View Rule

Under the Clean Air Act, the EPA is required to review and, if appropriate, revise the air quality criteria and national ambient air quality standards (NAAQS) every 5 years. On April 3, 2012, the EPA published a final rule in which the Agency determined to retain the current secondary standards (welfare-based) for oxides of nitrogen and for sulfur oxides (SOx). On January 15, 2013, the EPA published a final rule in which the Agency retained the secondary standards for particulate matter. The current review of the air quality criteria and secondary standards for ecological effects of SOx, nitrogen oxides and particulate matter included the preparation of an Integrated Review Plan, an Integrated Science Assessment, a Risk/Exposure Assessment, and a Policy Assessment by the EPA, with opportunities for review by the EPA's Clean Air Scientific Advisory Committee (CASAC) and the public. Opportunities for public participation in the development of these assessments included through the offering of comments to the CASAC at public CASAC meetings and directly to the EPA. These assessments informed the Administrator's proposed decision to strengthen the secondary NAAQS for SOx to 10-15 ppb, as an annual average, averaged over 3 years; while proposing to retain existing secondary standards for oxides of nitrogen and particulate matter (PM). The proposed decision was published in the Federal Register and announced on EPA’s website, with opportunity provided for public participation, including submission of comments to the rulemaking docket and public hearings. Additionally, opportunity was provided for Tribal consultation. The Administrator’s final decisions took into consideration published documents, CASAC advice, and public comment on the proposed decision. EPA was under a consent decree for this review to sign the final rule by December 10, 2024. EPA met this deadline and published the final rule on December 27, 2024, revising the existing secondary SO2 standard to an annual standard of 10 ppb, averaged over 3 consecutive years. In addition, EPA retained without revision the secondary standards for oxides of nitrogen and particulate matter. The final rule became effective on January 27, 2025.