The Clean Air Act: Summary and Impact

Has the CAA improved air quality in the US?

Smog over Los Angeles, California
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The Clean Air Act (CAA, or the Act) is a federal law aimed at combating issues facing air quality nationwide. In addition to regulating stationary sources of air pollution, like petroleum refineries, the Act regulates mobile sources such as motor vehicles and aircraft. Emission standards are focused on criteria pollutants that pose the most risk to human health, but the CAA has been expanded to include provisions regarding issues like ozone depletion and acid rain.

The CAA’s precursor, the Air Pollution Control Act, was passed in 1955 to provide funds to research the health impacts of air pollution following a severe air pollution event in Donora, Pennsylvania. Originally signed into law in December 1963, the CAA was created to develop emission standards and a regulatory framework to limit air pollution.

The Clean Air Act of 1970 was subsequently enacted to further control emissions through creation of the National Ambient Air Quality Standards (NAAQS), New Source Performance Standards (NSPS), and National Emission Standards for Hazardous Air Pollutants (NESHAPs). The Act was ultimately amended in 1977 and 1990 to establish new attainment deadlines and to reflect evolving regulatory needs.

What Is Air Pollution?

Air pollution is the release and presence of solid particles and gases in the air that can cause damage to the health of humans and other living things. Likewise, air pollution may cause damage to the climate—as is the case with chlorofluorocarbons (CFCs)—or to buildings and other materials, as with acid rain caused by sulfur dioxide (SO2).

Summary of the Law

With a goal of reducing the risks to public health and welfare caused by air pollution, the Clean Air Act relies on a series of air quality and emission standards that apply to both stationary and mobile sources.  

What Are NAAQS?

Under Title I, section 109 of the CAA, the United States Environmental Protection Agency (EPA) is required to establish National Ambient Air Quality Standards (NAAQS) that limit emissions from major stationary sources like petroleum refineries, food processing plants, and coal-powered plants. NAAQS apply to the six criteria pollutants covered in the Act—carbon monoxide, lead, nitrogen dioxide, ozone, particle pollution, and sulfur dioxide—and may be either primary or secondary.

Primary standards are those that protect public health—especially that of more sensitive parts of the population, including children, the elderly, and people with asthma. Secondary standards are aimed at limiting air pollution that endangers the public welfare; for example, levels of criteria pollutants shouldn’t cause decreased visibility or cause harm to animals, buildings, or agricultural resources.

Limits must be reviewed by the EPA every five years and may be revised at this time, though the review timeline is more extended in practice. Revisions are recommended based on the most up-to-date scientific information as well as factors like exposure risks and policy implications. The original NAAQS had an attainment deadline of 1977, but states struggled to comply and the deadline was subsequently extended.

State Implementation Plans (SIPs)

In order to meet NAAQS, states are directed to develop State Implementation Plans (SIPs) under section 110 of the Act. To do this, states must review emission inventories and computer models to predict whether violations of the NAAQS are likely to occur. States then create SIPs to prevent exceedances of the NAAQS.

Pursuant to the 1990 amendments, states are subject to sanctions for failing to submit or implement an adequate SIP; in these cases, a Federal Implementation Plan may also be imposed. The Menu of Control Measures (MCM) also assists state, local, and tribal agencies with attainment by providing details about the cost and efficiency of various control measures.

Emission Standards for Moving Sources

Denver Colorado vehicle tail pipe auto exhaust at busy intersection
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Title II of the Act regulates emission standards for moving sources, including motor vehicles and aircraft. Strengthening emission standards under the 1990 amendments, Title II addresses the fact that cars and trucks account for up to 90% of carbon monoxide (CO) emissions in urban areas.

More specifically, Title II of the Clean Air Act sets forth emission standards for new vehicles and vehicle engines and places requirements on fuel composition and quality. Manufacturers must comply with vehicle emission limits for hydrocarbons (HC), carbon monoxide, nitrogen oxides (NOx), and—with respect to diesel vehicles—particulates.

1977 and 1990 Revisions to the CAA

The 1977 and 1990 revisions to the Clean Air Act were primarily aimed at setting new goals, or dates by which NAAQS should be attained. However, both sets of amendments also ushered in new programs to strengthen the Act.

Clean Air Act Amendments of 1977

Beyond extending NAAQS attainment deadlines, the Clean Air Act Amendments of 1977 created the Prevention of Significant Deterioration (PSD) program. The PSD applies to new major sources or major modifications at existing sources, and is intended to ensure that economic growth is consistent with the requirements of the Clean Air Act. The PSD also ensures that permitted increases in air pollution are only allowed after careful evaluation of the potential consequences.

The 1977 amendments also modified the required auto emission reductions using technology-forcing standards that were initially introduced in the amendments of 1970. These 1977 amendments required that gas-powered automobiles meet hydrocarbon emissions standards by the 1980 model year, a nitrogen oxide standard by 1981, and a carbon monoxide standard by model year 1983.

Clean Air Act Amendments of 1990

Signed into law by President George H. W. Bush on Nov. 15, 1990, the Clean Air Act Amendments of 1990 are the most recent major change to the CAA. Not only did the 1990 amendments create the Acid Rain Program, they laid the groundwork for stratospheric ozone protection and expanded research programs under the Act.

  • Acid Rain Program. The EPA’s Acid Rain Program (ARP) was created under Title IV of the 1990 Clean Air Act Amendments, and mandated substantial reductions in sulfur dioxide and nitrogen oxide emissions from the energy industry. As the first national cap and trade program, the ARP set a permanent cap on sulfur dioxide emissions in the U.S. (to 50% below 1980 levels), and subsequently met the 2010 reduction goal in 2007. 
  • Stationary source operating permits. The 1990 amendments created an operating permits program aimed at ensuring compliance with the Act, strengthening enforcement actions, and clarifying source control requirements. Under the permitting requirement, air pollution sources must obtain a Title V operating permit and states have to implement the program. The EPA reviews state permitting programs, issues regulations and guidance materials, and oversees state implementation, making it easier for states to administer requirements. 
  • Stratospheric ozone protection. Title VI of the 1990 amendments expanded upon the existing requirements and market-based approach by introducing a total phase-out of ozone depleting substances, including CFCs and halons. Pursuant to the amendment, the EPA listed regulated substances along with information about their potential for ozone depletion and global warming, as well as a list of safe and unsafe alternatives to those restricted chemicals. 
  • NESHAPs. The 1990 amendments incorporated the National Emission Standards for Hazardous Air Pollutants (NESHAPs). These standards are set for stationary sources and pertain to hazardous air pollutants (HAPs) that are known—or suspected—to cause serious health problems or cancer.

Compliance Monitoring Strategy

Compliance monitoring is accomplished by the EPA in conjunction with state, local, and tribal regulators. In general, compliance monitoring involves on-site inspections and record reviews to ensure regulated entities are following relevant environmental laws. The EPA has provided a number of guidance documents aimed at helping regulators better monitor compliance with the CAA:

  • CAA Stationary Source Compliance Monitoring Strategy. Compliance with the Clean Air Act is monitored via the EPA’s CAA Stationary Source Compliance Monitoring Strategy, or CMS. The EPA monitors facilities, activities, and entities that are regulated under the CAA with respect to a number of areas, including acid rain inspection, asbestos demolition and renovation, mobile sources, new source review, and prevention of accidental releases.
  • CAA National Stack Testing Guidance. Stack testing, or source testing, is the process of measuring the amount of specific pollutants to determine whether a facility is capable of complying with the Clean Air Act. The CAA National Stack Testing Guidance aims to provide guidance around this process by detailing how stack test results should be interpreted and evaluating related legal and policy issues.
  • Risk Management Plan Rule (RMP Rule). Pertaining specifically to facilities that use highly hazardous substances, the RMP Rule requires these companies to develop a risk management program and submit a summary to the EPA. These plans, which include a strategy for chemical accident prevention, must be reviewed, revised, and resubmitted every five years.
  • Area Source Rule Implementation Guidance. The Clean Air Act Area Source Rules cover any stationary sources of emissions that are not classified as major sources. To help clarify implementation of these rules, the EPA published the Area Source Rule Implementation Guidance document.

Clean Air Act Enforcement

The Clean Air Act, including emissions from stationary and mobile sources, is also enforced by the Environmental Protection Agency. Generally speaking, there are three types of enforcement actions available to the EPA: civil administrative actions, which don’t involve a court; civil judicial actions, which take the shape of a formal lawsuit that’s filed in court; and criminal actions, which are reserved for willful or knowing violations. 

Just as enforcement actions range in seriousness, so do the results of an enforcement action. In the case of a civil enforcement, the EPA can rely on settlements, civil penalties (fines that compensate for the severity of the violation), and injunctive relief (requiring an entity to take an action or refrain from an act). Supplemental environmental projects (SEPs) and mitigation efforts may also be required to reduce or make up for damage caused by a violation. Criminal penalties are more serious and can include federal, state, and local fines or prison time. 

Has the Clean Air Act Worked?

The Clean Air Act has enjoyed a great deal of success since it was originally signed into law, and subsequent amendments have made an even more substantial impact on air quality in the United States. In fact, there has been a 74% reduction in emissions of key air pollutants since the Act was enacted in 1970.

Between enactment of the law and the 1990 amendments, the Act was responsible for the prevention of almost 700,000 cases of chronic bronchitis and over 200,000 premature deaths. Then, between 1990 and 2010—and in spite of a more than 64% increase in Gross Domestic Product—there was an over 41% decrease in criteria pollutant emissions. In 2020, the EPA estimated that restrictions put in place under Clean Air Act amendments prevented over 230,000 early deaths, 120,000 emergency room visits, and 17,000,000 lost work days. 

Current Status

The Clean Air Act has not been reauthorized since the 1990 amendments, which authorized appropriations through fiscal year 1998. While reauthorization is technically required under House rules, the requirement has been waived and CAA programs continue to receive funding—and make a difference—over 30 years later.

That said, because NAAQS and attainment deadlines are revised over time, counties sometimes fail to meet these requirements. For example, in 2018, the EPA reported that 209 counties across 22 states were not in attainment for the ozone NAAQS set in 2008. Criticism of the Act focuses on whether its economic benefits are measured in a meaningful way, with some arguing that using the avoidance of premature mortality as a metric presents substantial uncertainties. 

Current issues facing air quality include conventional air pollution like that addressed by the CAA and ozone layer depletion. Climate change will continue to be a growing threat to air quality in the United States (and worldwide), due in part to an increase in wildfires, which increase particulate concentrations.

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