The Clean Air Act, as amended by Congress most recently in 1990, is the legislative cornerstone of the United States' air pollution policies. It outlines specific clean air objectives and standards that individuals, businesses, and government agencies must comply with, and provides for the regulatory framework through which the rules are enforced. The Environmental Protection Agency (EPA), itself created coinciding with the 1970 Clean Air Act, holds the primary responsibility for overseeing compliance with the act, among other responsibilities. Under the statute, the EPA has the power to create certain air pollution standards and regulations that businesses must observe; however, enforcement is shared with other federal agencies and many state agencies. While the original Clean Air Act became law in 1955, there have been a half dozen revisions and amendments since then, with the three most recent and influential on current policy occurring in 1970, 1977, and 1990.


The 1970 act charged the EPA to establish two kinds of air quality standards, ambient air quality standards and emissions standards. The National Ambient Air Quality Standards (NAAQS), last revised in 1997, govern the maximal allowable concentrations of six priority pollutants: carbon monoxide, lead, sulfur dioxide, nitrogen oxides, ground-level ozone (smog), and particulate matter. Primary standards are grounded in concerns for human health and welfare, and are set to protect the most sensitive members of human society, e.g. infants and the elderly. Secondary standards, usually either the same or more lenient than primary standards, are intended to protect the environment. The primary standards are reviewed every five years and changed if new evidence suggests the accepted levels of pollutants are set inappropriately as they relate to human health.


National Emission Standards for Hazardous Air Pollutants (NESHAPs) were also established by the 1970 law to regulate emissions from industrial sources and motor vehicles. They applied to such toxins as inorganic arsenic, asbestos, benzene, mercury, beryllium, vinyl chloride, coke oven emissions, and radionuclides. While early goals under the NESHAPs were achieved, the EPA found this pollutant-based approach ineffective when dealing with industry. In response to the difficulties implementing the NESHAPS, the 1990 amendments moved enforcement to a technology-based model under which the EPA creates facility-specific standards based on the kind of activities an industrial site is engaged in. For instance, a pulp mill must comply with standards set for pulp manufacturing plants, rather than attempting to conform to standards that also govern steel mills and oil refineries. The industry-specific standards are based on what the EPA terms Maximum Achievable Control Technology (MACT), or the level of emissions currently attained by some of an industry's least-polluting facilities. The EPA found this approach to be substantially more successful than the previous policy and expected to reduce emissions in the two decades following 1990 tenfold over that which had been achieved during the previous 20 years.


Pollution from mobile sources is addressed by the Motor Vehicle and Aircraft Emissions Standards. The 1970 Clean Air Act prescribed a 90 percent reduction in the amount of hydrocarbons, carbon monoxide, and nitrogen oxides emitted from automobiles, which was to be achieved by the mid-1970s. The standards were not met on time, and the 1977 amendments postponed their achievement even further. They were, however, reached in the 1990s, by which time the EPA reported that the average new car generated 60-80 percent less pollution than its predecessors of the 1960s.

Still, cars, trucks, and buses continue to be major sources of pollution in the United States, contributing to half of all smog and 90 percent of all carbon monoxide in urban areas. And, until the mid-1990s, truck and bus emissions remained comparatively unregulated. To combat these problems, the EPA has stepped up controls on both consumer and commercial vehicle pollution. Its policies include much stricter particulate emissions controls—a 90 percent reduction—for all large trucks made since 1994, and expanded automotive emissions testing in metropolitan areas. Auto manufacturers are also now required to include longer-lasting pollution controls on cars. The old law only required such devices to last for 50,000 miles, but this is considered only half of the average car's useful life. Under the 1990 act the life of pollution control devices must be doubled to 100,000 miles.

Another area of heightened scrutiny has been in the composition of fuels used by motor vehicles. EPA regulations in the 1990s sought to promote cleaner fuels, particularly in urban areas already plagued by smog or high levels of air pollution. The less-damaging fuels feature lower levels of volatile organic compounds (VOCs) and more detergents.


The Clean Air Act also includes provisions for reducing acid rain. Sulfur dioxide (SO2) and nitrogen oxides (NOx) are the primary pollutants contributing to acid rain, and more than 70 percent of each (based on late 1980s figures) is generated by coal-burning electric plants and motor vehicles. The EPA acid rain program is mainly targeted at reducing electric plant emissions.

The law requires emissions of each to return to pre-1980 levels. It allows for trading of a gradually decreasing number of emission allowances which may be used to offset SO2 emissions. During Phase I, which started in 1995, a fixed number of allowances were issued to existing generators based on historical production figures. In the first year of Phase I, SO2 emissions from affected utilities were already 40 percent lower than the allowable limit. In Phase II, beginning in 2000, emissions are to be monitored continuously and sources must surrender one allowance for every one ton of SO2 emitted the previous year. Utilities likely to have emissions beyond their stock of allowances may choose to repower units, use cleaner fuel, install scrubbers, reduce the amount of electricity generated, or purchase additional allowances from "cleaner" utilities.

SEE ALSO : Clean Water Act ; Environmental Law and Business ; Environmental Protection Agency (EPA)


"Clean Air, Dirty Fight." Economist, 15 March 1997.

Environmental Protection Agency. "Acid Rain Program Overview." Washington, September 1997. Available from .

——. EPA's National Ambient Air Quality Standards: The Standard Review/Evaluation Process." Washington, 17 July 1997. Available from .

——. The Plain English Guide to the Clean Air Act. Washington, April 1993. Available from

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