"The Environmental Protection Agency (EPA) is under a court order to complete a review of its ambient air quality standard for ozone (the 'ozone NAAQS' [National Ambient Air Quality Standards]) by October 1, 2015. The agency must propose any change to the standard by December 1 of this year. A NAAQS is essentially EPA's definition of what constitutes clean air in the case of six of the most common pollutants. The agency conducts an exhaustive review of the published research on the health effects of these pollutants before determining the levels that (in the Clean Air Act's words) 'are requisite to protect the public health,' allowing an adequate margin of safety. If an area has monitor readings higher than the ozone NAAQS when averaged over a three-year period, the area is in 'non-attainment' of the standard. The NAAQS does not directly impose emission controls on industry or other pollution sources, but it sets in motion a process under which non-attainment areas are identified and states submit implementation plans to EPA to demonstrate how they will bring the areas into compliance."
CRS Insight, IN10165