SUPPLEMENTARY INFORMATION:

Section 108(a) of the Clean Air Act directs the Administrator to identify certain pollutants that “may reasonably be anticipated to endanger public health and welfare” and to issue air quality criteria for them. These air quality criteria are 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. * * *” Under section 109 of the Act, EPA is then to establish national ambient air quality standards (NAAQS) for each pollutant for which EPA has issued criteria. Section 109(d) of the Act requires subsequent periodic review and, if appropriate, revision of existing air quality criteria to reflect advances in scientific knowledge on the effects of the pollutant on public health and welfare. EPA is also to revise the NAAQS, if appropriate, based on the revised air quality criteria.

Sulfur oxides are one of six principal (or “criteria”) pollutants for which EPA has established NAAQS. Periodically, EPA reviews the scientific basis for these standards by preparing an Integrated Science Assessment (ISA), formerly called an Air Quality Criteria Document (AQCD). The ISA and supplementary annexes, in conjunction with additional technical and policy assessments, provide the scientific basis for EPA decisions on the adequacy of a current NAAQS and the appropriateness of new or revised standards. The Clean Air Scientific Advisory Committee (CASAC), an independent science advisory committee mandated by the Clean Air Act and part of the EPA's Science Advisory Board (SAB), is charged with expert scientific review of EPA's draft ISAs.

On May 16, 2006 (71 FR 28023), EPA formally initiated its current review of the criteria for Sulfur Oxides, requesting the submission of recent scientific information on specified topics. A draft of EPA's “Integrated Plan for Review of the Primary National Ambient Air Quality Standard for Sulfur Oxides” was made available in February 2007 for ( printed page 53003) public comment and was discussed by the CASAC via a publicly accessible teleconference consultation on May 11, 2007 (72 FR 20336). This plan was then finalized and made available to the public in October 2007. A review of the secondary (welfare-based) NAAQS for Sulfur Oxides is being conducted separately, in conjunction with the review of the secondary NAAQS for Oxides of Nitrogen. ( http://www.epa.gov/​ttn/​naaqs/​standards/​so2/​s_​so2_​cr_​pd.html). In February 2007 (72 FR 6238), a workshop was held to discuss, with invited scientific experts, initial draft materials prepared in the development of the ISA and supplementary annexes for sulfur oxides. The first external review draft of this ISA was released for public comment and review by the CASAC on September 28, 2007 (72 FR 55207), and was reviewed by CASAC at a public meeting held on December 5-6, 2007 (72 FR 64216). The second draft of this ISA was released for public comment and review by the CASAC in June 2008 (73 FR 31113), and was reviewed by CASAC at a public meeting held on July 30-31, 2008. EPA has considered comments by CASAC and by the public in preparing this final ISA.