The Clean Air Act requires the Environmental Protection Agency (EPA) to set national ambient air quality standards for six "criteria" air pollutants , including ground-level ozone (O3), as mandated by Sec. 110  of the Act. Ozone, which is the main component of smog, is created by the reaction of oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. While solar radiation-blocking ozone is beneficial in the upper atmosphere, its presence near ground-level has adverse environmental and public health effects, such as pulmonary and cardiovascular ailments, including asthma and decreased lung function.
In the NAAQS  program, two standards are required for ozone and the other criteria pollutants. Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings. For ozone, the primary and secondary standards are determined to be at the same level of ozone parts per billion (ppb). EPA has established ozone standards that are not to be exceeded in eight-hour periods and one-hour periods. These ozone standards are subject to review every five years through a public process that includes public and industry stakeholders and a Clean Air Scientific Advisory Committee (CASAC).
Rather than applying to specific facilities or vehicles that emit ozone, NAAQS apply to geographic areas. Jurisdictions must have air quality that meets all the NAAQS. Jurisdictions are in judged to be in "attainment" if they meet the standards, or "nonattainment," if they fail to. Once a jurisdiction is ruled to be in nonattainment, the state and local governments have three years to develop implementation plans outlining how areas will attain and maintain the standards by reducing air pollutant emissions contributing to ozone concentrations. These implementation plans can affect the types of pollution controls required for power plants and manufacturing facilities, as well as motor vehicle emissions testing requirements.
A nonattainment area with improved air quality will become a "maintenance" area after some years. Failure to improve air quality to meet the standards can result in the imposition of penalties or sanctions by EPA.
The first ozone NAAQS were established by EPA in 1971 after the Clean Air Act entered into law. Initially, hundreds of jurisdictions were judged to be in nonattainment, but the subsequent implementation plans lead to attainment for many jurisdictions and healthier air quality. In 1979 and 1997, those standards were gradually strengthened, causing a temporary increase in the numbers of nonattainment areas.
The most recent ozone NAAQS of 75 ppb was established in 2008 under the Bush Administration. In response to a legal challenge arguing that the standard did not adequately protect human health as required by law, in January 2010, EPA announced that it would revisit the 2008 standard. It proposed a new standard of between 60 and 70 ppb. The most recent ozone NAAQS of 75 ppb was established in 2008 under the Bush Administration. In response to a legal challenge arguing that the standard did not adequately protect human health as required by law, in January 2010, EPA announced that it would revisit the 2008 standard. It proposed a new standard of between 60 and 70 ppb, and ultimately submitted a standard of 65 ppb to the White House Office of Management and Budget for the last step in the regulatory approval process. In September 2011, President Obama overruled EPA had the EPA Administrator withdraw the proposed final rule.
In September 2014, EPA staff released a policy assessment that there is adequate evidence for again lowering the existing ozone standard from 75 ppb to somewhere between 70 ppb and 60 ppb. On November 26, 2014, EPA proposed lowering the ozone standard to between 65 ppb and 70 ppb. EPA must finalize a new rule by October 1, 2015 under court order.
Read more from EPA  on ozone NAAQS.