National Ambient Air Quality Standards

The U.S. Clean Air Act of 1970 called for the establishment of outdoor standards for those air pollutants for which adverse health effects have been shown to occur as a result of outdoor air pollution. Currently, six pollutants are regulated under this heading: ozone, sulfur dioxide, particulates, nitrogen dioxide, carbon monoxide, and lead. For each of these pollutants an ambient standard has been established by the Administrator of the U.S. Environmental Protection Agency (EPA), following a thorough review of the scientific evidence.

This review is overseen by the Clean Air Scientific Advisory Committee (CASAC), whose congressionally mandated advisory role requires it to publicly evaluate the science underlying the association between an air pollutant and its adverse effects, paying attention to the basic toxicology, animal studies, controlled human exposure studies, and the epidemiological evidence. CASAC then recommends to the EPA Administrator a pollutant level or range of levels and durations of exposure at which no adverse effects are believed to occur. The EPA Administrator then sets the standard based upon this information, including within the standard an adequate margin of safety for sensitive populations, as required by the Clean Air Act. The language of the act also requires the EPA Administrator to pay no attention to economic, social, political, or other considerations associated with the management activities required to meet the standard. These can be considered, however, when choices are made as to how to most effectively decrease air pollutant levels that exceed the ambient standard. The ability of the EPA to develop standards based on science has been recently reaffirmed by a U.S. Supreme Court decision upholding the new particulate and ozone standards.

The duration of exposure on which the National Ambient Air Quality Standards (NAAQS) are based is to a large extent dependent upon the toxicology of each particular compound: the eight- to twelve-hour equilibration period for carbon monoxide in the human body is the basis for the eight-hour carbon monoxide standard, and the recognition of the longer-term effects of ozone has been instrumental in the recent switch of the ozone standard from a one-hour to an eight-hour averaging time. An unusual but valuable aspect of the Clean Air Act is the requirement to revisit each standard every five years. This has spurred additional research and review, which has led to revision of air pollutant standards.

Management of air pollutant levels that exceed the standards is done by each state, with oversight by the EPA. This oversight includes guidance for the positioning of air pollutant monitoring equipment so as to ensure that potential hot spots are not missed. Once it is established that a state has an air pollutant measurement that exceeds the allowable standard, it is required to develop a state implementation plan that details the approach it will take to meet the standard. This is updated yearly as needed. The failure to develop and adhere to the implementation plan puts the state in jeopardy of losing federal funding, although this rarely occurs.

(SEE ALSO: Ambient Air Quality [Air Pollution]; Carbon Monoxide; Clean Air Act; Environmental Protection Agency; Lead; Sulfur-Containing Air Pollutants [Particulates])

— BERNARD D. GOLDSTEIN