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. 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. 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. EPA added regulations for PM2.5 in 1997 and updates the NAAQS from time to time based on emerging environmental and health science.
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. 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". 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. 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. 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 Rules for pollutants emitted from internal combustion engines in vehicles. Since 1965, Congress has mandated increasingly stringent controls on vehicle engine technology and reductions in tailpipe emissions. Mobile source pollutants are emissions produced by internal combustion engines in motor vehicles, including cars, trucks, and buses. Key pollutants regulated under the Clean Air Act (CAA) include carbon monoxide (CO), a colorless, odorless gas that impairs oxygen transport in the blood; nitrogen oxides (NOx), which contribute to the formation of ground-level ozone and acid rain, and can aggravate respiratory conditions; particulate matter (PM), tiny particles that can cause or worsen respiratory and cardiovascular diseases; volatile organic compounds (VOCs), which contribute to ozone formation and can lead to cancer and other health problems; hydrocarbons (HC), emitted from fuel combustion, which also contribute to ground-level ozone; and carbon dioxide (CO2), a greenhouse gas that contributes to global climate change. EPA has set increasingly stringent controls on these emissions from motor vehicles, aiming to reduce their impact on public health and the environment by incorporating advanced engine technologies that balance emission reductions with factors such as cost, energy use, and safety. Today, the law requires EPA to establish and regularly update regulations for pollutants that may threaten public health, from a wide variety of classes of motor vehicles, that incorporate technology to achieve the "greatest degree of emission reduction achievable", factoring in availability, cost, energy, and safety (). Since the introduction of vehicle emissions regulations in 1965 and the subsequent amendments to the Clean Air Act (CAA), significant reductions in mobile source pollutants have been achieved. Carbon monoxide (CO) emissions have decreased by more than 80% since 1970, primarily due to improvements in engine design and the introduction of catalytic converters. Nitrogen oxides (NOx) emissions have been reduced by over 50% since 1970, driven by advancements in engine technology and stricter emission standards. Particulate matter (PM) emissions have dropped by more than 90%, thanks to cleaner-burning engines and the use of diesel particulate filters. Volatile organic compounds (VOCs) have been reduced by around 85% since 1970, due to better fuel management and evaporative emission controls. Hydrocarbon (HC) emissions have decreased by approximately 80%, facilitated by improvements in fuel combustion technology and engine control strategies. While carbon dioxide (CO2) emissions per mile have been reduced by 30-40% since the 1970s due to increased fuel efficiency, total CO2 emissions remain a concern due to increased vehicle miles traveled and the growth of larger vehicles. These reductions are the result of technological advancements, such as catalytic converters, on-board diagnostics, and more efficient engines, along with stricter fuel standards and the use of alternative fuels. The EPA continues to update and enforce emission standards to further reduce the impact of vehicle emissions on air quality and public health.
On-road vehicles regulations EPA sets standards for exhaust gases, evaporative emissions, air toxics, refueling vapor recovery, and vehicle inspection and maintenance for several classes of vehicles that travel on roadways. EPA's "light-duty vehicles" regulations cover passenger cars, minivans, passenger vans, pickup trucks, and SUVs. "Heavy-duty vehicles" regulations cover large trucks and buses. EPA first issued motorcycle emissions regulations in 1977 () and updated them in 2004 (). ;Vehicle testing program The air pollution testing system for motor vehicles was originally developed in 1972 and used
driving cycles designed to simulate driving during rush-hour in
Los Angeles during that era. Until 1984, EPA reported the exact fuel economy figures calculated from the test. In 1984, EPA began adjusting city (aka
Urban Dynamometer Driving Schedule or
UDDS) results downward by 10% and highway (aka HighWay Fuel Economy Test or
HWFET) results by 22% to compensate for changes in driving conditions since 1972, and to better correlate the EPA test results with real-world driving. In 1996, EPA proposed updating the Federal Testing Procedures to add a new higher-speed test (
US06) and an air-conditioner-on test (SC03) to further improve the correlation of fuel economy and emission estimates with real-world reports. In December 2006 the updated testing methodology was finalized to be implemented in model year 2008 vehicles and set the precedent of a 12-year review cycle for the test procedures. In February 2005, EPA launched a program called "Your MPG" that allows drivers to add real-world fuel economy statistics into a database on EPA's fuel economy website and compare them with others and with the original EPA test results. EPA conducts fuel economy tests on very few vehicles. Two-thirds of the vehicles the EPA tests themselves are randomly selected and the remaining third is tested for specific reasons. Although originally created as a reference point for fossil-fueled vehicles,
driving cycles have been used for estimating how many miles an electric vehicle will get on a single charge.
Non-road vehicles regulations The 1970 CAA amendments provided for regulation of aircraft emissions (), and EPA began regulating in 1973. In 2012, EPA finalized its newest restrictions on NOx emissions from gas turbine aircraft engines with rated thrusts above 26.7
kiloNewton (3 short
ton-force), meaning primarily commercial jet aircraft engines, intended to match international standards. EPA has been investigating whether to regulate lead in fuels for small aircraft since 2010, but has not yet acted. The 1990 CAA Amendments ( § 222) added rules for a "nonroad" engine program (), which expanded EPA regulation to locomotives, heavy equipment and small equipment engines fueled by diesel (compression-ignition), and gas and other fuels (spark-ignition), and marine transport.
Voluntary programs EPA has developed a variety of voluntary programs to incentivize and promote reduction in transportation-related air pollution, including elements of the Clean Diesel Campaign, Ports Initiative, "SmartWay" program (for the freight transportation sector), and others.
Fuel controls The federal government has regulated the chemical composition of transportation fuels since 1967, with significant new authority added in 1970 to protect public health. One of EPA's earliest actions was the elimination of lead in U.S. gasoline beginning in 1971, a project that has been described as "one of the great public health achievements of the 20th century." EPA continues to regulate the chemical composition of
gasoline,
avgas, and
diesel fuel in the United States.
Stationary source operating permits The 1990 amendments authorized a national operating permit program, sometimes called the "Title V Program", covering thousands of large industrial and commercial sources. It required large businesses to address pollutants released into the air, measure their quantity, and have a plan to control and minimize them as well as to periodically report. This consolidated requirements for a facility into a single document. Most permits are issued by state and local agencies. If the state does not adequately monitor requirements, the EPA may take control. The public may request to view the permits by contacting the EPA. The permit is limited to no more than five years and requires a renewal. Enforcement cases are usually settled, with penalties assessed well below maximum statutory limits. Many of the largest Clean Air Act settlements have been reached with automakers accused of circumventing the Act's vehicle and fuel standards (e.g., the 2015 "
Dieselgate" scandal).
Greenhouse gas regulation Much of EPA's regulation of
greenhouse gas (GHG) emissions occurs under the programs discussed above. EPA began regulating GHG emissions following the 2007
Supreme Court ruling in
Massachusetts v. EPA, the EPA's subsequent endangerment finding, and development of specific regulations for various sources. The EPA's authority to regulate
carbon dioxide emissions was questioned by the court in
West Virginia v. EPA (2022) but restored by Congress with the
Inflation Reduction Act of 2022, which clarified that carbon dioxide is one of the pollutants covered by the Clean Air Act. Standards for mobile sources have been established pursuant to Section 202 of the CAA, and GHGs from stationary sources are controlled under the authority of Part C of Title I of the Act. The EPA's auto emission standards for greenhouse gas emissions issued in 2010 and 2012 are intended to cut emissions from targeted vehicles by half, double fuel economy of passenger cars and light-duty trucks by 2025 and save over $4 billion barrels of oil and $1.7 trillion for consumers. The agency has also proposed a two-phase program to reduce greenhouse gas emissions for medium and heavy duty trucks and buses. Following the Supreme Court decision in
West Virginia v. EPA, which ruled that Congress did not grant EPA the authority to require "outside the fence" options for limiting carbon dioxide at power plants, the Inflation Reduction Act of 2022 specifically defined carbon dioxide,
hydrofluorocarbons,
methane,
nitrous oxide,
perfluorocarbons, and
sulfur hexafluoride as greenhouse gases to be regulated by the EPA, as well as giving the EPA the ability to regulate the inclusion of renewable sources, notably, through a $27 billion
green bank, among other methods.
Others Other important but less foundational Clean Air Act regulatory programs tend to build on or cut across the above programs: •
Risk Assessment. Although not a regulatory program per se, many EPA regulatory programs involve risk assessment and management. Over the years, EPA has undertaken to unify and organize its many risk assessment processes. The 1990 CAA Amendments created a Commission on Risk Assessment and Management tasked with making recommendations for a risk assessment framework, and many subsequent reports have built on this work. •
Visibility and Regional Haze. EPA monitors visibility and air clarity (haze) at 156 protected parks and wilderness areas, and requires states to develop plans to improve visibility by reducing pollutants that contribute to haze. •
Interstate pollution control. The Clean Air Act's "good neighbor" provision requires states to control emissions that will significantly contribute to NAAQS nonattainment or maintenance in a downwind state. EPA has struggled to enact regulations that implement this requirement for many years. It developed the "Clean Air Interstate Rule" between 2003 and 2005, but this was overturned by the courts in 2008. EPA then developed the
Cross-State Air Pollution Rule between 2009 and 2011, and it continues to be litigated as EPA updates it. •
Startup, Shutdown, & Malfunction. EPA promulgates rules for states to address excess emissions during periods of startup, shutdown, and malfunction, when facility emissions may temporarily be much higher than standard regulatory limits. == Clean Air Act and states ==