National Ambient Air Quality Standard

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National Ambient Air Quality Standard

The key to national air pollution control policy since the passage of the Clean Air Act amendments of 1970 are the National Ambient Air Quality Standards (NAAQSs). Under this law, the Environmental Protection Agency (EPA) had to establish NAAQSs for six of the most common air pollutants, sometimes referred to as criteria pollutants, by April 1971. Included are carbon monoxide , lead (added in 1977), nitrogen oxides , ozone , particulate matter, and sulfur dioxide . Hydrocarbons originally appeared on the list of pollutants, but were removed in 1978 since they were adequately regulated through the ozone standard. The provisions of the law allow the EPA to identify additional substances as pollutants and add them to the list.

For each of these pollutants, primary and secondary standards are set. The primary standards are designed to protect human health. Secondary standards are to protect crops, forests, and buildings if the primary standards are not capable of doing so; a secondary standard presently exists only for sulfur dioxide. These standards apply uniformly throughout the country, in each of 247 Air Quality Control Regions. All parts of the country were required to meet the NAAQSs by 1975, but this deadline was extended, in some cases, to the year 2010. The states monitor air pollution , enforce the standards, and can implement stricter standards than the NAAQSs if they desire.

The primary standards must be established at levels that would "provide an adequate margin of safety ... to protect the public ... from any known or anticipated adverse effects associated with such air pollutant[s] in the ambient air." This phrase was based on the belief that there is a threshold effect of pollution : pollution levels below the threshold are safe, levels above are unsafe. Although such an approach to setting the standards reflected scientific knowledge at the time, more recent research suggests that such a threshold probably does not exist. That is, pollution at any level is unsafe. The NAAQSs are also to be established without consideration of how much it will cost to achieve them; they are to be based on the Best Available Control Technology (BAT). The secondary standards are to "protect the public welfare from any known or anticipated adverse effects."

The NAAQSs are established based on the EPA's "criteria documents," which summarize the effect on human health caused by each pollutant, based on current scientific knowledge. The standards are usually expressed in parts of pollutant per million parts of air and vary in the duration of time a pollutant can be allowed into the environment , so that only a limited amount of contaminant may be emitted per hour, week, or year, for example. The 1977 Clean Air Act amendments require the EPA to submit criteria documents to the Clean Air Scientific Advisory Committee and the EPA's Science Advisory Board for review. Several revisions of the criteria documents are usually required. Although standards should be based on scientific evidence, politics often become involved as environmentalists and public health advocates battle industrial powers in setting standards.

The six criteria pollutants come from a variety of sources and have a variety of health effects. Carbon monoxide is a gas produced by the incomplete combustion of fossil fuels . It can lead to damage of the cardiovascular, nervous, and pulmonary systems, and can also cause problems with short-term attention span and sensory abilities.

Lead, a heavy metal, has been traced to many health effects, mainly brain damage leading to learning disabilities and retardation in children. Most of the lead found in the air came from gasoline fumes until a 1973 court case, Natural Resources Defense Council v. EPA, prompted its inclusion as a criteria pollutant . Lead levels in gasoline are now monitored.

Nitrogen oxide is formed primarily by fossil fuel combustion. It not only contributes to acid rain and the formation of ground-level ozone, but it has been linked to respiratory illness.

Ground-level ozone is produced by a combination of hydrocarbons and nitrogen oxides in the presence of sunlight, and heat. It is the prime component of photochemical smog , which can cause respiratory problems in humans, reduce crop yields, and cause forest damage. In 1979, the first revision of an original NAAQS slightly relaxed the photochemical oxidant standard and renamed it the ozone standard. Experts are currently debating whether this standard is low enough, and in 1991 the American Lung Association sued the EPA for failure to review the ozone NAAQS in light of new evidence.

Particulate matter is composed of small pieces of solid and liquid matter, including soot, dust, and organic matter. It reduces visibility and can cause eye and throat irritation, respiratory ailments, and cancer . Through 1987, total suspended particulates were the basis of the NAAQS. In 1987, the standard was revised and based on particulate matter with an aerodynamic diameter of 10 micrometers or less (PM-10), which was identified as the main health risk.

Sulfur dioxide is a gas produced primarily by coal-burning utilities and other fossil fuel combustion. It is the chief cause of acid rain and can also cause respiratory problems.

To achieve the NAAQSs for these six pollutants, the Clean Air Act incorporated three strategies. First, the federal government would establish New Source Performance Standards (NSPSs) for stationary sources such as factories and power plants ; they would also establish emission standards for mobile sources. Finally, the states would develop State Implementation Plans (SIPs) to address existing sources of air pollution. If the federal government determined that an SIP was not adequate to assure that the state would meet the NAAQSs, it could impose federal controls to meet them. According to the EPA, the SIPs must be designed to bring sub-standard air quality regions up to the NAAQSs, or to make sure any area already meeting the requirements continued to do so. The SIPs should prevent increased air pollution in areas of noncompliance, either by preventing significant expansions of existing industries or the opening of new plants. In order to allow economic development and growth in such non-attainment areas while still working to reduce air pollution, the 1977 amendments to the Clean Air Act required that new sources of pollution in non-attainment areas must control emissions to the lowest achievable emission rate (LAER) for that type of source and pollution and demonstrate that the new pollution would be offset by new emission reductions from existing sources in the area, reductions that went beyond existing permits and compliance plans. So, new sources were allowed, but only if the additional pollution were offset by reductions at existing sources.

Between 1978 and 1987, data collected nationally at 84 to 1,726 sites (varying for each pollutant) indicate that annual average concentrations of total suspended particulates fell by 21%, sulfur dioxide levels fell by 35%, carbon monoxide levels fell by 32%, lead levels fell by 88%, nitrogen dioxide levels fell by 12%, and ozone levels fell by 9%. With the exception of ozone, the average concentrations are below the NAAQSs. It is unclear how these reductions have improved human health conditions, but illnesses due to chronic lead exposure are down. Ozone probably causes the most health problems in affected areas, since the NAAQS for it is most often exceeded. Due to other complex factors, though, it is unclear how much of a problem it is.

Though the Clean Air Act allows one violation per year for each of the one-hour, eight-hour, and twenty-four hour NAAQSs for each pollutant, most urban areas, called non-attainment areas, violate standards much more often. For example, in Los Angeles the ozone standard was exceeded an average of 123 times annually in 1983, 1984, and 1985. As of 1989, 341 counties had ozone non-attainment, 317 had particulate non-attainment, 123 had carbon monoxide non-attainment, sixty-seven had sulfur dioxide non-attainment, and four had nitrogen dioxide non-attainment. A total of 529 counties had not met standards on at least one criteria pollutant.

The 1990 amendments to the Clean Air Act dealt with the problem of such non-attainment areas. Six categories of ozone non-attainment were established, ranging from marginal to extreme, and two categories for both carbon monoxide and particulate matter were established. Deadlines to achieve the NAAQSs were extended from three to twenty years. Increased restrictions were required in non-attainment areas; existing controls were tightened and smaller sources were made subject to regulation. Also, annual reduction goals were mandated. Areas considered to have made inadequate progress toward reaching attainment are subjected to stringent regulations on new plants and limited use of federal highway funds.

See also Air pollution control; Air pollution index; Air quality criteria; Heavy metals and heavy metal poisoning; Nonpoint source; Point source

[Christopher McGrory Klyza ]



Bryner, G. C. Blue Skies, Green Politics: The Clean Air Act of 1990. Washington, DC: CQ Press, 1993.

Melnick, R. S. Regulation and the Courts: The Case of the Clean Air Act. Washington, DC: Brookings Institution, 1983.

Portney, P. R. "Air Pollution Policy." In Public Policies for Environmental Protection, edited by P. R. Portney. Washington, DC: Resources for the Future, 1990.

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National Ambient Air Quality Standard

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National Ambient Air Quality Standard