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((Short description|1963 United States federal law to control air pollution))
((other uses|Clean Air Act (disambiguation)))
((Use mdy dates|date=April 2019))
((Infobox U.S. legislation
| shorttitle = 
| othershorttitles = 
| longtitle = An Act to improve, strengthen, and accelerate programs for the prevention and abatement of air pollution, as amended.
| colloquialacronym = CAA
| nickname = 
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| title amended = 
| sections created = ((Usc-title-chap|42|85)) (§§ 7401-7671q)
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| amendments = Clean Air Act of 1963 (((usstat|77|392)), ((uspl|88|206)))<br>Motor Vehicle Air Pollution Control Act of 1965 (79 Stat. 992, ((uspl|89|272)))<br>Air Quality Act of 1967 (81 Stat. 485, ((uspl|90|148)))<br>Clean Air Amendments of 1970 (84 Stat. 1676, ((uspl|91|604)))<br>Clean Air Act Amendments of 1977 (91 Stat. 685, ((uspl|95|95)))<br>Clean Air Act Amendments of 1990 (104 Stat. 2468, ((uspl|101|549)))<br>[[Inflation Reduction Act]] (136 Stat. 2063, ((uspl|117|169)))
| SCOTUS cases = ((ubl|''[[Union Electric Co. v. EPA|Union Elec. Co. v. EPA]]'', ((ussc|427|246|1976))|''[[Chevron USA v. Natural Resources Defense Council|Chevron v. NRDC]]'', ((ussc|467|837|1984))|''[[Whitman v. American Trucking Ass'ns, Inc.|Whitman v. Am. Trucking Ass'ns]]'', ((ussc|531|457|2001))|''[[Alaska Department of Environmental Conservation v. EPA]]'', ((ussc|540|461|2004))|''[[Engineer Manufacturers Association v. South Coast Air Quality Management District]]'', ((ussc|541|246|2004))|''[[Department of Transportation v. Public Citizen]]'', ((ussc|541|752|2004))|''[[Massachusetts v. EPA]]'', ((ussc|549|497|2007))|''[[Environmental Defense v. Duke Energy Corp.]]'', ((ussc|549|561|2007))|''[[American Electric Power Co. v. Connecticut]]'', ((ussc|564|410|2011))|''[[EPA v. EME Homer City Generation, L. P.]]'', ((ussc|572|489|2014))|''[[Utility Air Regulatory Group v. Environmental Protection Agency]]'', ((ussc|573|302|2014))|''[[Michigan v. EPA]]'', ((ussc|docket=14-46|volume=576|year=2015))|''[[HollyFrontier Cheyenne Refining, LLC v. Renewable Fuels Assn.]]'', ((ussc|docket=20-472|volume=594|year=2021))|''[[West Virginia v. EPA]]'', ((ussc|docket=20-1530|volume=597|year=2022))|''[[Ohio v. EPA]]'', ((ussc|docket=23A349|volume=601|year=2024))))
))

The '''Clean Air Act''' ('''CAA''') is the United States' primary federal [[air quality law]], intended to reduce and control [[Air pollution in the United States|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 law]]s.

As with many other major [[United States environmental law|U.S. federal environmental statutes]], the Clean Air Act is administered by the [[United States Environmental Protection Agency|U.S. Environmental Protection Agency]] (EPA), in coordination with state, local, and tribal governments.<ref name="EPA Plain English Guide">((cite web |title=The Plain English Guide to the Clean Air Act |url=https://www.epa.gov/sites/default/files/2015-08/documents/peg.pdf |date=April 2007 |publisher=US Environmental Protection Agency (EPA) |location=Washington, DC |page=19 |id=EPA 456/K-07-001))</ref>((rp|2–3)) EPA develops extensive [[rulemaking|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 [[United States Code|codified]] statute at ((usctc|42|85)). That statute is the product of multiple acts of [[United States Congress|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.<ref>((cite book|last=Nelson|first=Garrison|year=2017|title=John William McCormack: A Political Biography|place=New York|publisher=Bloomsbury|page=663|isbn=978-1628925166))</ref><ref>((cite book|last=Nelson|first=Garrison|year=2017|title=John William McCormack: A Political Biography|place=New York|publisher=Bloomsbury|page=756|isbn=978-1628925166))</ref> 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).<ref>((cite web |title=Clean Air Act Text |date=2021-03-24 |url=https://www.epa.gov/clean-air-act-overview/clean-air-act-text |publisher=EPA))</ref> 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 [[Title 40 of the Code of Federal Regulations|40 C.F.R.]] Subchapter C, Parts 50–98.<ref>40 C.F.R. ((cite web |title=Subchapter C (Air Programs)|url=https://www.law.cornell.edu/cfr/text/40/chapter-I/subchapter-C))</ref> These Parts more often correspond to the Clean Air Act's major regulatory programs.

Today, 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((sub|3))), [[carbon monoxide]] (CO), [[particulate matter]] (PM((sub|10)), PM((sub|2.5))), [[lead]] (Pb), [[sulfur dioxide]] (SO((sub|2))), and [[nitrogen dioxide]] (NO((sub|2))) are allowed in the outdoor air.<ref>''NAAQS Program'':
* First enacted: 1970 CAA amendments, ((uspl|91|604)) § 4.
* Location in statute: CAA § 109, ((USC|42|7409)).
* Major regulations: ((CodeFedReg|40|50)).
* EPA pages: ((cite web |title=Criteria Air Pollutants |url=https://www.epa.gov/criteria-air-pollutants |date=April 9, 2014 )) ((cite web |title=NAAQS |date=September 9, 2016 |url=https://www.epa.gov/naaqs))</ref> 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 (((Federal Register|36|22384))). Initially, EPA did not list [[lead]] as a criteria 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 (((Federal Register|43|46258))). 

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.<ref>See ((cite web |title=EPA Clean Air Scientific Advisory Committee |url=https://yosemite.epa.gov/sab/sabpeople.nsf/WebCommittees/CASAC))</ref> EPA added regulations for PM2.5 in 1997 (((Federal Register|62|38652))), 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.<ref>''NESHAPS Program'':
* First enacted: 1970 CAA amendments, ((uspl|91|604)) § 4.
* Location in statute: CAA § 112, ((USC|42|7412)).
* Major regulations: ((CodeFedReg|40|61)), ((CodeFedReg|40|63)).
* EPA pages: ((cite web |title=Hazardous Air Pollutants |url=https://www.epa.gov/haps |date=December 2, 2015 )) ((cite web | title=EPA HAPs List |date=December 16, 2015 |url=https://www.epa.gov/haps/initial-list-hazardous-air-pollutants-modifications)) ((cite web |title=EPA NESHAP Source Categories|date=May 6, 2015|url=https://www.epa.gov/stationary-sources-air-pollution))</ref> 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 (((uspl|101|549)) 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.<ref>((cite web |title=HAPs Mobile Source Rule |date=August 3, 2016 |url=https://www.epa.gov/regulations-emissions-vehicles-and-engines/final-rule-control-emissions-hazardous-air-pollutants-0 |publisher=EPA))</ref> 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.<ref>((cite web |title=NESHAPs Risk and Technology Review |date=December 15, 2016 |url=https://www.epa.gov/stationary-sources-air-pollution/risk-and-technology-review-national-emissions-standards-hazardous |publisher=EPA))</ref>

===New Source Performance Standards ===
The [[New Source Performance Standards#Clean Air Act|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".<ref>''NSPS Program'':
* First enacted: 1970 CAA amendments, ((uspl|91|604)) § 4.
* Location in statute: CAA § 111, ((USC|42|7411)).
* Major regulations: ((CodeFedReg|40|60)).
* EPA pages: ((cite web |title=NSPS Regulations |date= November 25, 2016|url=https://www.epa.gov/stationary-sources-air-pollution/new-source-performance-standards))</ref> 

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." EPA issued its first NSPS regulation the next year, covering steam generators, incinerators, Portland cement plants, and nitric and sulfuric acid plants (((Federal Register|36|24876|))). 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 (((CodeFedReg|40|52)), subpart I) to determine whether maintenance and other activities rises to the level of modification requiring application of NSPS.<ref>((Cite web |url=https://www.epa.gov/nsr/nsr-regulatory-actions |title=NSR Regulatory Actions |date=December 9, 2015 |publisher=EPA))</ref>

===Acid Rain Program===
The [[Acid Rain Program]] (ARP) is an [[emissions trading]] program for [[power plants]] to control the pollutants that cause [[acid rain]].<ref>''Acid Rain Program'':
* First enacted: 1990 CAA amendments, ((uspl|101|549)) § 401.
* Location in statute: CAA Title IV-A, ((usctc|42|85|IV-A)).
* Major regulations: ((CodeFedReg|40|72)) through ((CodeFedReg|40|78)).
* EPA pages: ((cite web |title=Clean Air Markets |date=August 12, 2014 |url=https://www.epa.gov/airmarkets)) ((cite web |title=Acid Rain Program |date=August 21, 2014|url=https://www.epa.gov/acidrain/acid-rain-program)) ((cite web |title=Acid Rain |date=December 2, 2015|url=https://www.epa.gov/acidrain))</ref> The 1990 CAA Amendments created a new title to address the issue of acid rain, and particularly [[nitrogen oxides]] (NO((sub|x))) and [[sulfur dioxide]] (SO((sub|2))) emissions from electric power plants powered by fossil fuels, and other industrial sources. The Acid Rain Program was the first emissions trading program in the United States, setting a cap on total emissions that was reduced over time by way of traded emissions credits, rather than direct controls on emissions. The program evolved in two stages: the first stage required more than 100 electric generating facilities larger than 100 megawatts to meet a 3.5 million ton SO((sub|2)) emission reduction by January 1995. The second stage gave facilities larger than 75 megawatts a January 2000 deadline. The program has achieved all of its statutory goals.<ref>((cite web |title=ARP Progress Reports |url=https://www3.epa.gov/airmarkets/progress/reports/index.html |publisher=EPA))</ref>

===Ozone layer protection===
((main|Section 608))
((see also|Significant New Alternatives Policy))
The CAA ozone program is a technology transition program intended to phase out the use of chemicals that harm the [[ozone layer]].<ref>''Ozone Layer Protection Program'':
* First enacted: 1990 CAA amendments, ((uspl|101|549)) § 601.
* Location in statute: CAA Title VI, ((usctc|42|85|VI)).
* Major regulations: ((CodeFedReg|40|72)) through ((CodeFedReg|40|82)).
* EPA pages: ((cite web |title=Ozone Layer Protection |date=February 15, 2013 |url=https://www.epa.gov/ozone-layer-protection)) ((cite web |title=Ozone Layer Protection under the CAA |date=July 14, 2015|url=https://www.epa.gov/ozone-layer-protection/ozone-protection-under-title-vi-clean-air-act))</ref> 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. 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, [[Significant New Alternatives Policy|identifying new alternatives to those still in use]], and [[Section 608 EPA Certification|licensing]] technicians to use such chemicals.

===Mobile source programs===
Rules for pollutants emitted from internal combustion engines in vehicles.<ref>
''Mobile Source Programs''
* First enacted: Motor Vehicle Air Pollution Control Act of 1965, ((uspl|89|272)) § 101. 
*Location in statute: CAA Title II, ((usctc|42|85|II)).
* Major regulations: ((CodeFedReg|40|85)), ((CodeFedReg|40|86)), ((CodeFedReg|40|87)), ((CodeFedReg|40|89)), ((CodeFedReg|40|1039)), ((CodeFedReg|40|1048)), ((CodeFedReg|40|1054))), ((CodeFedReg|40|1051))), ((CodeFedReg|40|1068)).
* EPA pages: ((cite web |title=Regulations for Emissions from Vehicles and Engines |date=April 15, 2016 |url=https://www.epa.gov/regulations-emissions-vehicles-and-engines)) ((cite web |title=Regulations for Onroad Vehicles and Engines |date=August 10, 2016|url=https://www.epa.gov/regulations-emissions-vehicles-and-engines/regulations-onroad-vehicles-and-engines)) ((cite web |title=Regulations for Smog, Soot, and Other Air Pollution from Passenger Cars & Trucks |date=September 9, 2016|url=https://www.epa.gov/regulations-emissions-vehicles-and-engines/regulations-smog-soot-and-other-air-pollution-passenger)) ((cite web |title=Regulations for Smog, Soot, and Other Air Pollution from Commercial Trucks & Buses |date=September 12, 2016 |url=https://www.epa.gov/regulations-emissions-vehicles-and-engines/regulations-smog-soot-and-other-air-pollution-commercial)) ((cite web |title=Regulations for Smog, Soot, and Other Air Pollution from Non-Electric Motorcycles |date=September 22, 2016 |url=https://www.epa.gov/regulations-emissions-vehicles-and-engines/regulations-smog-soot-and-other-air-pollution-non)) ((cite web |title=Regulations for Nitrogen Oxide Emissions from Aircraft |date=September 28, 2016 |url=https://www.epa.gov/regulations-emissions-vehicles-and-engines/regulations-nitrogen-oxide-emissions-aircraft)) ((cite web |title=Regulations for Lead Emissions from Aircraft |date=September 9, 2016 |url=https://www.epa.gov/regulations-emissions-vehicles-and-engines/regulations-lead-emissions-aircraft)) ((cite web |title=Regulations for Emissions from Locomotives |date=September 13, 2016 |url=https://www.epa.gov/regulations-emissions-vehicles-and-engines/regulations-emissions-locomotives)) ((cite web |title=Voluntary Programs to Reduce Mobile Source Pollution |date=June 16, 2016 |url=https://www.epa.gov/mobile-source-pollution/voluntary-programs-reduce-mobile-source-pollution))</ref> Since 1965, Congress has mandated increasingly stringent controls on vehicle engine technology and reductions in tailpipe emissions. 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 (((USC|42|7521))).

====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 (((Federal Register|42|1122))) and updated them in 2004 (((Federal Register|69|2397))).

;Vehicle testing program
The air pollution testing system for motor vehicles was originally developed in 1972 and used [[driving cycle]]s 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. ((citation needed|date=March 2017)) In 1984, EPA began adjusting city (aka [[UDDS|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<ref>((cite web|url=http://www.epa.gov/OMS/sftp.htm|title=Federal Test Procedure Revisions |date=October 22, 1996 |publisher=EPA |access-date=October 3, 2009|archive-url=https://web.archive.org/web/20090914065629/http://www.epa.gov/OMS/sftp.htm|archive-date=September 14, 2009|url-status=live))</ref> 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.<ref name="epafe">((cite web |url=http://www.epa.gov/fueleconomy/420f06069.htm |title=Regulatory Announcement: EPA Issues New Test Methods for Fuel Economy Window Stickers |date=December 2006 |publisher=EPA |archive-url=https://web.archive.org/web/20080907013023/http://www.epa.gov/fueleconomy/420f06069.htm |archive-date=September 7, 2008|url-status=dead |access-date=July 16, 2011 |id= EPA 420-F-06-069. Fact sheet.))</ref>

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.<ref>((cite web|url=http://www.fueleconomy.gov/mpg/MPG.do |title=Welcome to Your MPG! |publisher=US Department of Energy and EPA |access-date=October 21, 2012 |archive-url=https://web.archive.org/web/20120913080618/https://www.fueleconomy.gov/mpg/MPG.do |archive-date=September 13, 2012 |url-status=live))</ref>

EPA conducts fuel economy tests on very few vehicles.<ref>((cite magazine |url=http://www.caranddriver.com/features/09q3/the_truth_about_epa_city_highway_mpg_estimates-feature |title=The Truth About EPA City / Highway MPG Estimates |author=Dave Vanderwerp |quote=Just 18 of the EPA's 17,000 employees work in the automobile-testing department in Ann Arbor, Michigan, examining 200 to 250 vehicles a year, or roughly 15 percent of new models. As to that other 85 percent, the EPA takes automakers at their word—without any testing-accepting submitted results as accurate. |date=August 2009 |magazine=Car and Driver |archive-url=https://web.archive.org/web/20100821232506/http://www.caranddriver.com/features/09q3/the_truth_about_epa_city_highway_mpg_estimates-feature|archive-date=August 21, 2010|url-status=dead|access-date=August 19, 2010))</ref> 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 cycle]]s have been used for estimating how many miles an electric vehicle will get on a single charge.<ref>((cite magazine |url=https://www.wired.com/autopia/2010/11/honda-finds-evs-a-perfect-fit |title=Honda Finds EVs a Perfect Fit &#124; Autopia |author=Chuck Squatriglia |date=November 17, 2010|access-date=March 12, 2017 |magazine=Wired |archive-url=https://web.archive.org/web/20140307180553/http://www.wired.com/autopia/2010/11/honda-finds-evs-a-perfect-fit/ |archive-date=March 7, 2014 |url-status=live))</ref>

====Non-road vehicles regulations====
The 1970 CAA amendments provided for regulation of aircraft emissions (((UnitedStatesCode|42|7571))), 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 (((uspl|101|549)) § 222) added rules for a "nonroad" engine program (((USC|42|7547))), 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,<ref>((cite web |title=National Clean Diesel Campaign; Fact Sheet |url=https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1001YU6.txt |date=November 2005 |publisher=EPA |id=EPA 420-F-05-012))</ref> Ports Initiative,<ref>((cite web |title=Ports Initiative |url=https://www.epa.gov/ports-initiative |date=2023-11-08 |publisher=EPA))</ref> SmartWay program (for the freight transportation sector),<ref>((cite web |title=SmartWay |url=https://www.epa.gov/smartway |date=2023-12-20 |publisher=EPA))</ref> 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.<ref>''Fuel Controls Programs'':
* First enacted: Air Quality Act of 1967, ((uspl|90|148)) § 210.
* Location in statute: CAA § 211, ((usc|42|7545)).
* Major regulations: ((CodeFedReg|40|1090)).</ref> One of EPA's earliest actions was the elimination of lead in U.S. gasoline beginning in 1971,<ref>((FederalRegister|36|1486)) (1971-01-30), ((FederalRegister|37|3882)) (1972-02-23), ((FederalRegister|38|33734)) (1973-12-06)</ref> a project that has been described as "one of the great public health achievements of the 20th century."<ref>((cite journal |last1=Bridbord |first1=Kenneth |last2=Hanson |first2=David |title=A Personal Perspective on the Initial Federal Health-Based Regulation to Remove Lead from Gasoline |journal=Environmental Health Perspectives |date=Aug 2009 |volume=117 |issue=8 |pages=1195–1201 |doi=10.1289/ehp.0800534 |pmid=19672397 |pmc=2721861 |doi-access=free))</ref> EPA continues to regulate the chemical composition of [[gasoline]], [[avgas]], and [[diesel fuel]] in the United States.<ref>EPA. ((cite web |title=Final Rulemaking: Streamlining and Consolidating of Existing Gasoline and Diesel Fuel Programs |date=October 7, 2020 |url=https://www.epa.gov/diesel-fuel-standards/final-rulemaking-streamlining-and-consolidating-existing-gasoline-and-diesel)) ((cite web |title=Gasoline Standards |date=2023-12-08 |url=https://www.epa.gov/gasoline-standards)) ((cite web |title=Diesel Fuel Standards |date=2023-12-08 |url=https://www.epa.gov/diesel-fuel-standards)) ((cite web |title=Renewable Fuel Standard Program | date=2023-11-09 |url=https://www.epa.gov/renewable-fuel-standard-program))</ref>

===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.<ref name="EPA Plain English Guide" />((rp|19)) In non-attainment areas, permits were required for sources that emit as little as 50, 25, or 10 tons per year of VOCs depending on the severity of the region's non-attainment status.<ref name="Clean Air Act Summary">((cite web |last=McCarthy |first=James |title=Clean Air Act: A Summary of the Act and its Major Requirements |url=https://crsreports.congress.gov/product/pdf/RL/RL30853 |publisher=Congressional Research Service |location=Washington, DC |date=February 25, 2020 |id=RL30853))</ref> Most permits are issued by state and local agencies.<ref>EPA (February 1998). [http://www.epa.gov/air/oaqps/permits/permitupdate/permits.pdf "Air Pollution Operating Permit Program Update: Key Features and Benefits."] Document no. EPA/451/K-98/002. p. 1.</ref> 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.<ref name="Clean Air Act Summary" />

===Monitoring and enforcement===
One of the most public aspects of the Clean Air Act, EPA is empowered to monitor compliance with the law's many requirements, seek penalties for violations, and compel regulated entities to come into compliance.<ref>Monitoring and Enforcement EPA pages: ((cite web |title=CAA Compliance Monitoring |date=July 17, 2013 |url=https://www.epa.gov/compliance/clean-air-act-caa-compliance-monitoring |publisher=EPA)), ((cite web |title=Air Enforcement |date=May 3, 2013|url=https://www.epa.gov/enforcement/air-enforcement)), ((cite web |title=Air Enforcement Policy, Guidance and Publications|date=May 3, 2013|url=https://www.epa.gov/enforcement/air-enforcement-policy-guidance-and-publications)), ((cite web |title=Enforcement Data and Results|date=May 3, 2013|url=https://www.epa.gov/enforcement/enforcement-data-and-results))</ref> Enforcement cases are usually settled, with penalties assessed well below maximum statutory limits.((Citation needed|date=November 2021)) Recently, 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===
((main|Regulation of greenhouse gases under the Clean Air Act))
Much of EPA's regulation of [[Greenhouse gas emissions by the United States|greenhouse gas (GHG) emissions]] occurs under the programs discussed above. EPA began regulating GHG emissions following the 2007 [[United States Supreme Court|Supreme Court]] ruling in ''[[Massachusetts v. EPA]]'', the EPA's subsequent endangerment finding, and development of specific regulations for various sources.<ref>((Cite web |url=https://www.epa.gov/sites/production/files/2015-12/documents/20101223factsheet.pdf |title=Fact Sheet: Clean Air Act Permitting for Greenhouse Gas Emissions–Final Rules |publisher=EPA |date=2010-12-23))</ref> 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.<ref>United States. Inflation Reduction Act of 2022. ((USPL|117|169)) Approved August 16, 2022.</ref><ref name="nytimes.com">((cite news |first1=Lisa |last1=Friedman |title=Democrats Designed the Climate Law to Be a Game Changer. Here's How. |url=https://www.nytimes.com/2022/08/22/climate/epa-supreme-court-pollution.html |newspaper=The New York Times |date=2022-08-22))</ref>

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.((Update inline|reason=Proposed rule published in 2021|date=October 2021))<ref name=":3" /> In addition, EPA oversees the national greenhouse gas inventory reporting program.<ref>((cite web |title=Greenhouse Gas Reporting Program |url=https://www.epa.gov/ghgreporting |date=2023-12-08 |publisher=EPA))</ref>

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.<ref name="nytimes.com" /><ref name="Cherny 2022" /><ref name="EESI 2022">((cite web |last=Yañez-Barnuevo |first=Miguel |title=New Climate Law Jumpstarts Clean Energy Financing |publisher=Environmental and Energy Study Institute |location=Washington, DC|date=12 September 2022 |url=https://www.eesi.org/articles/view/new-climate-law-jumpstarts-clean-energy-financing))</ref>

===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.<ref>((cite web |title=Risk Assessment |url=https://www.epa.gov/risk |date=2021-08-24 |publisher=EPA))<br>((cite web |title=History of Risk at EPA |url=https://www.epa.gov/risk/about-risk-assessment#tab-2 |website=Risk Assessment |date=2021-08-24 |publisher=EPA))</ref> 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,<ref>((uspl|101|549)) § 303. Not codified.</ref> 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.<ref>''Regional Haze Program'': 
*First enacted: 1977 Amendments, ((uspl|95|95)) § 128, 1990 Amendments, ((uspl|101|549)) § 816.
*Location in statute: ((usc|42|7491)), ((usc|42|7492)).
*Major regulations: ((CodeFedReg|40|51)), Subpart P.
*EPA Pages: ((cite web |title=Visibility and Regional Haze |date=May 4, 2015|url=https://www.epa.gov/visibility)).</ref>
*'''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.<ref>• ((cite web |title=Interstate Air Pollution Transport |url=https://www.epa.gov/interstate-air-pollution-transport/interstate-air-pollution-transport |date=2021-02-23 |publisher=EPA)) <br>• ((cite web |first1=Kate C. |last1=Shouse |title=The Clean Air Act's Good Neighbor Provision: Overview of Interstate Air Pollution Control |url=https://fas.org/sgp/crs/misc/R45299.pdf |date=2018-08-30 |publisher=Congressional Research Service |id=R45299)) <br>• ((cite web |title=Clean Air Interstate Rule (Archive) |url=https://archive.epa.gov/airmarkets/programs/cair/web/html/index.html |date=2016-02-21 |publisher=EPA))</ref><ref>((usc|42|7410))(a)(2)(D)(i)(I)/</ref> 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.<ref>((cite web |title=Emissions During Periods of Startup, Shutdown, & Malfunction (SSM) |url=https://www.epa.gov/air-quality-implementation-plans/emissions-during-periods-startup-shutdown-malfunction-ssm |date=2021-09-30 |publisher=EPA |website=Air Quality Implementation Plans |id=Guidance Memorandum: Withdrawal of the October 9, 2020, Memorandum |quote=EPA is returning to its 2015 policy explaining that State Implementation Plan provisions that provide exemptions from air emissions limits during periods of startup, shutdown and malfunction (SSM) or that provide affirmative defense provisions are not consistent with the Clean Air Act and would not generally be approvable.))</ref>

== The Clean Air Act and states ==

===State implementation plans===
The 1963 act required development of [[State Implementation Plan]]s (SIPs) as part of a cooperative federalist program for developing pollution control standards and programs.<ref>((CodeFedReg|40|51)), ((CodeFedReg|40|52))</ref> Rather than create a solely national program, the CAA imposes responsibilities on the U.S. states to create plans to implement the Act's requirements. EPA then reviews, amends, and approves those plans. EPA first promulgated SIP regulations in 1971 and 1972.<ref>((USFR|36|15486)), ((USFR|37|19807)))</ref>

The 1970 Amendments imposed eight criteria that an implementation plan must meet.<ref>§110 (a)(2)</ref> The EPA is required to approve plans that adhere to the Senate's three-year mandate for primary air quality standards even if the Agency feels the plan does not appear feasible. In ''Union Electric Co. v. Environmental Protection Agency'' the Supreme Court considered whether the Agency was required to reject plans that were not technologically or economically feasible. The court decided that states could adopt emission standards stricter than national standards and "force" technology (i.e. require installation of more advanced technologies).<ref>https://www.govinfo.gov/content/pkg/STATUTE-84/pdf/STATUTE-84-Pg1676.pdf ((bare URL PDF|date=February 2024))</ref><ref>((ussc|name=Union Electric Co. v. Environmental Protection Agency |volume=427|page=246|date=1976))</ref>

====Non-attainment areas====
The 1977 CAA Amendments added SIP requirements for areas that had not attained the applicable NAAQS ("nonattainment areas"). In these areas, states were required to adopt plans that made "reasonable further progress" toward attainment until all "reasonably available control measures" could be adopted. As progress on attainment was much slower than Congress originally instructed, major amendments to SIP requirements in nonattainment areas were part of the 1990 CAA Amendments.<ref>((cite web |last1=Garrett & Winner |title=Chapter 4. Nonattainment |url=https://elr.info/sites/default/files/articles/22.10173.htm |website=ELR))</ref>

====Prevention of significant deterioration====
The 1977 CAA Amendments modified the SIP requirements by adding "Prevention of Significant Deterioration" (PSD) requirements. These requirements protect areas, including particularly wilderness areas and national parks, that already met the NAAQS. The PSD provision requires SIPs to preserve good quality air in addition to cleaning up bad air. The new law also required New Source Review (investigations of proposed construction of new polluting facilities) to examine whether PSD requirements would be met.<ref>Section 127, adding CAA Title I Part C, codified at ((usc-title-chap|42|85|I)). ((CodeFedReg|40|52|21))</ref>

===Federalism===
The Constitution contains no provisions listing environmental standards as an enumerated Federal power, and until 1970 these were essentially handled at the state and local level. However, legislators of the 1960s had been heavily influenced by [[New Deal]]-era ideologies of government, allowing considerable expansion of Federal authority, often in excess of what was strictly allowed in the Constitution. The Clean Air Act provided the EPA with enforcement authority and requiring states to develop State Implementation Plans for how they would meet new national ambient air quality standards by 1977.<ref>"Early Implementation of the Clean Air Act of 1970 in California." EPA Alumni Association. [http://www.epaalumni.org/history/video/interview.cfm?id=38 Video], [https://www.epaalumni.org/userdata/pdf/2B9E3C6816EC9466.pdf#page=6 Transcript] (see p. 6). July 12, 2016.</ref> This cooperative federal model continues today. The law recognizes that states should lead in carrying out the Clean Air Act, because [[pollution control]] problems often require special understanding of local conditions such as geography, industrial activity, transportation and housing patterns. However, states are not allowed to have weaker controls than the national minimum criteria set by EPA. EPA must approve each SIP, and if a SIP is not acceptable, EPA can retain CAA enforcement in that state. For example, California was unable to meet the new standards set by the 1970 amendments, which led to a lawsuit and a federal state implementation plan for the state.<ref>"Early Implementation of the Clean Air Act of 1970 in California." EPA Alumni Association. [http://www.epaalumni.org/history/video/interview.cfm?id=38 Video], [https://www.epaalumni.org/userdata/pdf/2B9E3C6816EC9466.pdf#page=1 Transcript]. July 12, 2016.</ref> The federal government also assists the states by providing scientific research, expert studies, engineering designs, and money to support clean air programs.

The law also prevents states from setting standards that are more strict than the federal standards, but carves out a special exemption for California due to its past issues with smog pollution in the metropolitan areas. In practice, when [[California]]'s environmental agencies decide on new vehicle emission standards, they are submitted to the EPA for approval under this waiver, with the most recent approval in 2009.<ref>((cite web |date=11 February 2019 |title=Shifting Gears: The Federal Government's Reversal on California's Clean Air Act Waiver |url=https://www.acslaw.org/issue_brief/shifting-gears-the-federal-governments-reversal-on-californias-clean-air-act-waiver/ |archive-url=https://web.archive.org/web/20190223001139/https://www.acslaw.org/wp-content/uploads/2019/02/CA-Car-Standards-IB-2019.pdf |archive-date=23 February 2019 |access-date=19 September 2019 |publisher=American Constitution Society))</ref> The California standard was adopted by twelve other states, and established the ''de facto'' standard that automobile manufacturers subsequently accepted, to avoid having to develop different emission systems in their vehicles for different states. However, in September 2019, President [[Donald Trump]] attempted to revoke this waiver, arguing that the stricter emissions have made cars too expensive, and by removing them, will make vehicles safer. EPA's [[Andrew R. Wheeler|Andrew Wheeler]] also stated that while the agency respects federalism, it could not allow one state to dictate standards for the entire nation. California's governor [[Gavin Newsom]] considered the move part of Trump's "political vendetta" against California and stated his intent to sue the federal government.<ref>((cite web |last=Liptak |first=Kevin |date=September 18, 2019 |title=Trump revokes waiver for California to set higher auto emissions standards |url=https://www.cnn.com/2019/09/18/politics/epa-trump-california/index.html |access-date=September 18, 2019 |publisher=CNN))</ref> Twenty-three states, along with the District of Columbia and the cities of New York City and Los Angeles, joined California in a federal lawsuit challenging the administration's decision.<ref>((cite web |last=Davenport |first=Coral |date=September 20, 2019 |title=California Sues the Trump Administration in Its Escalating War Over Auto Emissions |url=https://www.nytimes.com/2019/09/20/climate/california-auto-emissions-lawsuit.html |access-date=September 20, 2019 |work=The New York Times))</ref> In March 2022 the [[Biden administration]] reversed the Trump-era rule, allowing California to again set stricter auto emissions standards.<ref>((cite news |last=Davenport |first=Coral |date=2022-03-09 |title=Biden Restores California's Power to Set Stringent Tailpipe Rules |newspaper=The New York Times |url=https://www.nytimes.com/2022/03/09/climate/biden-california-tailpipe-waiver.html))</ref>

==History==
Between the [[Second Industrial Revolution]] and the 1960s, the United States experienced increasingly severe [[air pollution]]. Following the [[1948 Donora smog]] event, the public began to discuss air pollution as a major problem, states began to pass a series of laws to reduce air pollution, and Congress began discussing whether to take further action in response. At the time, the primary federal agencies interested in air pollution were the [[United States Bureau of Mines]], which was interested in "smoke abatement" (reducing smoke from coal burning), and the [[United States Public Health Service]], which handled [[industrial hygiene]] and was concerned with the causes of lung health problems.<ref name="Orford">((Cite journal |last1=Orford |first1=Adam |title = The Clean Air Act of 1963: Postwar Environmental Politics and the Debate Over Federal Power| journal=Hastings Environmental Law Journal | date=2021 | volume=27 | issue=2 |pages=1–77 |url = https://repository.uchastings.edu/cgi/viewcontent.cgi?article=1598&context=hastings_environmental_law_journal|access-date = April 26, 2021|issn = 1080-0735))</ref>

After several years of proposals and hearings, Congress passed the first federal legislation to address air pollution in 1955. The [[Air Pollution Control Act of 1955]] authorized a research and training program, sending $3 million per year to the U.S. Public Health Service for five years, but did not directly regulate pollution sources. The 1955 Act's research program was extended in 1959, 1960, and 1962 while Congress considered whether to regulate further.

Beginning in 1963, Congress began expanding federal air pollution control law to accelerate the elimination of air pollution throughout the country. The new law's programs were initially administered by the U.S. [[Secretary of Health, Education, and Welfare]], and the Air Pollution Office of the [[U.S. Public Health Service]], until they were transferred to the newly created EPA immediately before major amendments in 1970. EPA has administered the Clean Air Act ever since, and Congress added major regulatory programs in 1977 and 1990.<ref>((cite book |last1=Bailey |first1=Christopher J. |title=Congress and Air Pollution |date=1998 |publisher=Manchester University Press))</ref> Most recently, the U.S. Supreme Court's ruling in ''[[Massachusetts v. EPA]]'' resulted in an expansion of EPA's CAA regulatory activities to cover [[greenhouse gases]].

===Clean Air Act of 1963 and early amendments===
The Clean Air Act of 1963 (((USPL|88|206))) was the first federal legislation to permit the U.S. federal government to take direct action to control air pollution. It extended the 1955 research program, encouraged cooperative state, local, and federal action to reduce air pollution, appropriated $95 million over three years to support the development of state pollution control programs, and authorized the HEW Secretary to organize conferences and take direct action against interstate air pollution where state action was deemed to be insufficient.<ref name="Orford" />

The Motor Vehicle Air Pollution Control Act (((USPL|89|272))) amended the 1963 Clean Air Act and set the first federal [[Automobile emissions control|vehicle emissions]] standards, beginning with the 1968 models. These standards were reductions from 1963 emissions levels: 72% reduction for [[hydrocarbon]]s, 56% reduction for [[carbon monoxide]], and 100% reduction for crankcase hydrocarbons.((Citation needed|date=April 2021)). The law also added a new section to authorize abatement of international air pollution.<ref name="Stern">((cite journal |last1=Stern |first1=Arthur C. |title=History of Air Pollution Legislation in the United States |journal=Journal of the Air Pollution Control Association |date=1982 |volume=32 |issue=1 |pages=44–61 |doi=10.1080/00022470.1982.10465369 |pmid=7033323 |doi-access=free ))</ref>

[[File:Clean Air Act Signing.jpg|thumb|President [[Lyndon B. Johnson]] signs the 1967 Air Quality Act in the East Room of [[the White House]], November 21, 1967.]]
The Air Quality Act of 1967 (((uspl|90|148))) authorized planning grants to state air pollution control agencies, permitted the creation of interstate air pollution control agencies, and required HEW to define air quality regions and develop technical documentation that would allow states to set ambient air quality and pollution control technology standards, and required states to submit implementation plans for improvement of air quality, and permitted HEW to take direct abatement action in air pollution emergencies. It also authorized expanded studies of air pollutant emission inventories, ambient monitoring techniques, and control techniques.<ref>((cite journal | title = The Air Quality Act of 1967 | year = 1968 | journal = Journal of the Air Pollution Control Association | volume = 18 | issue = 2 | pages = 62–71 | doi = 10.1080/00022470.1968.10469096 | pmid = 5637413 | doi-access = free ))</ref><ref name="Stern" /> This enabled the federal government to increase its activities to investigate enforcing interstate air pollution transport, and, for the first time, to perform far-reaching ambient monitoring studies and stationary source inspections. The 1967 act also authorized expanded studies of air pollutant emission inventories, ambient monitoring techniques, and control techniques.<ref name="CAA-history">((USGovernment|url=http://www.epa.gov/air/caa/amendments.html|title=History of the Clean Air Act|agency=U.S. Environmental Protection Agency| date =August 8, 2013| access-date =August 23, 2014))</ref> While only six states had air pollution programs in 1960, all 50 states had air pollution programs by 1970 due to the federal funding and legislation of the 1960s.<ref name=":3">John Bachmann, David Calkins, Margo Oge. [http://www.epaalumni.org/hcp/air.pdf "Cleaning the Air We Breathe: A Half Century of Progress.]" EPA Alumni Association. September 2017.</ref>

[[File:Richard Nixon signs Clean Air Act of 1970 12-31-1970.jpg|thumb|President [[Richard Nixon]] signs the Clean Air Amendments of 1970 at the White House, December 31, 1970.]]

===1970 and 1977 amendments===

In the Clean Air Amendments of 1970 (((uspl|91|604))), Congress greatly expanded the federal mandate by requiring comprehensive federal and state regulations for both industrial and mobile sources. The law established the [[National Ambient Air Quality Standards]] (NAAQS), [[New Source Performance Standard]]s (NSPS); and [[National Emissions Standards for Hazardous Air Pollutants]] (NESHAPs), and significantly strengthened federal enforcement authority, all toward achieving aggressive air pollution reduction goals.

To implement the strict amendments, EPA Administrator [[William Ruckelshaus]] spent 60% of his time during his first term on the automobile industry, whose emissions were to be reduced 90% under the new law. Senators had been frustrated at the industry's failure to cut emissions under previous, weaker air laws.<ref>EPA Alumni Association: William Ruckelshaus in a 2013 interview discusses his first-term efforts at implementing the Clean Air Act of 1970, [http://www.epaalumni.org/history/video/interview.cfm?id=16 Video], [http://www.epaalumni.org/userdata/pdf/605EA9C8EF2DD115.pdf#page=14 Transcript] (see p14).</ref>

Major amendments were added to the ''Clean Air Act'' in 1977 (1977 CAAA) (91 Stat. 685, ((uspl|95|95))). The 1977 Amendments primarily concerned provisions for the Prevention of Significant Deterioration (PSD) of air quality in areas attaining the NAAQS. The 1977 CAAA also contained requirements pertaining to sources in non-attainment areas for NAAQS. A non-attainment area is a geographic area that does not meet one or more of the federal air quality standards. Both of these 1977 CAAA established major permit review requirements to ensure attainment and maintenance of the NAAQS.<ref name="CAA-history" /> These amendments also included the adoption of an offset trading policy originally applied to Los Angeles in 1974 that enables new sources to offset their emissions by purchasing extra reductions from existing sources.<ref name=":3" />

The Clean Air Act Amendments of 1977 required Prevention of Significant Deterioration (PSD) of air quality for areas attaining the NAAQS and added requirements for non-attainment areas.<ref>Clean Air Act Amendments of 1977, [http://www.epw.senate.gov/envlaws/cleanair.pdf P.L. 95-95], ((usstat|91|685)), August 7, 1977.</ref>

=== 1990 amendments===
[[File:President George H. W. Bush signs the Clean Air Act Amendments.jpg|thumb|President [[George H. W. Bush]] signs the Clean Air Act Amendments of 1990 at the White House, November 15, 1990.]]
Another set of major amendments to the Clean Air Act occurred in 1990 (1990 CAAA) (104 Stat. 2468, ((uspl|101|549))). The 1990 CAAA substantially increased the authority and responsibility of the federal government. New regulatory programs were authorized for control of acid deposition (acid rain)<ref>Former Deputy Administrator Hank Habicht talks about management at EPA. An Interview with Hank Habicht [http://www.epaalumni.org/history/video/interview.cfm?id=17 Video], [https://www.epaalumni.org/userdata/pdf/60772C611F145A4D.pdf#page=6 Transcript] (see p6). December 21, 2012.</ref> and for the issuance of stationary source operating permits. The NESHAPs were incorporated into a greatly expanded program for controlling toxic air pollutants. The provisions for attainment and maintenance of NAAQS were substantially modified and expanded. Other revisions included provisions regarding stratospheric ozone protection, increased enforcement authority, and expanded research programs.<ref name="CAA-history" />

The 1990 Clean Air Act added regulatory programs for control of acid deposition (acid rain) and stationary source operating permits. The provisions aimed at reducing sulfur dioxide emissions included a cap-and-trade program, which gave power companies more flexibility in meeting the law's goals compared to earlier iterations of the Clean Air Act. The amendments moved considerably beyond the original criteria pollutants, expanding the NESHAP program with a list of 189 hazardous air pollutants to be controlled within hundreds of source categories, according to a specific schedule.<ref name="EPA Plain English Guide" />((rp|16)) The NAAQS program was also expanded. Other new provisions covered stratospheric ozone protection, increased enforcement authority and expanded research programs.<ref>Clean Air Act Amendments of 1990, [http://www.epw.senate.gov/envlaws/cleanair.pdf P.L. 101-549], ((usstat|104|2399)), November 15, 1990.</ref>

Further amendments were made in 1990 to address the problems of [[acid rain]], [[ozone depletion]], and toxic air pollution, and to establish a national permit program for stationary sources, and increased enforcement authority. The amendments also established new auto gasoline reformulation requirements, set [[Reid vapor pressure]] (RVP) standards to [[Evaporative emission#Evaporative emissions control|control evaporative emissions]] from gasoline, and mandated new gasoline formulations sold from May to September in many states. Reviewing his tenure as EPA Administrator under President [[George H. W. Bush]], [[William K. Reilly]] characterized passage of the 1990 amendments to the Clean Air Act as his most notable accomplishment.<ref>EPA Alumni Association: EPA Administrator William K. Reilly describes why passage of the 1990 Clean Air Act amendments was vitally important. Reflections on US Environmental Policy: An Interview with William K. Reilly [http://www.epaalumni.org/history/video/interview.cfm?id=37 Video], [http://www.epaalumni.org/userdata/pdf/3E0FC143699CD31B.pdf#page=3 Transcript] (see p10).</ref>

The 1990 amendments also included a requirement for the Department of Labor to issue, no later than 12 months from the date of publication of the amendments, and in collaboration with the EPA, a "process safety standard". The text also highlighted the 14 principles on which this should be based. These were implemented in 1992 in OSHA's [[Process Safety Management (OSHA regulation)|Process Safety Management regulation]] ([https://www.ecfr.gov/current/title-29/subtitle-B/chapter-XVII/part-1910/subpart-H/section-1910.119 Title 29 CFR Part 1910, Subpart H § 1910.119]), as well as in EPA's 1996 [[Risk management program (RMP)|Risk Management Program]] (RMP) rule ([https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-68?toc=1 Title 40 CFR Part 68]).

===2022 amendments===
The [[Inflation Reduction Act]], the [[budget reconciliation]] bill signed by President [[Joe Biden]] in August 2022, amends the Clean Air Act to allow the EPA to administer $27 billion in grants to [[green bank]]s nationwide, through a competitive funding mechanism to be called the Greenhouse Gas Reduction Fund. On April 4, 2024, the Biden administration announced the eight recipients of the first $20 billion of the Greenhouse Gas Reduction Fund. For the $14 billion National Clean Investment Fund, the recipients are the consumer-focused Climate United Fund ($6.97 billion to a consortium of Calvert Impact, [[Center for Community Self-Help|Self-Help Ventures Fund]] and Community Preservation Corporation), the Coalition for Green Capital ($5 billion), and Power Forward Communities ($2 billion to a consortium of [[Enterprise Community Partners]], [[Local Initiatives Support Corporation]], [[United Way]], [[Habitat for Humanity]] and Rewiring America); collectively they have pledged 60 percent of funds would go to low-income and marginalized communities, well above the 40 percent required by Biden. For the $6 billion Clean Communities Investment Accelerator program to disburse money exclusively and deep into such communities, they are four [[Community development financial institution|CDFI]] Intermediaries (Opportunity Finance Network, Inclusiv, Native CDFI Network and Appalachian Community Capital) receiving a total of roughly $5.1 billion, and a coalition of community organizations called the Justice Climate Fund receiving $940 million. The Biden administration projects that they will leverage $7 from the private sector for every dollar of public investment, and slash emissions by up to 40 million metric tons by 2032 through a very wide variety of projects.<ref name="The White House GGRF announcement 2024">((cite web | author=The White House | title=Biden-Harris Administration Announces Historic $20 Billion in Awards to Expand Access to Clean Energy and Climate Solutions and Lower Energy Costs for Communities Across the Nation | website=The White House | date=April 4, 2024 | url=https://www.whitehouse.gov/briefing-room/statements-releases/2024/04/04/biden-harris-administration-announces-historic-20-billion-in-awards-to-expand-access-to-clean-energy-and-climate-solutions-and-lower-energy-costs-for-communities-across-the-nation/ | access-date=April 5, 2024))</ref> The remaining $7 billion will go to 60 [[distributed generation]] projects in low-income communities, and is forecast to collectively save 900,000 households $8 billion in energy bills by the IRA's expiration.<ref name="Solar for All announcement 2024">((cite web | title=Biden-Harris Administration Announces $7 Billion Solar for All Grants to Deliver Residential Solar, Saving Low-Income Americans $350 Million Annually and Advancing Environmental Justice Across America | website=US EPA | date=April 22, 2024 | url=https://www.epa.gov/newsreleases/biden-harris-administration-announces-7-billion-solar-all-grants-deliver-residential | access-date=April 22, 2024))</ref> <ref name="St. John April 2024 ">((cite web | last=St. John | first=Jeff | title=EPA’s new $20B ‘green bank’ will benefit disadvantaged communities… | website=Canary Media | date=April 4, 2024 | url=https://www.canarymedia.com/articles/climatetech-finance/epas-new-20b-green-bank-will-benefit-disadvantaged-communities-most | access-date=April 5, 2024))</ref><ref name="Roberts 2024">((cite web | last=Roberts | first=David | title=How the EPA will spend $27 billion in carbon-reduction funds | website=Volts | date=April 5, 2024 | url=https://www.volts.wtf/p/how-the-epa-will-spend-27-billion | access-date=April 5, 2024))</ref> <ref name="Morgan Lewis 2022">((cite web | title=Inflation Reduction Act Creates $27B 'Green Bank' Fund for Clean Energy Projects, but False Claims Risks Exist | last1=Avery|first1=W. Barron A.|last2=McCarthy|first2=Ryan A.|last3=Lorenzo|first3=Sarah-Jane|website=Morgan Lewis | date=August 16, 2022 | url=https://www.morganlewis.com/pubs/2022/08/inflation-reduction-act-creates-27b-green-bank-fund-for-clean-energy-projects-but-false-claims-risks-exist | access-date=October 22, 2022))</ref><ref name="Burr & Forman LLP 2022">((cite web | title=Three Ways the Inflation Reduction Act Advances Green Banking |last1=Penny|first1=William|last2=Snow|first2=Ed| website=Burr & Forman LLP | date=August 19, 2022 | url=https://www.burr.com/newsroom/articles/three-ways-the-inflation-reduction-act-advances-green-banking | access-date=October 22, 2022))</ref><ref name="Cherny 2022">((cite web | last=Cherny | first=Andrei | title=Commentary: This little-discussed provision in the Inflation Reduction Act just established the world's largest green lending program | website=Fortune | date=August 22, 2022 | url=https://fortune.com/2022/08/22/climate-inflation-reduction-act-national-green-bank-environment-biden-andrei-cherny/ | access-date=October 22, 2022))</ref><ref>((Cite web|url=https://www.epa.gov/greenhouse-gas-reduction-fund/about-greenhouse-gas-reduction-fund|website=US EPA|date=June 5, 2023|access-date=June 13, 2023|title=About the Greenhouse Gas Reduction Fund))</ref>

The Act is expected to generate high rates of return for the government on private sector investments from the Fund. The Act also designates [[carbon dioxide]] and other greenhouse gases as substances to be regulated by the EPA, in reaction to the Supreme Court case ''[[West Virginia v. EPA]]'', which limited the EPA's authority to institute a program such as the Obama-era [[Clean Power Plan]]. The IRA also allows the EPA more leeway to promote renewable energy.<ref>((cite news | url=https://www.nytimes.com/2022/08/22/climate/epa-supreme-court-pollution.html | title=Democrats Designed the Climate Law to be a Game Changer. Here's How | newspaper=The New York Times | date=August 22, 2022 | last1=Friedman | first1=Lisa ))</ref>

==Effects==
[[File:National Air Quality Concentration Averages 1990-2015 US EPA.png|thumb|Graph showing decreases in US air pollution concentrations during 1990 to 2015]]

According to a 2022 review study in the ''[[Journal of Economic Literature]]'', there is overwhelming causal evidence that shows that the CAA improved air quality.<ref>((Cite journal |last1=Aldy |first1=Joseph E. |last2=Auffhammer |first2=Maximilian |last3=Cropper |first3=Maureen |last4=Fraas |first4=Arthur |last5=Morgenstern |first5=Richard |date=2022 |title=Looking Back at 50 Years of the Clean Air Act |url=http://www.nber.org/papers/w26687.pdf |journal=Journal of Economic Literature |publisher=American Economic Association |language=en |volume=60 |issue=1 |pages=179–232 |doi=10.1257/jel.20201626 |s2cid=247303159 |issn=0022-0515))</ref>

According to the most recent study by EPA, when compared to the baseline of the 1970 and 1977 regulatory programs, by 2020 the updates initiated by the 1990 Clean Air Act Amendments would be costing the United States about $60 billion per year, while benefiting the United States (in monetized health and lives saved) about $2 trillion per year.<ref>EPA (2011).[http://www.epa.gov/air/sect812/feb11/fullreport.pdf "The Benefits and Costs of the Clean Air Act from 1990 to 2020. Final Report."] [https://web.archive.org/web/20110603010504/http://www.epa.gov/air/sect812/feb11/fullreport.pdf archived] (also known as the [http://www.epa.gov/air/sect812/prospective2.html "Second Prospective Study."] [https://web.archive.org/web/20131014203524/http://www.epa.gov/air/sect812/prospective2.html archived])</ref> In 2020, a study prepared for the [[Natural Resources Defense Council]] estimated annual benefits at 370,000 avoided premature deaths, 189,000 fewer hospital admissions, and net economic benefits of up to $3.8 trillion (32 times the cost of the regulations).<ref>((cite web |last1=Mui |first1=Simon |last2=Levin |first2=Amanda |publisher=Natural Resources Defense Council |location=New York, NY |title=Clearing the Air: The Benefits of the Clean Air Act |url=https://www.nrdc.org/resources/clean-air-acts-benefits-map |date=2020-05-05))</ref> Other studies have reached similar conclusions.<ref>((Cite journal |last1=Ross |first1=Kristie |last2=Chmiel |first2=James F. |last3=Ferkol |first3=Thomas |date=November 2012 |title=The impact of the Clean Air Ac t|journal=The Journal of Pediatrics |volume=161 |issue=5 |pages=781–786 |doi=10.1016/j.jpeds.2012.06.064 |issn=0022-3476|pmc=4133758 |pmid=22920509)) ((Cite journal |last1=Isen |first1=Adam |last2=Rossin-Slater |first2=Maya |last3=Walker |first3=W. Reed |date=May 1, 2017 |title=Every Breath You Take—Every Dollar You'll Make: The Long-Term Consequences of the Clean Air Act of 1970 |journal=Journal of Political Economy |volume=125 |issue=3 |pages= 848–902 |doi=10.1086/691465 |issn=0022-3808 |citeseerx=10.1.1.589.9755 |s2cid=222433000 ))</ref>

Mobile sources including automobiles, trains, and boat engines have become 99% cleaner for pollutants like hydrocarbons, carbon monoxide, nitrogen oxides, and particle emissions since the 1970s. The allowable emissions of volatile organic chemicals, carbon monoxide, nitrogen oxides, and lead from individual cars have also been reduced by more than 90%, resulting in decreased national emissions of these pollutants despite a more than 400% increase in total miles driven yearly.<ref name=":3" /> Since the 1980s, 1/4th of ground level ozone has been cut, mercury emissions have been cut by 80%, and since the change from leaded gas to unleaded gas 90% of atmospheric lead pollution has been reduced.<ref>((Cite web |url=https://www.ucsusa.org/global-warming/solutions/reduce-emissions/the-clean-air-act.html#.W5MPF5NKjow |title=The Clean Air Act |publisher=Union of Concerned Scientists |location=Cambridge, MA |date=2012-02-01))</ref> A 2018 study found that the Clean Air Act contributed to the 60% decline in pollution emissions by the manufacturing industry between 1990 and 2008.<ref>((Cite news |url=http://news.berkeley.edu/2018/08/09/environmental-regulations-drove-steep-declines-in-u-s-factory-pollution/ |title=Environmental regulations drove steep declines in U.S. factory pollution |date=August 9, 2018 |work=Berkeley News |access-date=August 11, 2018))</ref><ref>((Cite journal |last1=Shapiro |first1=Joseph S. |last2=Walker |first2=Reed |title=Why is Pollution from U.S. Manufacturing Declining? The Roles of Environmental Regulation, Productivity, and Trade |journal=American Economic Review |volume=108 |issue=12 |pages=3814–3854 |doi=10.1257/aer.20151272 |issn=0002-8282 |year=2018 |doi-access=free))</ref>

==Legal challenges==
Since its inception, the authority given to the EPA by Congress and the EPA's rulemaking within the Clean Air Act has been subject to numerous lawsuits. Some of the major suits where the Clean Air Act has been focal point of litigation include the following:

;''[[Train v. Natural Resources Defense Council, Inc.]]'', 421 U.S. 60 (1975)
:Under the Clean Air Act, states were required to submit their implementation plans within nine months of the EPA's promulgation of the new standards. The EPA approved several state plans that allowed for variances in their emissions limitations, and the Natural Resources Defense Council challenged that approval. The Supreme Court held that the EPA's approval was valid, and that as long as the “ultimate effect of a State's choice of emission limitations is compliance with the national standards for ambient air,” a state is “at liberty to adopt whatever mix of emission limitations it deems best suited to its particular situation.”<ref>((Cite web |url=https://www.westlaw.com/Document/I5cfef0da792711dd93e7a76b30106ace/View/FullText.html?transitionType=Default&contextData=(sc.Default)&VR=3.0&RS=cblt1.0 |title=The Important, Yet Limited, Role of State Environmental Actions |website=Westlaw |url-access=subscription |access-date=2021-11-11))</ref>

;''[[Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc.]]'', 467 U.S. 837 (1984)
:The Clean Air Act instructed the EPA to regulate emissions from sources of air pollution, but did not define what should be considered a source for the emission of air pollution, so the EPA had interpreted what a source was based on the legislation. The EPA's interpretation was challenged, but after review, the Supreme Court ruled in a 7–0 decision that the EPA had [[judicial deference]] to establish their own interpretation of law when the law is ambiguous and the interpretation is reasonable and consistent. This principle has come to be known as the [[Chevron deference]] applying to any executive agency granted powers from Congress.<ref>((cite book |last1=Brannon |first1=Valerie C. |last2=Cole |first2=Jared P. |title=Chevron Deference: A Primer |date=September 19, 2017 |publisher=Congressional Research Service |url=https://fas.org/sgp/crs/misc/R44954.pdf |id=R44954))</ref>

;''[[Whitman v. American Trucking Ass'ns, Inc.]]'', 531 U.S. 457 (2001)
:Following the EPA's rulemaking related to setting NAAQS standards related to diesel truck emissions, the trucking industry challenged the EPA's rule in lower courts, asserting the EPA's rule failed to justify reasoning for these new levels and violated the [[nondelegation doctrine]]. The [[United States Court of Appeals for the District of Columbia Circuit|D.C. Circuit Court]] found in favor of the trucking industry, determining that the EPA's rule did not consider the costs of implementing emissions regulations and controls. The Supreme Court reversed the D.C. District Court's ruling, affirming that not only was the delegation of power from Congress to the EPA by the Clean Air Act constitutional, but that the law did not require the EPA to consider costs as part of its determination for air quality controls.<ref>((cite journal |last=Bressman |first=Lisa Schultz |title=Disciplining Delegation After Whitman v. American Trucking Assns. |url=https://scholarship.law.cornell.edu/clr/vol87/iss2/9/ |journal=Cornell Law Review |volume=87 |issue=52 |date=2002 |pages=452–485))</ref>

;''[[Massachusetts v. Environmental Protection Agency]]'', 549 U.S. 497 (2007)
:With pressure from states and environmental groups on the EPA to regulate carbon dioxide and other greenhouse gas emissions from motor vehicles, the EPA determined in 2003 that the language of the Clean Air Act did not allow them to regulate emissions from motor vehicles, nor were they motivated to set such regulations even if they were able to. Multiple states and agencies sued the EPA for failing to act on what they considered to be harmful air pollutants. The Supreme Court ruled 5–4 that not only did the Clean Air Act mandate the EPA to regulate carbon dioxide and other greenhouse gases as air pollutants, but that failing to regulate these emissions would leave the EPA liable to further litigation.<ref>((cite web | url = https://www.nytimes.com/2007/04/03/washington/03scotus.html |title=Justices Say E.P.A. Has Power to Act on Harmful Gases |first=Linda |last=Greenhouse |date=April 3, 2007 |accessdate=November 1, 2021 |work=The New York Times))</ref> While the decision has remained contentious, the Court's decision in ''Massachusetts v. EPA'' was considered [[List of landmark court decisions in the United States|landmark]], as it opened up the courts for further environmental lawsuits to force entities to respond to climate change.<ref>((cite web |url=https://thehill.com/opinion/energy-environment/490813-mass-v-epa-after-13-years-its-time-for-climate-policy-review |title=Massachusetts v. EPA: After 13 years, it's time for climate policy review |first=William |last=Kovacs |date = April 2, 2020 |accessdate=November 1, 2021 |work=The Hill))</ref>

;''[[American Electric Power Co. v. Connecticut]]'', 564 U.S. 410 (2011)
:Several states and cities sued electric utility companies to force them to use a cap-and-trade system to reduce their emissions under a claim their emissions were a [[public nuisance]]. The Supreme Court ruled in an 8–0 decision that private companies cannot be sued by other parties for emissions-related issues, as this is a power specifically delegated to the EPA through the Clean Air Act under [[federal common law]].<ref>((cite journal |last=Brody |first=David R. |title=American Electric Power Co. v. Connecticut |url=https://harvardelr.com/wp-content/uploads/sites/12/2012/04/Brody.pdf |journal=Harvard Environmental Law Review |volume=36 |date=2012 |pages = 297–304))</ref>

;''[[Utility Air Regulatory Group v. Environmental Protection Agency]]'', 573 U.S. 302 (2014)
:The EPA issued new rules in 2010 that expanded emissions regulations for both regulated air pollutants and greenhouse gases to light and heavy engines and smaller stationary sources. These expanded rules were challenged by several power companies and regulatory groups, as they greatly expanded what types of facilities would need to acquire environmental permits prior to construction. The Supreme Court generally upheld the EPA's powers through the Clean Air Act, through it vacated portions of the EPA's new rules affecting smaller sources.<ref>((cite news |url=https://online.wsj.com/articles/supreme-court-reins-in-some-of-epas-greenhouse-gas-efforts-1403534416 | title=Supreme Court Ruling Backs Most EPA Emission Controls |work=The Wall Street Journal |date=June 23, 2014 |access-date=July 1, 2014))</ref>

;''[[Michigan v. EPA]]'', 576 U.S. 743 (2015)
:In 2012, the EPA issued new rules that identified new pollutants such as mercury as hazardous materials to be regulated in power plant emissions. The cost of implementing mercury pollution controls on new and existing plants can be expensive, and several states, companies, and other organizations sued the EPA that their analysis leading to these new rules did not consider the cost factor involved. The Supreme Court ruled 5–4 that the EPA's failure to consider the costs of these pollution controls was inappropriate, and that cost must be a factor in any finding that the EPA finds "necessary and appropriate" under the Clean Air Act.<ref>((cite web |url=https://www.nytimes.com/2015/06/30/us/supreme-court-blocks-obamas-limits-on-power-plants.html |title=Supreme Court Blocks Obama's Limits on Power Plants |first=Adam |last=Liptak |date=June 29, 2015 |work=The New York Times))</ref>

;''[[West Virginia v. EPA]]'', 597 U.S. ___ (2022)
:In 2014, the EPA under the Obama administration proposed the [[Clean Power Plan]] (CPP), which aimed to combat climate change by reducing carbon dioxide emissions from coal-burning power plants. Using its authority under ((USC|42|7411(d))), the agency sought to regulate emissions from existing power plants.<ref name=harv>[http://harvardlawreview.org/2016/02/the-clean-power-plan/ ''Recent Regulation: The Clean Power Plan''], 129 Harv. L. Rev. 1152 (February 10, 2016).</ref> After the final plan was announced in 2015, a number of states successfully petitioned federal courts for an emergency stay to block the implementation of the plan. In 2017, the CPP never having been implemented, the EPA under the Trump administration enacted the Affordable Clean Energy (ACE) rule, which repealed the CPP.<ref>((cite news |last1=Eilperin |first1=Juliet |title=EPA's Pruitt signs proposed rule to unravel Clean Power Plan |url=https://www.washingtonpost.com/politics/epas-pruitt-signs-proposed-rule-to-unravel-clean-power-plan/2017/10/10/96c83d2c-add2-11e7-a908-a3470754bbb9_story.html |newspaper=The Washington Post |date=October 10, 2017))</ref> A different set of states and environmental advocacy groups challenged the ACE rule in federal court, and on January 19, 2021, the D.C. Circuit Court of Appeals vacated the rule because it relied on a flawed interpretation of § 7411(d).<ref>((cite web |last1=Frazin |first1=Rachel |title=Court strikes down Trump coal power plant rule |url=https://thehill.com/policy/energy-environment/534772-court-strikes-down-trump-coal-power-plant-rule/ |website=The Hill |date=January 19, 2021))</ref> A number of states petitioned the Supreme Court, arguing that the D.C. Circuit's interpretation of § 7411(d) was too broad, and four petitions were consolidated as ''West Virginia v. EPA''. On June 30, 2022, the Supreme Court ruled in a 6–3 decision that the CPP was invalid because it fell under the [[major questions doctrine]] and thus required more specific Congressional approval than could reasonably be argued to be present in the statute.<ref>((cite web |last1=Totenberg |first1=Nina |title=Supreme Court restricts the EPA's authority to mandate carbon emissions reductions |url=https://www.npr.org/2022/06/30/1103595898/supreme-court-epa-climate-change |website=NPR |date=June 30, 2022))</ref>

== Current challenges ==
As poor air quality is still an issue in the United States, it has been noted that it is 1.5 times more likely for an ethnic minority person to live in an area burdened with air pollution than it is for an ethnic majority person.<ref>((Cite web |last=Air |first=Healthy |title=Environmental Justice: Addressing the Burden of Air Pollution |url=https://www.lung.org/blog/environmental-justice-air-pollution |access-date=2022-05-30 |website=www.lung.org |language=en))</ref> While the United States is still being affected by the [[COVID-19 pandemic]], research has shown that if someone lives in an area that experiences air pollution, they are 8% more likely to experience long-term COVID effects, potentially with a fatal outcome. This creates a disparity between minority and majority communities of Americans, even though the air pollution that these communities face is often a result of [[redlining]], which places minority communities in areas where industries that heavily pollute the air, as well as high-traffic roadways, reside.<ref>((Cite web |date=2020-01-08 |title=Why Air Quality is an Environmental Justice Issue |url=https://www.sierraclub.org/toiyabe/blog/2020/01/why-air-quality-environmental-justice-issue |access-date=2022-05-30 |website=Sierra Club |language=en))</ref>

Solutions proposed by the [[Sierra Club]] and the [[United States Environmental Protection Agency|Environmental Protection Agency]] (EPA) include funding clean energy transportation services, lowering acceptable emission levels, and enforcing the Clean Air Act that is already in place more intensely.<ref name="US EPA">((Cite web |last=US EPA |first=REG 09 |date=2013-03-07 |title=EPA Activities for Cleaner Air |url=https://www.epa.gov/sanjoaquinvalley/epa-activities-cleaner-air |access-date=2022-05-30 |website=www.epa.gov |language=en))</ref> While the Clean Air Act has been generally effective since it was enacted in 1970, there have been efforts from the fossil fuel industry to override it which have led to worse air pollution in many communities.<ref>((Cite web |last1=June 22 |last2=Turrentine |first2=2021 Jillian Mackenzie Jeff |title=Air Pollution: Everything You Need to Know |url=https://www.nrdc.org/stories/air-pollution-everything-you-need-know |access-date=2022-05-30 |website=NRDC |language=en))</ref>

An example of an area where the Clean Air Act has not been too effective is the [[San Joaquin Valley]] which experiences poor air quality conditions that stem from harmful agricultural practices, heavy traffic on major roadways, and the oil industry.<ref name="US EPA"/> Research shows that the leading air pollutant in the region is known as PM 2.5, or fine particulate matter, and causes health issues in pregnant women exposed to it, such as more severe asthma, decreased [[Forced expiratory volume|FEV1]], compromised immunity, and an increased risk of premature birth.<ref>((Cite web |date=2020-12-04 |title=Air Pollution in the San Joaquin Valley Has Dropped 60% Since 2000 |url=https://publichealth.berkeley.edu/news-media/air-pollution-in-san-joaquin-valley-has-dropped-60-percent-since-2000/ |access-date=2022-05-30 |website=UC Berkeley Public Health |language=en-US))</ref> Other symptoms that come from exposure to PM 2.5 include chronic bronchitis, reduced lung function in children, and heart and lung-related hospitalizations that can potentially lead to premature death especially if the individual has previous health concerns. In 2004, a mass decrease in PM 2.5 pollution occurred because of a decline in agricultural and biomass burning practices; however, it is still a common occurrence today and PM 2.5 levels have increased. Organizations such as the [[California Air Resources Board]] (CARB) have recommended that more sufficient regulations be implemented in the San Joaquin Valley to reduce toxic emissions into the atmosphere.<ref>((Cite web |title=Clean-air plan for San Joaquin Valley first to meet all federal standards for fine particle pollution ((!)) California Air Resources Board |url=https://ww2.arb.ca.gov/news/clean-air-plan-san-joaquin-valley-first-meet-all-federal-standards-fine-particle-pollution |access-date=2022-05-30 |website=ww2.arb.ca.gov))</ref> There are now efforts from the EPA, the CARB, and the San Joaquin Valley Air Pollution Control District to enforce regulations from the Clean Air Act and fund further efforts to live in a more sustainable fashion.

== Future challenges ==
As of 2017, some US cities still do not meet all national ambient air quality standards. It is likely that tens of thousands of premature deaths are still being caused by fine-particle pollution and ground-level ozone pollution.<ref name=":3" />

Climate change poses a challenge to the management of conventional air pollutants in the United States due to warmer, dryer summer conditions that can lead to increased air stagnation episodes. Prolonged droughts that may contribute to wildfires would also result in regionally high levels of air particles.<ref name=":2">John Bachmann, David Calkins, Margo Oge. [http://www.epaalumni.org/hcp/air.pdf "Cleaning the Air We Breathe: A Half Century of Progress."] EPA Alumni Association. September 2017. pp. 32–33.</ref>

Transboundary air pollution (both entering and exiting the United States) is not directly regulated by the Clean Air Act, requiring international negotiations and ongoing agreements with other nations, particularly Canada and Mexico.<ref>((cite web |title=Transboundary Air Pollution |date=March 27, 2014 |url=https://www.epa.gov/international-cooperation/transboundary-air-pollution |publisher=EPA))</ref>

[[Environmental justice]] continues to be an ongoing challenge for the Clean Air Act. By promoting pollution reduction, the Clean Air Act helps reduce heightened exposure to air pollution among minority and low-income communities.<ref>((Cite web |url=https://www.epa.gov/clean-air-act-overview/air-pollution-current-and-future-challenges |title=Air Pollution: Current and Future Challenges |publisher=EPA |date=2021-09-23))</ref> But African American populations are "consistently over represented" in areas with the poorest air quality.<ref>((Cite journal |last1=Miranda |first1=Marie Lynn |last2=Edwards |first2=Sharon E. |last3=Keating |first3=Martha H. |last4=Paul |first4=Christopher J. |date=April 18, 2017 |title=Making the Environmental Justice Grade: The Relative Burden of Air Pollution Exposure in the United States |journal=International Journal of Environmental Research and Public Health |volume=8 |issue=6 |pages=1755–1771 |doi=10.3390/ijerph8061755|issn=1661-7827|pmc=3137995|pmid=21776200|doi-access=free))</ref> Dense populations of low-income and minority communities inhabit the most polluted areas across the United States, which is considered to exacerbate health problems among these populations.<ref>((Cite web |url=http://www.ase.tufts.edu/gdae/education_materials/modules/Environmental_Justice.pdf |title=Environmental Justice: Income, Race, and Health |last=Massey |first=Rachel |date=2004 |website=Tufts University Global Development And Environment Institute))</ref> High levels of exposure to air pollution is linked to several health conditions, including asthma, cancer, premature death, and infant mortality, each of which disproportionately impact minority and low-income communities.<ref name=":1">((Cite web |url=https://www.epa.gov/sites/production/files/2015-04/documents/advancingejthroughp2.pdf |title=Advancing Environmental Justice Through Pollution Prevention |id=Report developed from the National Environmental Justice Advisory Council Meeting of December 9–13, 2002|author=EPA |date=2002))</ref> The pollution reduction achieved by the Clean Air Act is associated with a decline in each of these conditions and can promote environmental justice for communities that are disproportionately impacted by air pollution and diminished health status.<ref name=":1" />

==See also==
* [[Diesel Emissions Reduction Act]] (2005; amended 2010)

==References==
((Reflist|30em))

==External links==
((wikisource|United States Code/Title 42/Chapter 85|Clean Air Act))
* [https://www.law.cornell.edu/uscode/text/42/chapter-85 As codified in 42 U.S.C. chapter 85] of the [[United States Code]] from the [[Legal Information Institute|LII]]
* [https://uscode.house.gov/browse/prelim@title42/chapter85&edition=prelim As codified in 42 U.S.C. chapter 85] of the [[United States Code]] from the [[United States House of Representatives|US House of Representatives]]
* [https://www.govinfo.gov/content/pkg/COMPS-8160/uslm/COMPS-8160.xml Clean Air Act] as amended ([https://www.govinfo.gov/content/pkg/COMPS-8160/pdf/COMPS-8160.pdf PDF]/[https://www.govinfo.gov/app/details/COMPS-8160/ details]) in the [[United States Government Publishing Office|GPO]] [https://www.govinfo.gov/help/comps Statute Compilations collection]
* [https://www.epa.gov/laws-regulations/summary-clean-air-act Summary of the Clean Air Act] from the EPA
* [https://echo.epa.gov/ EPA Enforcement and Compliance History Online]
* [https://crsreports.congress.gov/product/pdf/RL/RL30853 Clean Air Act: A Summary of the Act and Its Major Requirements]. [[Congressional Research Service]] report, 2022. 
* [https://www.epaalumni.org/history/interview.cfm?id=38 EPA Alumni Association Oral History Video "Early Implementation of the Clean Air Act of 1970 in California"]

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