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Under the Clean Air Act, the EPA is required to review and, if appropriate, revise the air quality criteria for the primary (health-based) and secondary (welfare-based) national ambient air quality standards (NAAQS) every 5 years. On March 23, 2008, the EPA published a final rule to revise the primary and secondary NAAQS for ozone to provide increased protection of public health and welfare. With regard to the primary standard for ozone, the EPA revised the level of the 8-hour ozone standard to 0.075 ppm. With regard to the secondary ozone standard, the EPA made it identical in all respects to the primary ozone standard, as revised. The D.C. Circuit upheld the primary standard, but remanded the secondary standard back to the EPA. The EPA initiated the current review in October 2008 with a workshop to discuss key policy-relevant issues around which the EPA would structure the review. This review includes the preparation of an Integrated Science Assessment, Risk/Exposure Assessment, and a Policy Assessment Document by the EPA, with opportunities for review by EPA's Clean Air Scientific Advisory Committee and the public. The EPA proposed revisions to the primary and secondary ozone NAAQS on December 17, 2014.