Summaries of Environmental Laws
Administered by the EPA
Congressional Research Service Report  RL30022
Redistributed as a service of the National Library for the Environment

Clean Air Act

Prepared by James E. McCarthy, Larry B. Parker,
Linda Schierow, and Claudia Copeland,
Senior Analysts, Environment and Natural Resources Policy Division.


Table 1. Clean Air Act and Amendments
National Ambient Air Quality Standards
State Implementation Plans
Nonattainment Requirements
Table 2. Ozone Nonattainment Classifications

Requirements for Ozone Nonattainment Areas
Requirements for Carbon Monoxide Nonattainment Areas
Requirements for Particulate Nonattainment Areas

Emission Standards for Mobile Sources
Hazardous Air Pollutants
New Source Performance Standards
Solid Waste Incinerators
Prevention of Significant Deterioration / Regional Haze
Acid Deposition Control
Stratospheric Ozone Protection
Selected References
Table 3. Major U.S. Code Sections of the Clean Air Act

The Clean Air Act, codified as 42 U.S.C. 7401 et seq., seeks to protect human health and the environment from emissions that pollute ambient, or outdoor, air. It requires the Environmental Protection Agency to establish minimum national standards for air quality, and assigns primary responsibility to the states to assure compliance with the standards. Areas not meeting the standards, referred to as nonattainment areas, are required to implement specified air pollution control measures. The Act establishes federal standards for mobile sources of air pollution, for sources of 188 hazardous air pollutants, and for the emissions that cause acid rain. It establishes a comprehensive permit system for all major sources of air pollution. It also addresses the prevention of pollution in areas with clean air and protection of the stratospheric ozone layer.



Like many other programs administered by the Environmental Protection Agency, federal efforts to control air pollution have gone through several phases, beginning with information collection, research, and technical assistance, before being strengthened to establish federal standards and enforcement. Federal legislation addressing air pollution was first passed in 1955, prior to which, air pollution was the exclusive responsibility of state and local levels of government.

Table 1. Clean Air Act and Amendments
(codified generally as 42 U.S.C. 7401-7671)

Year Act Public Law Number
1955 Air Pollution Control Act P.L. 84-159
1959 Reauthorization P.L. 86-353
1960 Motor vehicle exhaust study P.L. 86-493
1963 Clean Air Act Amendments P.L. 88-206
1965 Motor Vehicle Air Pollution Control Act P.L. 89-272, title I
1966 Clean Air Act Amendments of 1966 P.L. 89-675
1967 Air Quality Act of 1967 P.L. 90-148
1970 Clean Air Act Amendments of 1970 P.L. 91-604
1973 Reauthorization P.L. 93-13
1974 Energy Supply and Environmental Coordination Act of 1974 P.L. 93-319
1977 Clean Air Act Amendments of 1977 P.L. 95-95
1980 Acid Precipitation Act of 1980 P.L. 96-294, title VII
1981 Steel Industry Compliance Extension Act of P.L. 97-23
1987 Clean Air Act 8-month Extension P.L. 100-202
1990 Clean Air Act Amendments of 1990 P.L. 101-549
1995-6 Relatively minor laws amending the Act P.L. 104-6, P.L. 104-59, P.L. 104-70, P.L. 104-260

The federal role was strengthened in subsequent amendments, notably the Clean Air Act Amendments of 1970, 1977, and 1990.

The 1970 amendments established procedures under which EPA sets national standards for air quality, required a 90% reduction in emissions from new automobiles by 1975, established a program to require the best available control technology at major new sources of air pollution, established a program to regulate air toxics, and greatly strengthened federal enforcement authority. The 1977 amendments extended deadlines and added the Prevention of Significant Deterioration program to protect air cleaner than national standards.

Changes to the Act in 1990 included provisions to (1) classify non-attainment areas according to the extent to which they exceed the standard, tailoring deadlines, planning, and controls to each area's status; (2) tighten auto emission standards and require reformulated and alternative fuels in the most polluted areas; (3) revise the air toxics section, establishing a new program of technology-based standards and addressing the problem of sudden, catastrophic releases of toxics; (4) establish an acid rain control program, with a marketable allowance scheme to provide flexibility in implementation; (5) require a state-run permit program for the operation of major sources of air pollutants; (6) implement the Montreal Protocol to phase out most ozone-depleting chemicals; and (7) update the enforcement provisions so that they parallel those in other pollution control acts, including authority for EPA to assess administrative penalties.

The remainder of this section describes major programs required by the Act, with an emphasis on the changes established by the 1990 amendments.

National Ambient Air Quality Standards

In section 109, the Act requires EPA to establish National Ambient Air Quality Standards (NAAQS) for several types of air pollutants. The NAAQS must be designed to protect public health and welfare with an adequate margin of safety. Using this authority, EPA has promulgated NAAQS for six air pollutants: sulfur dioxide (SO2), particulate matter (PM2.5 and PM10), nitrogen dioxide (NO2), carbon monoxide (CO), ozone,(2) and lead. The Act requires EPA to review the scientific data upon which the standards are based, and revise the standards, if necessary, every 5 years. More often than not, however, EPA has taken more than 5 years in reviewing and revising the standards.

Originally, the Act required that the NAAQS be attained by 1977 at the latest, but the states experienced widespread difficulty in complying with these deadlines. As a result, the deadlines have been extended several times. Under the 1990 amendments, areas not in attainment with NAAQS must meet special compliance schedules, staggered according to the severity of an area's air pollution problem. The amendments also established specific requirements for each nonattainment category, as described below.

State Implementation Plans

While the Act authorizes the EPA to set NAAQS, the states are responsible for establishing procedures to attain and maintain the standards. Under Section 110 of the Act, the states adopt plans, known as State Implementation Plans (SIPs), and submit them to EPA to ensure that they are adequate to meet statutory requirements.

SIPs are based on emission inventories and computer models to determine whether air quality violations will occur. If these data show that standards would be exceeded, the state imposes additional controls on existing sources to ensure that emissions do not cause "exceedances" of the standards. Proposed new and modified sources must obtain state construction permits in which the applicant shows how the anticipated emissions will not exceed allowable limits. In nonattainment areas, emissions from new or modified sources must also be offset by reductions in emissions from existing sources.

The 1990 amendments require EPA to impose sanctions in areas which fail to submit a SIP, fail to submit an adequate SIP, or fail to implement a SIP: unless the state corrects such failures, a 2-to-1 emissions offset for the construction of new polluting sources is imposed 18 months after notification to the state, and a ban on most federal highway grants is imposed 6 months later. An additional ban on air quality grants is discretionary. Ultimately, a Federal Implementation Plan may be imposed if the state fails to submit or implement an adequate SIP.

Nonattainment Requirements

In a major departure from the prior law, the 1990 Clean Air Act Amendments group nonattainment areas into classifications based on the extent to which the NAAQS is exceeded, and establish specific pollution controls and attainment dates for each classification. These requirements are spelled out in Sections 171-193 of the Act. The most extensive requirements apply to areas failing to attain the 1-hour ozone standard of 0.12 parts per million. Because they are specified in the statute, these provisions continue in place, even though EPA modified the ozone standard through regulations promulgated in July 1997.

Nonattainment areas are classified on the basis of a "design value," which is derived from the pollutant concentration (in parts per million) recorded by air quality monitoring devices. The design value for the 1-hour ozone standard is the fourth highest reading measured over a 3-year period. The Act creates five classes of ozone nonattainment, as shown in Table 2. Only Los Angeles falls into the "extreme" class, but 97 other areas were classified in one of the other four ozone categories. A simpler classification system establishes moderate and serious nonattainment areas for carbon monoxide and particulate matter with correspondingly more stringent control requirements for the more polluted class.

Table 2. Ozone Nonattainment Classifications

Class Marginal Moderate Serious Severe Extreme
Deadline 3 years 6 years 9 years 15-17 yrs.* 20 years
Areas** 42 areas 32 areas 14 areas 9 areas 1 area


0.121 ppm-

0.138 ppm

0.138 ppm-

0.160 ppm

0.160 ppm-

0.180 ppm

0.180 ppm-

0.280 ppm

>0.280 ppm

*Areas with a 1988 design value between 0.190 and 0.280 ppm have 17 years to attain; others have 15 years.
** Number of areas in each category as of the date of enactment.

For ozone nonattainment areas, the deadlines stretch from 1993 to 2010, depending on the severity of the problem. For carbon monoxide, the attainment date for moderate areas was December 31, 1995, and for serious areas is December 31, 2000. For particulate matter, the deadline for areas designated moderate nonattainment as of 1990 was December 31, 1994; for those areas subsequently designated as moderate, the deadline is 6 years after designation. For serious areas, the respective deadlines are December 31, 2001 or 10 years after designation.

Requirements for Ozone Nonattainment Areas.

Although areas with more severe air pollution problems have a longer time to meet the standards, more stringent control requirements are imposed in areas with worse pollution. A summary of the primary ozone control requirements for each nonattainment category follows.

Primary Ozone Requirements
Marginal Areas
  • Inventory emissions sources (to be updated every 3 years).
  • Require 1.1 to 1 offsets (i.e., industries must reduce emissions from existing facilities by 10% more than the emissions of any new facility opened in the area).
  • Impose reasonably available control technology (RACT) on all major sources emitting more than 100 tons per year for the nine industrial categories where EPA had already issued control technique guidelines describing RACT prior to 1990.
Moderate Areas
  • Meet all requirements for marginal areas.
  • Impose a 15% reduction in volatile organic compounds (VOCs) in 6 years.
  • Adopt a basic vehicle inspection and maintenance program.
  • Impose RACT on all major sources emitting more than 100 tons per year for all additional industrial categories where EPA will issue control technique guidelines describing RACT.
  • Require vapor recovery at gas stations selling more than 10,000 gallons per month.
  • Require 1.15 to 1 offsets.
Serious Areas
  • Meet all requirements for moderate areas.
  • Reduce definition of a major source of VOCs from emissions of 100 tons per year to 50 tons per year for the purpose of imposing RACT.
  • Reduce VOCs 3% annually for years 7 to 9 after the 15% reduction already required by year 6.
  • Improve monitoring.
  • Adopt an enhanced vehicle inspection and maintenance program.
  • Require fleet vehicles to use clean alternative fuels.
  • Adopt transportation control measures if the number of vehicle miles traveled in the area is greater than expected.
  • Require 1.2 to 1 offsets.
  • Adopt contingency measures if the area does not meet required VOC reductions.
Severe Areas
  • Meet all requirements for serious areas.
  • Reduce definition of a major source of VOCs from emissions of 50 tons per year to 25 tons per year for the purpose of imposing RACT.
  • Adopt specified transportation control measures.
  • Implement a reformulated gasoline program.
  • Require 1.3 to 1 offsets.
  • Impose $5,000 per ton penalties on major sources if the area does not meet required reductions.
Extreme Areas
  • Meet all requirements for severe areas.
  • Reduce definition of a major source of VOCs from emissions of 25 tons per year to 10 tons per year for the purpose of imposing RACT.
  • Require clean fuels or advanced control technology for boilers emitting more than 25 tons per year of NOx.
  • Require 1.5 to 1 offsets.

As noted, EPA promulgated a new, 8-hour ozone standard in July 1997. Under legislation adopted subsequent to this promulgation (P.L. 105-178, Title VI), the Agency will designate nonattainment areas for the new standard in July 2000. State Implementation Plans must be submitted within 3 years of an area's designation.

Requirements for Carbon Monoxide Nonattainment Areas.

As with ozone nonattainment areas, carbon monoxide (CO) nonattainment areas are subjected to specified control requirements, with more stringent requirements in Serious nonattainment areas. A summary of the primary CO control requirements for each nonattainment category follows.

Primary CO Requirements
Moderate Areas
  • Conduct an inventory of emissions sources.
  • Forecast total vehicle miles traveled in the area.
  • Adopt an enhanced vehicle inspection and maintenance program.
  • Demonstrate annual improvements sufficient to attain the standard.
Serious Areas
  • Adopt specified transportation control measures.
  • Implement an oxygenated fuels program for all vehicles in the area.
  • Reduce definition of a major source of CO from emissions of 100 tons per year to 50 tons per year if stationary sources contribute significantly to the CO problem.

Serious areas failing to attain the standard by the deadline have to revise their SIP and demonstrate reductions of 5% per year until the standard is attained.


2. Unlike the other NAAQS pollutants, ozone is not directly emitted, but rather is formed in the atmosphere by the interaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. The control of ozone is thus based on regulating emissions of VOCs and NOx.

3. The 1990 amendments specified 189 pollutants, but Public Law 102-187, enacted on December 4, 1991, deleted hydrogen sulfide from the list of toxic pollutants, leaving only 188.

4. NOTE: This table shows only the major U.S. Code sections. For more detail and to determine when a section was added, the reader should consult the official printed version of the U.S. Code.

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