairment appropriate for consideration in setting secondary PM NAAQS to protect the public welfare, the Visibility Assessment comprehensively examined information that was available in this review regarding people's stated preferences regarding acceptable and unacceptable visual air quality.

Light extinction is an atmospheric property that by itself does not directly translate into a public welfare effect. Instead, light extinction becomes meaningful in the context of the impact of differences in visibility on the human observer. This has been studied in terms of the acceptability or unacceptability expressed for the visibility impact of a given level of light extinction by a human observer. The perception of the visibility impact of a given level of light extinction occurs in conjunction with the associated characteristics and lighting conditions of the viewed scene. [154] Thus, a given level of light extinction may be perceived differently by observers looking at different scenes or the same scene with different lighting characteristics. Likewise, different observers looking at the same scene with the same lighting may have different preferences regarding the associated visual air quality. When scene and lighting characteristics are held constant, the perceived appearance of a scene (i.e., how well the scenic features can be seen and the amount of visible haze) depends only on changes in light extinction. This has been demonstrated using the WinHaze model (Molenar et al., 1994) that uses image processing technology to apply user-specified changes in light extinction values to the same base photograph with set scene and lighting characteristics.

Much of what is known about the acceptability of levels of visibility comes from survey studies in which participants were asked questions about their preference or the value they place on various visibility levels as displayed to them in scenic photographs and/or WinHaze images with a range of known light extinction levels. The Visibility Assessment (U.S. EPA, 2010b, chapter 2) reviewed the limited number of urban visibility preference studies currently available (i.e., four studies) to assess the light extinction levels judged by the participant to have acceptable visibility for those particular scenes.

The reanalysis of urban preference studies conducted in the Visibility Assessment for this review included three completed western urban visibility preference survey studies plus a pair of smaller focus studies designed to explore and further develop urban visibility survey instruments. The three western studies included one in Denver, Colorado (Ely et al., 1991), one in the lower Fraser River valley near Vancouver, British Columbia (BC), Canada (Pryor, 1996), and one in Phoenix, Arizona (BBC Research Consulting, 2003). A pilot focus group study was also conducted for Washington, DC (Abt Associates Inc., 2001). In response to an EPA request for public comment on the Scope and Methods Plan (74 FR 11580, March 18, 2009), comments were received (Smith, 2009) about the results of a new focus group study of scenes from Washington, DC, that had been conducted on subjects from both Houston, Texas, and Washington, DC, using scenes, methods and approaches similar to the method and approach employed in the EPA pilot study (Smith and Howell, 2009). When taken together, these studies from the four different urban areas included a total of 852 individuals, with each individual responding to a series of questions while viewing a set of images of various urban visual air quality conditions.

The approaches used in the four studies were similar and were all derived from the method first developed for the Denver urban visibility study. In particular, the studies all used a similar group interview type of survey to investigate the level of visibility impairment that participants described as “acceptable.” In each preference study, participants were initially given a set of “warm up” exercises to familiarize them with how the scene in the photograph or image appears under different VAQ conditions. The participants next were shown 25 randomly ordered photographs (images), and asked to rate each one based on a scale of 1 (poor) to 7 (excellent). They were then shown the same photographs or images again, in the same order, and asked to judge whether each of the photographs (images) would violate what they would consider to be an appropriate urban visibility standard (i.e. whether the level of impairment was “acceptable” or “unacceptable”). The term “acceptable” was not defined, so that each person's response was based on his/her own values and preferences for VAQ. However, when answering this question, participants were instructed to consider the following three factors: (1) The standard would be for their own urban area, not a pristine national park area where the standards might be stricter; (2) The level of an urban visibility standard violation should be set at a VAQ level considered to be unreasonable, objectionable, and unacceptable visually; and (3) Judgments of standards violations should be based on visibility only, not on health effects. While the results differed among the four urban areas, results from a rating exercise show that within each preference study, individual survey participants consistently distinguish between photos or images representing different levels of light extinction, and that more participants rate as acceptable images representing lower levels of light extinction than they do images representing higher levels.

Given the similarities in the approaches used, the EPA staff concluded that it was reasonable to compare the results to identify overall trends in the study findings and to conclude that this comparison can usefully inform the selection of a range of levels for use in further analyses. However, the staff also noted that variations in the specific materials and methods used in each study introduce uncertainties that should also be considered when interpreting the results of these comparisons. Key differences between the studies include the following: (1) Scene characteristics; (2) image presentation methods (e.g., projected slides of actual photos, projected images generated using WinHaze (a significant technical advance in the method of presenting visual air quality conditions), or use of a computer monitor screen; (3) number of participants in each study; (4) participant representativeness of the general population of the relevant metropolitan area; and (5) specific wording used to frame the questions used in the group interview process.

In the Visibility Assessment, each study was evaluated separately and figures developed to display the percentage of participants that rated the visual air quality depicted in each photograph as “acceptable.” Ely et al. (1991) introduced a “50% acceptability” criterion analysis of the Denver preference study results. The 50 percent acceptability criterion is designed to identify the visual air quality level (defined in terms of deciviews or light extinction) that best divides the photographs into two groups: Those with a visual air quality rated as acceptable by the majority of the participants, and those rated not acceptable by the majority of participants. The Visibility Assessment adopted this criterion as a useful index for comparison between studies. The results of each analysis were then combined graphically to allow for visual comparison. This information was then carried forward into the Policy Assessment. Figure 5 presents the graphical summary of the results of the studies in the four cities and draws on results previously presented in Figures 2-3, 2-5, 2-7, and 2-11 of chapter 2 in the Visibility Assessment. Figure 5 also contains lines at 20 dv and 30 dv that generally identify a range where the 50 percent acceptance criteria occur across all four of the urban preference studies (U.S. EPA, 2011a, p. 4-24). Out of the 114 data points shown in Figure 5, only one photograph (or image) with a visual air quality below 20 dv was rated as acceptable by less than 50 percent of the participants who rated that photograph. [155] Similarly, only one image with a visual air quality above 30 dv was rated acceptable by more than 50 percent of the participants who viewed it. [156]

As Figure 5 above shows, each urban area has a separate and unique response curve that appears to indicate that it is distinct from the others. [158] These curves are the result of a logistical regression analysis using a logit model of the greater than 19,000 ratings of haze images as acceptable or unacceptable. The model results can be used to estimate the visual air quality in terms of dv values where the estimated response functions cross the 50 percent acceptability level, as well as any alternative criteria levels. Selected examples of these are shown in Table 4- 1 of the Policy Assessment (U.S. EPA, 2011a; U.S. EPA, 2010b, Table 2-4). This table shows that the logit model results also support the upper and lower ends of the range of 50th percentile acceptability values (e.g., near 20 dv for Denver and near 30 dv for Washington, DC) already identified in Figure 5.

Based on the composite results and the effective range of 50th percentile acceptability across the four urban preference studies shown in Figure 5 and Table 4-1 of the Policy Assessment, benchmark levels of (total) light extinction were selected in a range from 20 dv to 30 dv (75 to 200 Mm −1) [159] for the purpose of provisionally assessing whether visibility conditions would be considered acceptable (i.e., less than the low end of the range), unacceptable (i.e., greater than the high end of the range), or potentially acceptable (within the range) based on the very limited public preference information. A midpoint of 25 dv (120 Mm −1) was also selected for use in the assessment. This level is also very near to the 50th percentile criterion value from the Phoenix study (i.e., 24.2 dv), which is by far the best of the four studies in terms of the fit of the data to the response curve and the representativeness of study participants. Based on the currently available information, the Policy Assessment concluded that the use of 25 dv to represent the middle of the distribution of results seemed well supported (U.S. EPA, 2011a, p. 4-25).

These three benchmark values provide a low, middle, and high set of light extinction conditions that are used to provisionally define daylight hours with urban haze conditions that have been judged unacceptable by at least 50 percent of the participants in one or more of these preference studies. As discussed above, PM light extinction is taken to be (total) light extinction minus the Rayleigh scatter, [160] such that the low, middle, and high levels correspond to PM light extinction levels of about 65 Mm −1, 110 Mm −1, and 190 Mm −1. In the Visibility Assessment, these three light extinction levels were called Candidate Protection Levels (CPLs). This term was also used in the Policy Assessment and in the proposal notice. It is important to note, however, that the degree of protection provided by a secondary NAAQS is not determined solely by any one component of the standard but by all the components (i.e., indicator, averaging time, form, and level) being applied together. Therefore, the Policy Assessment noted that the term CPL is meant only to indicate target levels of visibility within a range that the EPA staff felt appropriate for consideration that could, in conjunction with other elements of the standard, including indicator, averaging time, and form, potentially provide an appropriate degree of visibility protection.

In characterizing the Policy Assessment's confidence in each CPL and across the range, a number of issues were considered (U.S. EPA, 2011a, p. 4-26). Looking first at the two studies that define the upper and lower bounds of the range, the Policy Assessment considered whether they represent a true regional distinction in preferences for urban visibility conditions between western and eastern U.S. There was little information available to help evaluate the possibility of a regional distinction especially given that there have been preference studies in only one eastern urban area. Smith and Howell (2009) found little difference in preference response to Washington, DC, haze photographs between the study participants from Washington, DC, and those from Houston, Texas. [161] This provides some limited evidence that the value judgment of the public in different areas of the country may not be an important factor in explaining the differences in these study results.

In further considering what factors could explain the observed differences in preferences across the four urban areas, the Policy Assessment noted that the urban scenes used in each study had different characteristics (U.S. EPA, 2011a, p. 4-26). For example, each of the western urban visibility preference study scenes included mountains in the background while the single eastern urban study did not. It is also true that each of the western scenes included objects at greater distances from the camera location than in the eastern study. There is no question that objects at a greater distance have a greater sensitivity to perceived visibility changes as light extinction is changed compared to otherwise similar scenes with objects at a shorter range. This alone might explain the difference between the results of the eastern study and those from the western urban studies. Having scenes with the object of greatest intrinsic value nearer and hence less sensitive in the eastern urban area compared with more distant objects of greatest intrinsic value in the western urban areas could further explain the difference in preference results.

Another question considered was whether the high CPL value that is based on the eastern preference results is likely to be generally representative of urban areas that do not have associated mountains or other valued objects visible in the distant background. Such areas would include the middle of the country, many areas in the eastern U.S., and possibly some areas in the western U.S. as well. [162] Based on the currently available information, the Policy Assessment concluded that the high end of the CPL range (30 dv) is an appropriate level to consider (U.S. EPA, 2011a, p. 4-27).

With respect to the low end of the range, the Policy Assessment considered factors that might further refine its understanding of the robustness of this level. The Policy Assessment concluded that additional urban preference studies, especially with a greater variety in types of scenes, could help evaluate whether the lower CPL value of 20 dv is generally supportable (U.S. EPA, 2011a, p. 4-27). Further, the reason for the noisiness in data points around the curves apparent in both the Denver and British Columbia results compared to the smoother curve fit of Phoenix study results could be explored. One possible explanation discussed in the Policy Assessment is that these older studies use photographs taken at different times of day and on different days to capture the range of light extinction levels needed for the preference studies. In contrast, the use of WinHaze in the Phoenix (and Washington, DC) study reduced variations that affect scene appearance preference rating and avoided the uncertainty inherent in using ambient measurements to represent sight path-averaged light extinction values. Reducing these sources of noisiness and uncertainty in the results of future studies of sensitive urban scenes could provide more confidence in the selection of a low CPL value.

Based on the above considerations, and recognizing the limitations in the currently available information, the Policy Assessment concluded that it is reasonable to consider a range of CPL values including a high value of 30 dv, a mid-range value of 25 dv, and a low value of 20 dv (U.S. EPA, 2011a, p. 4-27). Based on its review of the second draft Policy Assessment, CASAC also supported this set of CPLs for consideration by the EPA in this review. CASAC noted that these CPL values were based on all available visibility preference data and that they bound the study results as represented by the 50 percent acceptability criteria. While recommending that further visibility preference studies be conducted to reduce remaining uncertainties, [163] CASAC concluded that this range of levels was “adequately supported by the evidence presented” (Samet, 2010d, p. iii).

c. Summary of Proposed Conclusions

i. Adequacy of the Current Standards for PM-Related Visibility Impairment

At the time of proposal, the Administrator provisionally concluded that the current suite of secondary PM standards is not sufficiently protective of visual air quality, and that consideration should be given to an alternative secondary standard that would provide additional protection against PM-related visibility impairment, with a focus primarily in urban areas. This proposed conclusion was based on the information presented in the proposal with regard to the nature of PM-related visibility impairment, the results of public perception surveys on the acceptability of varying degrees of visibility impairment in urban areas, analyses of the number of days that are estimated to exceed a range of candidate protection levels under conditions simulated to just meet the current standards, and the advice of CASAC. This section summarizes key points from section VI.C of the proposal regarding visibility under current conditions, the degree of protection afforded by the current standards, and CASAC's advice regarding the adequacy of the current standards.

As discussed in section VI.C.1 of the proposal, to evaluate visibility under current conditions the Visibility Assessment and Policy Assessment estimated PM-related light extinction [164] levels for 15 urban areas [165] in the United States. Consistent with the emphasis in this review on the hourly or multi-hour time periods that might reasonably characterize the visibility effects experienced by various segments of the population, these analyses focused on using maximum 1-hour and 4-hour values of PM light extinction during daylight hours for purposes of evaluating the degree of visibility impairment. Hourly average PM-related light extinction was analyzed in terms of both PM 10 and PM 2.5 light extinction. For reasons discussed above, hours with relative humidity greater than 90 percent were excluded from consideration. Recent visibility conditions in these urban areas were then compared to the CPLs identified above. The Visibility Assessment, which focused on PM 10 light extinction in 14 of the 15 urban areas during the 2005 to 2007 time period, [166] found that all 14 areas had daily maximum hourly PM 10 light extinction values estimated to exceed even the highest CPL some of the days. Except for the two Texas areas and the non-California western urban areas, all of the other urban areas were estimated to have maximum hourly PM 10 concentrations that exceeded the high CPL on about 20 percent to over 60 percent of the days. All 14 of the urban areas were estimated to have maximum hourly PM 10 concentrations that exceeded the low CPL on about 40 percent to over 90 percent of the days. In general, areas in the East and in California tend to have a higher frequency of hourly visibility conditions estimated to be above the high CPL compared with those in the western U.S.

The Policy Assessment repeated the Visibility Assessment-type modeling based on PM 2.5 light extinction and data from the more recent 2007 to 2009 time period for the same 15 study areas (including St. Louis). While the estimates of the percentage of daily maximum hourly PM 2.5 light extinction values exceeding the CPLs were somewhat lower than for PM 10 light extinction, the patterns of these estimates across the study areas was found to be similar. More specifically, except for the two Texas and the non-California western urban areas, all of the other urban areas were estimated to have maximum hourly PM 2.5 concentrations that exceeded the high CPL on about 10 percent up to about 50 percent of the days based on PM 2.5 light extinction, while all 15 areas were estimated to have maximum hourly PM 2.5 concentrations that exceeded the low CPL on over 10 percent to over 90 percent of the days.

To evaluate how PM-related visibility would be affected by just meeting the current suite of PM 2.5 secondary standards, the Policy Assessment applied the proportional rollback approach described in section VI.C.2 of the proposal to all the PM 2.5 monitoring sites in each study area. [167] After adjusting for composition, the Policy Assessment applied the original IMPROVE algorithm to calculate the PM 10 light extinction, using “rolled back” PM 2.5 component concentrations, the current conditions PM 10-2.5 concentration for the day and hour, and relative humidity for the day and hour.

In these analyses, the Policy Assessment estimated both PM 2.5 and PM 10 light extinction in terms of both daily maximum 1-hour average values and multi-hour (i.e., 4-hour) average values for daylight hours. Figure 4-7 and Table 4-6 of the Policy Assessment displayed the results of the rollback procedures as a box and whisker plot of daily maximum daylight 1-hour PM 2.5 light extinction and the percentage of daily maximum hourly PM 2.5 light extinction values estimated to exceed the CPLs when just meeting the current suite of PM 2.5 secondary standards for all 15 areas considered in the Visibility Assessment (including St. Louis) (excluding hours with relative humidity greater than 90 percent). These displays showed that the daily maximum 1-hour average PM 2.5 light extinction values in all of the study areas other than the three western non-California areas were estimated to exceed the high CPL on about 8 percent up to over 30 percent of the days and to exceed the middle CPL on about 30 percent up to about 70 percent of the days, while all areas except Phoenix were estimated to have daily maximum 1-hour average PM 2.5 light extinction values that exceeded the low CPL on over 15 percent to about 90 percent of the days. Figure 4-8 and Table 4-7 of the Policy Assessment present results based on daily maximum 4-hour average values. These displays show that the daily maximum 4-hour average PM 2.5 light extinction values in all of the study areas other than the three western non-California areas and the two areas in Texas were estimated to exceed the high CPL on about 4 percent up to over 15 percent of the days and to exceed the middle CPL on about 15 percent up to about 45 percent of the days, while all areas except Phoenix were estimated to have daily maximum 4-hour average PM 2.5 light extinction values that exceeded the low CPL on over 10 percent to about 75 percent of the days. A similar set of figures and tables were developed in terms of PM 10 light extinction (U.S. EPA, 2011a, Figures 4-5 and 4-6, Tables 4-4 and 4-5).

Taking the results of these analyses focusing on 1-hour and 4-hour maximum light extinction values into account, the Policy Assessment concluded that the available information in this review clearly called into question the adequacy of the current suite of PM 2.5 standards in the context of public welfare protection from visibility impairment, primarily in urban areas, and supported consideration of alternative standards to provide appropriate protection (U.S. EPA, 2011a, p. 4-39). This conclusion was based in part on the large percentage of days, in many urban areas, that were estimated to have maximum 1-hour or 4-hour light extinction values that exceed the range of CPLs identified for consideration under simulations of conditions that would just meet the current suite of PM 2.5 secondary standards. In particular, for air quality that was simulated to just meet the current PM 2.5 standards, greater than 10 percent of the days were estimated to have peak light extinction values that exceed the highest, least protective CPL of 30 dv in terms of PM 2.5 light extinction for 9 of the 15 urban areas, based on 1-hour average values, and would thus likely fail to meet a 90th percentile-based standard at that level. For these areas, the percent of days estimated to have maximum 1-hour values that exceed the highest CPL ranged from over 10 percent to over 30 percent. Similarly, when the middle CPL of 25 dv was considered, greater than 30 percent up to approximately 70 percent of the days were estimated to have peak light extinction that exceeded that CPL in terms of PM 2.5 light extinction, for 11 of the 15 urban areas, based on 1-hour average values. Based on a 4-hour averaging time, 5 of the areas were estimated to have at least 10 percent of the days with peak light extinction exceeding the highest CPL in terms of PM 2.5 light extinction, and 8 of the areas were estimated to have at least 30 percent of the days with peak light extinction exceeding the middle CPL in terms of PM 2.5 light extinction. For the lowest CPL of 20 dv, the percentages of days with 4-hour maximum light extinction estimated to exceed that CPL are even higher for all cases considered. Based on all of the above, the Policy Assessment concluded that PM light extinction estimated to be associated with just meeting the current suite of PM 2.5 secondary standards in many areas across the country exceeded levels and percentages of days that could reasonably be considered to be important from a public welfare perspective (U.S. EPA, 2011a, p. 4-40).

Further, the Policy Assessment concluded that use of the current indicator of PM 2.5 mass, in conjunction with the current 24-hour and annual averaging times, is clearly called into question for a national standard intended to protect public welfare from PM-related visibility impairment (U.S. EPA, 2011a, p. 4-40). This is because such a standard is inherently variable in the degree of protection provided because of regional differences in relative humidity and species composition of PM 2.5, which are critical factors in the relationship between the mix of fine particles in the ambient air and the associated impairment of visibility. The Policy Assessment noted that this concern was one of the important elements in the court's decision to remand the PM 2.5 secondary standards set in 2006 to the Agency.

Thus, in addition to concluding that the available information clearly calls into question the adequacy of the protection against PM-related visibility impairment afforded by the current suite of PM 2.5 standards, the Policy Assessment also concluded that it clearly calls into question the appropriateness of each of the current standard elements: indicator, averaging time, form, and level (U.S. EPA, 2011a, p. 4-40).

After reviewing the information and analysis in the second draft Policy Assessment, CASAC concluded that the “currently available information clearly calls into question the adequacy of the current standards and that consideration should be given to revising the suite of standards to provide increased public welfare protection” (Samet, 2010d, p. iii). CASAC noted that the detailed estimates of hourly PM light extinction associated with just meeting the current standards “clearly demonstrate that current standards do not protect against levels of visual air quality which have been judged to be unacceptable in all of the available urban visibility preference studies.” Further, CASAC stated, with respect to the current suite of secondary PM 2.5 standards, that “[T]he levels are too high, the averaging times are too long, and the PM 2.5 mass indicator could be improved to correspond more closely to the light scattering and absorption properties of suspended particles in the ambient air” (Samet, 2010d, p. 9).

After considering the available evidence and the advice of CASAC, the Administrator concluded at the time of proposal that such information did provide an appropriate basis to inform a conclusion as to whether the current standards afford adequate protection against PM-related visibility impairment in urban areas. The Administrator took into account the information discussed above with regard to the nature of PM-related visibility impairment, the results of public perception surveys on the acceptability of varying degrees of visibility impairment in urban areas, analyses of the number of days on which peak 1-hour or 4-hour light extinction values are estimated to exceed a range of candidate protection levels under conditions simulated to just meet the current standards, and the advice of CASAC. She noted the clear causal relationship between PM in the ambient air and impairment of visibility, the evidence from the visibility preference studies, and the rationale for determining a range of candidate protection levels based on those studies. She also noted the relatively large number of days when maximum 1-hour or 4-hour light extinction values were estimated to exceed the three candidate protection levels, including the highest level of 30 dv, under the current standards. While recognizing the limitations in the available information on public perceptions of the acceptability of varying degree of visibility impairment and the information on the number of days estimated to exceed the CPLs, she concluded that such information provided an appropriate basis to inform a conclusion as to whether the current standards provide adequate protection against PM-related visibility impairment in urban areas. Based on these considerations, and placing great importance on the advice of CASAC, the Administrator provisionally concluded that the current standards are not sufficiently protective of visual air quality, and that consideration should be given to an alternative secondary standard that would provide additional protection against PM-related visibility impairment, with a focus primarily in urban areas.

Having reached this conclusion, the Administrator also stated at the time of proposal that the current indicator of PM 2.5 mass, in conjunction with the current 24-hour and annual averaging times, is not well suited for a national standard intended to protect public welfare from PM-related visibility impairment. As noted in the proposal, the current standards do not incorporate information on the concentrations of various species within the mix of ambient particles, nor do they incorporate information on relative humidity, both of which play a central role in determining the relationship between the mix of PM in the ambient air and impairment of visibility. Such considerations were reflected both in CASAC's advice to set a distinct secondary standard that would more directly reflect the relationship between ambient PM and visibility impairment and in the court's remand of the current secondary PM 2.5 standards. Based on the above considerations, at the time of proposal the Administrator provisionally concluded that the current secondary PM 2.5 standards, taken together, are neither sufficiently protective nor suitably structured to provide an appropriate degree of public welfare protection from PM-related visibility impairment, primarily in urban areas. This led the EPA to consider alternative standards by looking at each of the elements of the standards—indicator, averaging time, form, and level—as discussed below.

ii. Indicator

At the time of proposal, the EPA considered three alternative indicators for a PM 2.5 standard designed to protect against visibility impairment: The current PM 2.5 mass indicator; directly measured PM 2.5 light extinction; and calculated PM 2.5 light extinction. Directly measured PM 2.5 light extinction is a measurement (or combination of measurements) of the light absorption and scattering caused by PM 2.5 under ambient conditions. Calculated PM 2.5 light extinction uses the IMPROVE algorithm to calculate PM 2.5 light extinction using measured PM 2.5 mass, speciated PM 2.5 mass, and measured relative humidity. The Policy Assessment evaluated each of these alternatives, finally concluding that consideration should be given to establishing a new calculated PM 2.5 light extinction indicator (U.S. EPA, 2011a, p. 4-51).

As discussed in section VI.D.1 of the proposal, the Policy Assessment concluded that consideration of the use of either directly measured PM 2.5 light extinction or calculated PM 2.5 light extinction as an indicator is justified because light extinction is a physically meaningful measure of the characteristic of ambient PM 2.5 that is most relevant and directly related to PM-related visibility effects (U.S. EPA, 2011a, p. 4-41). Further, as noted above, PM 2.5 is the component of PM responsible for most of the visibility impairment in most urban areas. In these areas, the contribution of PM 10-2.5 is a minor contributor to visibility impairment most of the time. The Policy Assessment also indicated that the available evidence demonstrated a strong correspondence between calculated PM 2.5 light extinction and PM-related visibility impairment, as well as the significant degree of variability in visibility protection across the U.S. allowed by a PM 2.5 mass indicator. The Policy Assessment recognized that while in the future it would be appropriate to consider a direct measurement of PM 2.5 light extinction it was not an appropriate option in this review because a suitable specification of the equipment and associated performance verification procedures cannot be developed in the time frame for this review.

(a) PM 2.5 Mass

In terms of utilizing a PM 2.5 mass indicator, the proposal noted that PM 2.5 mass monitoring methods are in widespread use, including the FRM involving the collection of periodic (usually 1-day-in-6 or 1-day-in-3) 24-hour filter samples. However, these routine monitoring activities do not include measurement of the full water content of the ambient PM 2.5 that contributes, often significantly, to visibility impacts. Further, the PM 2.5 mass concentration monitors do not provide information on the composition of the ambient PM 2.5, which plays a central role in the relationship between PM-related visibility impairment and ambient PM 2.5 mass concentrations. Additional analyses discussed in the proposal that looked at the contribution of PM 2.5 to total PM-related light extinction (defined in terms of hourly PM 10 calculated light extinction) indicate that there is a poor correlation between hourly PM 10 light extinction and hourly PM 2.5 mass principally due to the impact of the water content of the particles on light extinction, which depends on both the composition of the PM 2.5 and the ambient relative humidity. Both composition and especially relative humidity vary during a single day, as well as from day-to-day, at any site and time of year. Also, there are systematic regional and seasonal differences in the distribution of ambient humidity and PM 2.5 composition conditions that make it impossible to select a PM 2.5 concentration that generally would correspond to the same PM-related light extinction levels across all areas of the nation. Analyses discussed in the proposal quantify the projected uneven protection that would result from the use of 1-hour average PM 2.5 mass as the indicator.

(b) Directly Measured PM 2.5 Light Extinction

PM light extinction has a nearly one-to-one relationship to light extinction, unlike PM 2.5 mass concentration. As explained above, PM 2.5 is the component responsible for the large majority of PM light extinction in most places and times. PM 2.5 light extinction can be directly measured using several instrumental methods, some of which have been used for decades to routinely monitor the two components of PM 2.5 light extinction (light scattering and absorption) or to jointly measure both as total light extinction (from which Rayleigh scattering is subtracted to get PM 2.5 light extinction). As noted at the time of proposal, there are a number of advantages to direct measurements of light extinction for use in a secondary standard relative to estimates of PM 2.5 light extinction calculated using PM 2.5 mass and speciation data. These include greater accuracy of direct measurements with shorter averaging times and overall greater simplicity when compared to the need for measurements of multiple parameters to calculate PM light extinction.

In evaluating whether direct measurement of PM 2.5 or PM 10 light extinction is appropriate to consider in the context of this PM NAAQS review, the EPA solicited comment from the Ambient Air Monitoring and Methods Subcommittee (AAMMS) of CASAC. The CASAC AAMMS recommended that consideration of direct measurement should be limited to PM 2.5 light extinction, and that although instruments suitable for this purpose are commercially available at present, research is expected to produce even better instruments in the near term. The CASAC AAMMS advised against choosing any currently available commercial instrument, or even a general measurement approach, as an FRM because to do so could discourage development of other potentially superior approaches. Instead, the CASAC AAMMS recommended that the EPA develop performance-based approval criteria for direct measurement methods in order to put all approaches on a level playing field.

At the present time, the EPA has not undertaken to develop and test such performance-base approval criteria. The EPA anticipates that if an effort were begun it would take at least several years before such criteria would be ready for regulatory use. Thus, the Policy Assessment concluded that while in the future it would be appropriate to consider a direct measurement of PM 2.5 light extinction, or the sum of separate measurements of light scattering and light absorption, as the indicator for the secondary PM 2.5 standard, this is not an appropriate option in this review because a suitable specification of the equipment or appropriate performance-based verification procedures cannot be developed in the time frame for this review (U.S. EPA, 2011a, p. 4-51, -52).

(c) Calculated PM 2.5 Light Extinction

For the reasons discussed above, the Policy Assessment concluded that a calculated PM 2.5 light extinction indicator would be the preferred approach. PM 2.5 light extinction can be calculated from PM 2.5 mass, combined with speciated PM 2.5 mass concentration data plus relative humidity data, as is presently routinely done on a 24-hour average basis under the Regional Haze Program using data from the rural IMPROVE monitoring network. This same calculation procedure, using a 24-hour average basis, could be used for a NAAQS focused on protecting against PM-related visibility impairment primarily in urban areas. This approach would use the type of data that is routinely collected from the urban CSN [168] in combination with monthly average relative humidity data based on long-term climatological means as used in the Regional Haze Program (U.S. EPA, 2011a, Appendix G, section G.2). The proposal discussed the complex approach utilized in the Visibility Assessment for calculating hourly PM 2.5 light extinction [169] and discussed various simplified approaches for calculating these hourly values that were analyzed in the Policy Assessment. The Policy Assessment concluded that each of these simplified approaches provided reasonably good estimates of PM 2.5 light extinction and each would be appropriate to consider as the indicator for a distinct hourly or multi-hour secondary standard (U.S. EPA, 2011a, p. 4-48). The proposal also recognized that the Policy Assessment identified a number of variations on these simplified approaches that it would be appropriate to consider, including:

(1) The use of the split-component mass extinction efficiency approach from the revised IMPROVE algorithm [170]

(2) The use of more refined value(s) for the organic carbon multiplier [171]

(3) The use of the reconstructed 24-hour PM 2.5 mass (i.e., the sum of the five PM 2.5 components from speciated monitoring) as a normalization value for the hourly measurements from the PM 2.5 instrument as a way of better reflecting ambi