On October 1, 2015, under the authority of Sections 108 and 109 of the Clean Air Act (CAA) as amended, the Environmental Protection Agency (EPA) finalized revisions to the National Ambient Air Quality Standards (NAAQS) for ground-level ozone from the then-current level of 75 parts per billion (ppb) to 70 ppb. As described in detail below (see " Implementation of the 2015 Standards "), under the CAA, revision of the standards sets in motion a process in which states and EPA identify areas that do not meet the standards ("nonattainment areas") and the states prepare implementation plans to demonstrate how emissions will be lowered sufficiently to reach attainment in those areas. The revised standards are based on a review of more than 1,000 scientific studies by EPA staff. The EPA science assessment was itself reviewed by a panel of 20 outside scientific experts, a group known as the Clean Air Scientific Advisory Committee (CASAC). The EPA Administrator was under a court order to sign any changes or reaffirm the standards by October 1, 2015. A comment period, public hearings, and interagency review had followed the publication of a proposed range of standards in December 2014. Ground-level ozone (often referred to as "smog") is associated with potentially serious health effects when present in high enough concentrations. These health effects include aggravated asthma, chronic bronchitis, and heart attacks, and in some cases premature death. In the Regulatory Impact Analysis (RIA) accompanying the 2015 rule , EPA stated that reducing concentrations to 70 ppb versus the previous standard of 75 ppb would avoid 320 to 660 premature deaths annually in 2025 nationwide (excluding California). California was excluded from EPA's estimate of both costs and benefits of the nationwide standard, because most areas of the state will have until the 2030s to reach attainment of the NAAQS. California costs and benefits are presented separately in the " Costs and Benefits of Control " section of this report. High ozone concentrations also affect the growth of plants, causing damage to both forests and field crops. In 2006, the U.S. Forest Service examined 380 monitoring sites in the Mid-Atlantic and Southeastern states and found visible injury to forest plants from ozone at 121 (32%) of them. At 20 of the sites, the damage was described as "severe." In addition, EPA found that "several economically important crop species are sensitive to ozone levels typical of those found in the United States," and estimated that crop losses could be reduced by $400 million to $620 million annually by implementation of a more stringent ozone standard. While EPA's analysis has found that there would be substantial health and welfare benefits to reducing ozone, the agency also has concluded that there could be substantial costs. In the RIA, EPA estimated the economic value of the benefits of reducing ozone concentrations to 70 ppb (for areas other than California) at $2.9-$5.9 billion annually by 2025, with costs at $1.4 billion annually in 2025. The ozone NAAQS are among EPA's most far-reaching standards. At the level set in March 2008 (75 ppb), 107 million people lived in areas that were designated nonattainment for the standards (see Figure 1 ), as of June 2018. These 38 areas (referred to as "nonattainment areas") include 168 counties in 17 states and the District of Columbia. The areas generally coincide with metropolitan areas, but may be larger or smaller. Of the 38 areas, 18 have ozone readings that indicate they have attained the standard, which would leave 20 areas with a combined population of nearly 78 million not attaining the standard. When it promulgated the 2015 NAAQS revision, EPA stated that if the most recent monitoring data then available (for the years 2012-2014) were used to identify nonattainment areas for the revised standards, the number of counties showing nonattainment would be 241 in 33 states and the District of Columbia. The actual nonattainment designations promulgated in 2018 are in most cases based on 2014-2016 monitoring data and include just over 200 counties or partial counties in 22 states and the District of Columbia, plus two tribal areas. (See Figure 2 .) In the intervening years, the emissions that contribute to ozone formation have declined, in response to already promulgated standards for motor vehicles, gasoline, power plants, and other sources of emissions. Thus, the number of counties showing nonattainment is lower than the EPA estimate released in 2015. The potential economic, health, and environmental impacts of a change in the ozone NAAQS have led to great interest in EPA's review and implementation of the standards. To assist Members and staff in evaluating the process and its potential impacts, this report provides background on NAAQS, the process used to establish them, the 2008 and 2015 ozone standards, and issues that have been raised as EPA implements the final rule. NAAQS are federal standards that apply to ambient (outdoor) air. Section 109 of the Clean Air Act directs EPA to set both primary NAAQS, which 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," and secondary NAAQS, which are standards necessary to protect public welfare, a broad term that includes damage to crops, vegetation, property, building materials, climate, etc. The pollutants for which NAAQS have been set 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 (or categories of 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 he determines that additional pollutants meet the definition, or delete pollutants from the list if they no longer meet the definition. There have been no changes to the list, however, since the late 1970s. 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 for the pollutant in question. Once a NAAQS has been set, the agency, using monitoring data and other information submitted by the states, identifies areas that exceed the standards and must reduce pollutant concentrations; these areas are referred to as "nonattainment areas." The designation process is often delayed by litigation over the standards, by EPA's agreement to reconsider aspects of them, or by consultations with the states regarding the specifics of the areas to be designated. Designation of nonattainment areas for the 1997 ozone NAAQS, for example, took seven years. Designations under the 2008 standards took four years. After nonattainment areas are designated, state and local governments have up to three years to produce State Implementation Plans (SIPs), which outline the measures that will reduce emission levels and attain the standards. Finalizing SIPs, through EPA review and approval, often takes longer. Under the CAA, actual attainment of the ozone NAAQS is allowed to stretch over a three-year to 20-year period, depending on the severity of the area's pollution. Ozone nonattainment areas are designated as Marginal, Moderate, Serious, Severe, or Extreme, depending on the level of pollution. Each of these classifications comes with required pollution control measures: the more severe the pollution, the more stringent are the required controls, and the longer the area is allowed before it must demonstrate attainment. Thus, establishment or revision of a NAAQS is not an event that requires immediate compliance with an air quality standard; rather, it sets in motion a long and complex implementation process. That process may ultimately have far-reaching impacts for public health and welfare, for sources of pollution in numerous economic sectors, and for states and local governments. EPA has several tools available to encourage nonattainment areas to comply with a NAAQS. The most frequently mentioned of these is the potential for highway-fund sanctions: failure to submit or implement a SIP adequate to attain or maintain compliance with the NAAQS can lead to the temporary suspension of federal highway funds for new nonsafety-related projects. Ultimately, EPA can impose a federal implementation or maintenance plan (a FIP) in an area that does not have an approved SIP. Imposition of sanctions or FIPs is relatively rare, however: generally the states avoid sanctions by submitting plans that require sufficient emission reductions to be deemed adequate by EPA. In addition to state and local actions to address ambient concentrations of NAAQS pollutants, EPA itself acts to control emissions and concentrations of criteria pollutants through national standards. These national standards apply to products that contribute to ozone and other criteria pollution (particularly mobile sources, such as automobiles) and to stationary emission sources (such as power plants). These standards lead to emission reductions that states can factor into their implementation plans, reducing the need for local air pollution control measures. Section 109(d) of the Clean Air 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 or complete a review. In June 2013, the Sierra Club and three other groups filed suit over EPA's failure to complete the required review of the 2008 ozone NAAQS by the March 2013 deadline, and a court subsequently ordered EPA to propose any changes to the standards by December 1, 2014, and complete the review, with promulgation of any revisions by October 1, 2015. A historical review of the ozone NAAQS and their revisions is presented in Table 1 . Reviewing an existing NAAQS is generally a long process.