The Clean Air Act (CAA) is the United States' primary federal air quality law, intended to reduce and control air pollution nationwide. Initially enacted in 1963 and amended many times since, it is one of the United States' first and most influential modern environmental laws.

As with many other major U.S. federal environmental statutes, the Clean Air Act is administered by the U.S. Environmental Protection Agency (EPA), in coordination with state, local, and tribal governments. EPA develops extensive administrative regulations to carry out the law's mandates. Associated regulatory programs, which are often technical and complex, implement these regulations. Among the most important, the National Ambient Air Quality Standards program sets standards for concentrations of certain pollutants in outdoor air, and the National Emissions Standards for Hazardous Air Pollutants program which sets standards for emissions of particular hazardous pollutants from specific sources. Other programs create requirements for vehicle fuels, industrial facilities, and other technologies and activities that impact air quality. Newer programs tackle specific problems, including acid rain, ozone layer protection, and climate change.

The CAA has been challenged in court many times, both by environmental groups seeking more stringent enforcement and by states and utilities seeking greater leeway in regulation.

Although its exact benefits depend on what is counted, the Clean Air Act has substantially reduced air pollution and improved US air quality—benefits which EPA credits with saving trillions of dollars and many thousands of lives each year.

Regulatory programs In the United States, the "Clean Air Act" typically refers to the codified statute at . That statute is the product of multiple acts of Congress, one of which—the 1963 act—was actually titled the Clean Air Act, and another of which—the 1970 act—is most often referred to as such. In the U.S. Code, the statute itself is divided into subchapters, and the section numbers are not clearly related to the subchapters. However, in the bills that created the law, the major divisions are called "Titles", and the law's sections are numbered according to the title (e.g., Title II begins with Section 201). In practice, EPA, courts, and attorneys often use the latter numbering scheme.

Although many parts of the statute are quite detailed, others set out only the general outlines of the law's regulatory programs, and leave many key terms undefined. Responsible agencies, primarily EPA, have therefore developed administrative regulations to carry out Congress's instructions. EPA's proposed and final regulations are published in the Federal Register, often with lengthy background histories. The existing CAA regulations are codified at 40 C.F.R. Subchapter C, Parts 50–98.40 C.F.R. These Parts more often correspond to the Clean Air Act's major regulatory programs.

The following are major regulatory programs under the Clean Air Act.

National Ambient Air Quality Standards The National Ambient Air Quality Standards (NAAQS) govern how much ground-level ozone (O), carbon monoxide (CO), particulate matter (PM, PM), lead (Pb), sulfur dioxide (SO), and nitrogen dioxide (NO) are allowed in the outdoor air.NAAQS Program: First enacted: 1970 CAA amendments, § 4. Location in statute: CAA § 109, . Major regulations: . EPA pages: The NAAQS set the acceptable levels of certain air pollutants in the ambient air in the United States. Prior to 1965, there was no national program for developing ambient air quality standards, and prior to 1970 the federal government did not have primary responsibility for developing them.

The 1970 CAA amendments required EPA to determine which air pollutants posed the greatest threat to public health and welfare and promulgate NAAQS and air quality criteria for them. The health-based standards were called "primary" NAAQS, while standards set to protect public welfare other than health (e.g., agricultural values) were called "secondary" NAAQS.

In 1971, EPA promulgated regulations for sulfur oxides, particulate matter, carbon monoxide, photochemical oxidants, hydrocarbons, and nitrogen dioxide (). Initially, EPA did not list lead as a criteria air pollutant, controlling it through mobile source authorities, but it was required to do so after successful litigation by Natural Resources Defense Council (NRDC) in 1976 ().

The 1977 CAA Amendments created a process for regular review of the NAAQS list, and created a permanent independent scientific review committee to provide technical input on the NAAQS to EPA.See EPA added regulations for PM2.5 in 1997 and updates the NAAQS from time to time based on emerging environmental and health science.EPA. (1997-07-18) "National Ambient Air Quality Standards for Particulate Matter." Final rule. Federal Register,

National Emissions Standards for Hazardous Air Pollutants The National Emissions Standards for Hazardous Air Pollutants (NESHAPs) govern how much of 187 toxic air pollutants are allowed to be emitted from industrial facilities and other sources.NESHAPS Program: First enacted: 1970 CAA amendments § 4. Location in statute: CAA § 112, . Major regulations: , . EPA pages: These pollutants are often referred to as "air toxics" and include chemicals that are known or suspected to cause cancer, birth defects, and other serious health issues. Some of the notable hazardous air pollutants include: Benzene (found in gasoline) Asbestos Formaldehyde Lead compounds Mercury compounds Chloroform Arsenic compounds Cadmium compounds Dioxins and furans Polychlorinated biphenyls (PCBs).

Under the CAA, hazardous air pollutants (HAPs, or air toxics) are air pollutants other than those for which NAAQS exist, which threaten human health and welfare. The NESHAPs are the standards used for controlling, reducing, and eliminating HAPs emissions from stationary sources such as industrial facilities.

The 1970 CAA required EPA to develop a list of HAPs, and then develop national emissions standards for each of them. The original NESHAPs were health-based standards.

The 1990 CAA Amendments ( Title III) codified EPA's list, and required creation of technology-based standards according to "maximum achievable control technology" (MACT). Over the years, EPA has issued dozens of NESHAP regulations, which have developed NESHAPs by pollutant, by industry source category, and by industrial process. There are also NESHAPs for mobile sources (transportation), although these are primarily handled under the mobile source authorities. The 1990 amendments (adding CAA § 112(d-f)) also created a process by which EPA was required to review and update its NESHAPs every eight years, and identify any risks remaining after application of MACT, and develop additional rules necessary to protect public health.

New Source Performance Standards The New Source Performance Standards (NSPS) are rules for the equipment required to be installed in new and modified industrial facilities, and the rules for determining whether a facility is "new".NSPS Program: First enacted: 1970 CAA amendments § 4. Location in statute: CAA § 111, . Major regulations: . EPA pages:

The 1970 CAA required EPA to develop standards for newly constructed and modified stationary sources (industrial facilities) using the "best system of emission reduction which (taking into account the cost of achieving such reduction) the [EPA] determines has been adequately demonstrated." The EPA issued its first NSPS regulation the next year, covering steam generators, which emit carbon dioxide (CO2), nitrogen oxides (NOx), sulfur dioxide (SO2), particulate matter (PM), carbon monoxide (CO), volatile organic compounds (VOCs), and mercury; incinerators, which release dioxins and furans, PM, heavy metals (e.g., lead, mercury, cadmium), CO, hydrochloric acid (HCl), SO2, and NOx; Portland cement plants, which emit PM, NOx, SO2, CO, VOCs, and heavy metals; nitric acid plants, which produce NOx, nitrous oxide (N2O), and ammonia (NH3); and sulfuric acid plants, which discharge SO2, sulfur trioxide (SO3), and acid mist (PM). (). Since then, EPA has issued dozens of NSPS regulations, primarily by source category. The requirements promote industrywide adoption of available pollution control technologies. However, because these standards apply only to new and modified sources, they promote extending the lifetimes of pre-existing facilities.

In the 1977 CAA Amendments, Congress required EPA to conduct a "new source review" process (, subpart I) to determine whether maintenance and other activities rises to the level of modification requiring application of NSPS.

Acid Rain Program The Acid Rain Program (ARP) is an emissions trading program for power plants to control the pollutants that cause acid rain.Acid Rain Program: First enacted: 1990 CAA amendments. § 401 Location in statute: CAA Title IV-A, . Major regulations: through . EPA pages: The 1990 CAA Amendments created a new title to address the issue of acid rain, and particularly nitrogen oxides (NO) and sulfur dioxide (SO) emissions from electric power plants powered by fossil fuels, and other industrial sources. The Acid Rain Program (ARP), established under the 1990 Clean Air Act Amendments, was the first cap-and-trade emissions program in the United States, aimed at reducing sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions, the primary contributors to acid rain. The program set a national cap on total emissions of SO2 and NOx from power plants, which was gradually reduced over time. Power plants were allocated emission allowances, each allowing the release of one ton of SO2, based on historical emissions. Plants that reduced their emissions below their allowances could trade excess allowances with other plants, creating a financial incentive to invest in cleaner technologies. Continuous emissions monitoring systems were required to ensure compliance, and plants that exceeded their allowances faced fines. The program's success is evident in the 50% reduction of SO2 emissions from power plants by 2000, significantly improving air quality and reducing acid rain. The Acid Rain Program became a model for subsequent cap-and-trade systems, including those targeting greenhouse gas emissions.

Ozone layer protection

The CAA ozone program is a technology transition program intended to phase out the use of chemicals that harm the ozone layer.Ozone Layer Protection Program: First enacted: 1990 CAA amendments. § 601. Location in statute: CAA Title VI, . Major regulations: through . EPA pages: Consistent with the US commitments in the Montreal Protocol, CAA Title VI, added by the 1990 CAA Amendments, mandated regulations regarding the use and production of chemicals that harm Earth's stratospheric ozone layer. Ozone-depleting chemicals (ODCs) are substances that contribute to the depletion of Earth's stratospheric ozone layer, which protects life on Earth from harmful ultraviolet (UV) radiation. The primary ODCs regulated under the Clean Air Act (CAA) and the Montreal Protocol include chlorofluorocarbons (CFCs), halons, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), methyl chloroform, and carbon tetrachloride. CFCs were commonly used in refrigeration, air conditioning, and foam-blowing agents, while halons were used in fire extinguishing systems. HCFCs were seen as less harmful alternatives to CFCs but still contributed to ozone depletion, and they are also being phased out. Under Title VI, EPA runs programs to phase out ozone-destroying substances, track their import and export, determine exemptions for their continued use, and define practices for destroying them, maintaining and servicing equipment that uses them, identifying new alternatives to those still in use, and licensing technicians to use such chemicals.

Mobile source programs