On January 19, 2010, the Environmental Protection Agency (EPA) proposed revisions to the National Ambient Air Quality Standards (NAAQS) for ozone. Because they have widespread implications for public health and for the pollution control measures that will be imposed on sectors of the economy, the revisions have stirred congressional interest and caused a variety of reactions among state and local officials. EPA held three public hearings (in Arlington, VA, Houston, TX, and Sacramento, CA) during the first week of February 2010, and accepted public comments in writing until March 22. The agency received more than 5,000 unique comments. Final standards were expected to be issued by the end of August 2010, but EPA has delayed promulgation, first to October, then to December, and now to July 2011. The delays have allowed the agency to analyze the information it received during the public comment period. Having done so, the agency says that it intends to seek additional advice from the Clean Air Scientific Advisory Committee (CASAC), a panel of outside experts who review agency decision-making regarding the setting of NAAQS. Specifically, the agency intends to take the following steps: During December 2010 and January 2011, EPA intends to prepare a set of questions for CASAC and provide them for CASAC's review. The questions are expected to request additional advice focused on the scientific evidence and other information before the Administrator. EPA anticipates that CASAC will hold a public meeting in February 2011 to discuss their response, and anticipates that CASAC will provide its additional advice to the Agency by letter shortly thereafter. The CASAC process includes an opportunity for the public to submit comments to CASAC and EPA. EPA intends to issue a final decision on the reconsideration by July 29, 2011. The final decision will be subject to interagency review under Executive Order No. 12866 before being promulgated. This report provides background on NAAQS, the process used to establish them, the pre-existing ozone standards, and EPA's proposed revisions, as well as information regarding the revisions' potential effects. As defined in Section 109 of the Clean Air Act, NAAQS are standards that apply to ambient (outdoor) air. The act directs EPA to set both primary and secondary standards. Primary NAAQS are standards, "the attainment and maintenance of which in the judgment of the [EPA] Administrator ... are requisite to protect the public health," with "an adequate margin of safety." Secondary NAAQS are standards necessary to protect public welfare, a broad term that includes damage to crops, vegetation, property, building materials, etc. NAAQS are at the core of the Clean Air Act, even though they do not directly regulate emissions. In essence, they are standards that define what EPA considers to be clean air. Once a NAAQS has been set, the agency, using monitoring data and other information submitted by the states, identifies areas that exceed the standard and must, therefore, reduce pollutant concentrations to achieve it. After these "nonattainment" areas are identified, state and local governments have up to three years to produce State Implementation Plans which outline the measures they will implement to reduce the pollution levels and attain the standards. Depending on the severity of the pollution, ozone nonattainment areas have anywhere from 3 to 20 years to actually attain the standard. EPA also acts to control many of the NAAQS pollutants wherever they are emitted, through national standards for products that emit them (particularly mobile sources, such as automobiles) and emission standards for new stationary sources, such as power plants. Thus, establishment or revision of a NAAQS sets in motion a long and complicated implementation process that has far-reaching impacts for public health, for sources of pollution in numerous economic sectors, and for states and local governments. The pollutants to which NAAQS apply are generally referred to as "criteria" pollutants. The act defines them as pollutants that "endanger public health or welfare," and whose presence in ambient air "results from numerous or diverse mobile or stationary sources." Six pollutants are currently identified as criteria pollutants: ozone, particulates, carbon monoxide, sulfur dioxide, nitrogen oxides, and lead. The EPA Administrator can add to this list if she determines that additional pollutants meet the act's criteria, or delete them if she concludes that they no longer do so. The act requires the agency to review each NAAQS every five years. That schedule is rarely met, but it often triggers lawsuits that force the agency to undertake a review. In the case of ozone, the American Lung Association filed suit over EPA's failure to complete a review in 2003, and a consent decree established a schedule under which the Bush Administration's EPA promulgated revisions to the NAAQS in March 2008. Numerous parties petitioned the courts for a review of that standard, and, with the change of Administrations, EPA itself concluded that it had concerns whether the 2008 standards "satisfy the requirements of the Clean Air Act." Thus, in September 2009, the agency suspended implementation of the 2008 standards and set a schedule for their review under which the January 2010 proposal was released. EPA is also in the process of reviewing several other NAAQS. For more information, see the "Air Quality Standards" section of CRS Report R40145, Clean Air Issues in the 111 th Congress . Reviewing an existing NAAQS is generally a long process. To begin the process, EPA scientists review the scientific literature published since the last NAAQS revision, and summarize it in a report known as a Criteria Document or Integrated Science Assessment. The review that culminated in the 2008 ozone revision identified 1,700 scientific studies on topics as wide-ranging as the physics and chemistry of ozone in the atmosphere; environmental concentrations, patterns, and exposure; dosimetry and animal-to-human extrapolation; toxicology; interactions with co-occurring pollutants; controlled human exposure studies; epidemiology; effects on vegetation and ecosystems; effects on UVB exposures and climate; and effects on man-made materials. A second document that EPA prepares, the Staff Paper or Policy Assessment, summarizes the information compiled in the Criteria Document and provides the Administrator with options regarding the indicators, averaging times, statistical form, and numerical level (concentration) of the NAAQS. To ensure that these reviews meet the highest scientific standards, the 1977 amendments to the Clean Air Act required the Administrator to appoint an independent Clean Air Scientific Advisory Committee (CASAC). CASAC has seven members, largely from academia and from private research institutions. In conducting NAAQS reviews, their expertise is supplemented by panels of the nation's leading experts on the health and environmental effects of the specific pollutants that are under review. These panels can be rather large. The review panel for the 2008 ozone standard, for example, had 23 members. CASAC and the public make suggestions regarding the membership of the panels on specific pollutants, with the final selections made by EPA. The panels review the agency's work during NAAQS-setting and NAAQS-revision, rather than conducting their own independent reviews. The ozone standard affects a larger percentage of the population than any of the other NAAQS. As of September 2010, 39% of the U.S. population lived in areas designated nonattainment under the 1997 standard for ozone, 119 million people in all. Since the standard has been strengthened as a result of a review completed in 2008 and the Administrator is now proposing a further strengthening, more areas will be affected, and those already considered nonattainment may have to impose more stringent emission controls. The primary (health-based) standard promulgated in 1997 was set at 0.08 parts per million (ppm), averaged over an 8-hour period. Allowing for rounding, EPA considered areas with readings as high as 0.084 ppm (84 parts per billion) to have attained the standard. The review completed in 2008 found evidence of health effects, including mortality, at levels of exposure below the 0.08 ppm standard. As a result, both EPA staff and CASAC recommended strengthening the standard. According to CASAC, "There is no scientific justification for retaining the current [0.08 ppm] primary 8-hr NAAQS." The panel unanimously recommended a range of 0.060 to 0.070 ppm (60 to 70 parts per billion) for the primary 8-hour standard. EPA staff also recommended strengthening the standard, in wording not quite so direct. The staff stated, "The overall body of evidence on ozone health effects clearly calls into question the adequacy of the current standard." They recommended "considering a standard level within the range of somewhat below 0.080 parts per million (ppm) to 0.060 ppm." Based on these recommendations, and his own judgment regarding the strength of the science, Stephen Johnson, the Bush Administration's last EPA Administrator, chose to finalize the standard at 0.075 ppm (75 parts per billion). That revision has not yet been implemented, however: the designation of nonattainment areas has not been completed, and the Obama Administration's EPA announced in September 2009 that it would undertake a further review of the NAAQS before completing the designations. As shown in Figure 1 , using 2004-2006 data, 85 counties had monitors showing violation of the old 0.08 ppm primary standard. Figure 2 shows what happens when the standard is strengthened to 0.075 ppm (75 ppb), again using 2004-2006 data: under the 0.075 ppm standard, 345 counties, more than four times as many, showed violations. Figure 3 , using more recent monitoring data, shows the number of counties that would exceed a standard in the range of 0.060 to 0.070 ppm. At 0.070 ppm, 515 counties (76% of those with monitors) exceeded the standard. At 0.060 ppm, 650 counties—virtually every county with a monitor—exceeded the proposed standard.