Clean Air Act, United States

July 4, 2012, 3:01 pm
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Source: EHS Journal/Nikita-Golovanov

The Clean Air Act is the law that most significantly regulates air quality in the United States. It has been a driving force behind changes in emission standards in the auto, airline and utility industries.

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

History caption (EPA)

Efforts by the United States federal government to control air pollution have gone through several phases, beginning with information collection, research, and technical assistance, before being strengthened to establish federal standards and enforcement. Federal legislation addressing air pollution was first passed in 1955, prior to which air pollution was the exclusive responsibility of state and local levels of government.

The Clean Air Act (CAA) ammendments of 1963 were passed with the purpose of reducing air pollution from stationary sources such as power plants and steel mills. The CAA sets emission standards for stationary sources while promoting public health and welfare of the United States population. Ninety-five million dollars over a three-year period financed state and local governments and air pollution control agencies, enabling them to conduct research and create control programs. Within the CAA, Congress also recognized the hazards of motor vehicle exhaust and mandated research, investigations, surveys, and experiments on interstate pollution from the use of high sulfur coal and oil. In 1965, the Motor Vehicle Air Pollution Control Act amended the 1963 Clean Air Act.

The federal role was strengthened in subsequent amendments, notably the Clean Air Act amendments of 1970, 1977, and 1990. The 1970 amendments established procedures under which EPA sets national standards for air quality, required a 90% reduction in emissions from new automobiles by 1975, established a program to require the best available control technology at major new sources of air pollution, established a program to regulate air toxics, and greatly strengthened federal enforcement authority. The CAA of 1970 also encompasses multiple levels of research funding and detailed control requirements that the federal government prescribes as regulation, and state governments apply and administer. The centerpiece of the Clean Air Act is the National Ambient Air Quality Standard (NAAQA) program. The NAAQS sets standards for six pollutants: sulfur dioxide; nitrogen dioxide; particulate matter; carbon monoxide, ozone; and lead. These substances are regulated under the CAA because they have been identified as dangers to public health and welfare. Each state was given primary responsibility for assuring that emissions sources from within their borders are consistent with the levels designated by the NAAQS. In order to achieve these goals, each state is required to submit a State Implementation Plan (SIP) to the EPA to ensure the implementation of primary and secondary air quality standards. The CAA of 1970 contains substantive procedural requirements governing the development and approval of each SIP.

Since many states failed to meet mandated air quality standards first set by the Clean Air Act, Congress created the 1977 amendments to aid states in achieving their original goals. One major revision tightens pollution control for newly built sources and brings older plants under the Clean Air Act’s regulations. It instituted the New Source Review (NSR), which requires companies to obtain permits before modifying equipment. The 1977 amendments extended deadlines and added the Prevention of Significant Deterioration program to protect air cleaner than national standards.

Congress enacted another major modification in 1990; the Clean Air Act of 1990 introduces a permit program for large sources that release pollutants into the air. The permit includes information on which pollutants are being released, how much may be released, and steps the source’s owner or operator are taking to reduce the pollution. This permit system simplifies and clarifies business’ obligations for reducing air pollution.

Changes to the act in 1990 included provisions to:

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

codified generally at 42 U.S.C. 7401-7671q)
Year Act Public Law Number

Air Pollution Control Act

P.L. 84-159



P.L. 86-353


Motor vehicle exhaust study

P.L. 86-493


Clean Air Act Amendments

P.L. 88-206


Motor Vehicle Air Pollution Control Act

P.L. 89-272, Title I


Clean Air Act Amendments of 1966

P.L. 89-675


Air Quality Act of 1967

P.L. 90-148


Clean Air Act Amendments of 1970

P.L. 91-604



P.L. 93-13


Energy Supply and Environmental Coordination Act of 1974

P.L. 93-319


Clean Air Act Amendments of 1977

P.L. 95-95


Acid Precipitation Act of 1980

P.L. 96-294, Title VII


Steel Industry Compliance Extension Act of 1981

P.L. 97-23


Clean Air Act 8-month Extension

P.L. 100-202


Clean Air Act Amendments of 1990

P.L. 101-549


Relatively minor laws amending the act

P.L. 104-6, P.L. 104-59,
P.L. 104-70, P.L. 104-260


Chemical Safety Information, Site Security and Fuels Regulatory Relief Act

P.L. 106-40


Amendments to §209 regarding small engines

P.L. 108-199, Division G,
Title IV, Section 428


Energy Policy Act of 2005 (amended §211 re fuels)

P.L. 109-58


Energy Independence and Security Act of 2007

P.L. 110-140

The 1990 Clean Air Act includes other provisions to reduce interstate air pollution, and covers pollution that originates in Mexico and Canada that is transported into the United States as well as pollution that originates in the United States and affects Canada and Mexico. The EPA's enforcement powers broadened with the new amendment; the EPA is now authorized to fine violators and increase penalties for violations of the Act. The section establishes federal authority to issue agency and court orders requiring compliance and to impose penalties for violations of Act requirements. Additionally the EPA is authorized to require sources to submit reports, monitor emissions, and certify compliance with the Act’s requirements, and authorizes EPA personnel to conduct inspections.

Primarily states or local governments enforce the Clean Air Act; they issue most permits, monitor compliance, and conduct the majority of inspections. However, the Act also provides for citizen suits both against persons (including corporations or government agencies) alleged to have violated emissions standards or permit requirements, and against EPA in cases where the Administrator has failed to perform an action that is not discretionary under the Act

One example, California’s Zero Emission Vehicle (ZEV) program, is intended to promote the development of alternative fuels and vehicles. The Clean Air Act grants California the authority to develop its own vehicle emissions standards if those standards are at least as stringent as the federal standards. In addition to setting more stringent standards for all vehicles, California used this authority to establish a program requiring auto manufacturers to sell ZEVs (electric or hydrogen fuel cell vehicles) in the state beginning in 2003. This program has been substantially modified since it was enacted, and now allows credit for hybrid and partial ZEV vehicles in addition to true ZEVs, but it has served as an incubator for lower emission technologies since its adoption. The Act allows other states to adopt California’s stricter standards: ten states (Connecticut, Maine, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington) have already adopted them or are in the process of doing so.

The 1990 amendment also addresses acid rain pollution by creating a market-based system as a means to reduce sulfur dioxide emissions from power plants. There are two phases; Phase I targets the highest emission producing plants, which were expected to achieve reductions by 1995, and Phase II, which began in 2000, affects smaller plants and calls for stricter reductions from Phase I sources. Companies are allowed to either bank their allowances or trade them with other companies.

On December 31, 2003, amendment votes on the 2003 budget bill changed New Source Review pollution control provisions. These amendments give companies the option to not update emission controls if their plant’s equipment has been reviewed in the last decade. Additionally, communities are no longer able to publicly comment when power plants expand production and increase emissions.

Within the 110th term of Congress, state governments and the courts have taken action on air issues that has stirred congressional interest. On April 2, 2007, the Supreme Court decided Massachusetts v. EPA, finding that EPA has authority under the Clean Air Act to regulate greenhouse gas emissions from new motor vehicles and requiring EPA to make a finding as to whether such emissions endanger public health or welfare. On February 8, 2008, the D.C. Circuit Court of Appeals, in New Jersey v. EPA, found EPA’s approach to the regulation of power plant mercury emissions to be unlawful. On July 11, 2008, in North Carolina v. EPA, the D.C. Circuit vacated the Clean Air Interstate Rule (CAIR), which would have controlled emissions from power plants affecting air quality in downwind states. Other cases involving climate change, clean air standards, and the regulation of power plants are pending at the D.C. Circuit Court of Appeals and in a number of federal and state courts. Decisions in these cases may prompt hearings or legislation.

In addition, the Act mandates emission controls for sources of 188 hazardous air pollutants, requires the prevention of significant deterioration of air quality in areas with clean air, requires a program to restore visibility impaired by regional haze in national parks and wilderness areas, and implements the Montreal Protocol on Substances that Deplete the Ozone Layer to phase out most ozone-depleting chemicals.

Title I - Air Pollution Prevention and Control

National Ambient Air Quality Standards

In Section 109, the act requires EPA to establish National Ambient Air Quality Standards (NAAQS) for air pollutants that endanger public health or welfare, in the Administrator’s judgment, and whose presence in ambient air results from numerous or diverse sources. The NAAQS must be designed to protect public health and welfare with an adequate margin of safety. Using this authority, EPA has promulgated NAAQS for six air pollutants: sulfur dioxide (SO2), particulate matter (PM2.5 and PM10), nitrogen dioxide (NO2), carbon monoxide (CO), ozone, and lead. Unlike the other NAAQS pollutants, ozone is not directly emitted, but rather is formed in the atmosphere by the interaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. The control of ozone is thus based on regulating emissions of VOCs and NOx. The act requires EPA to review the scientific data upon which the standards are based, and revise the standards, if necessary, every five years. More often than not, however, EPA has taken more than five years in reviewing and revising the standards.

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

Geographic areas that meet NAAQSs for a given pollutant are classified as attainment areas; those that do not meet NAAQSs are classified as non-attainment areas.

State Implementation Plans

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

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

The 1990 amendments require EPA to impose sanctions in areas which fail to submit a SIP, fail to submit an adequate SIP, or fail to implement a SIP: unless the state corrects such failures, a 2-to-1 emissions offset for the construction of new polluting sources is imposed 18 months after notification to the state, and a ban on most new federal highway grants is imposed six months later. An additional ban on air quality grants is discretionary. Ultimately, a Federal Implementation Plan may be imposed if the state fails to submit or implement an adequate SIP. The amendments also require that, in nonattainment areas, no federal permits or financial assistance may be granted for activities that do not “conform” to a State Implementation Plan. This requirement can cause a temporary suspension in funding for most new highway and transit projects, unless an area demonstrates that the emissions caused by such projects are consistent with attainment and maintenance of ambient air quality standards. Demonstrating conformity of transportation plans and SIPs is required in nonattainment areas whenever new plans are submitted.

Nonattainment Requirements

In a major departure from the prior law, the 1990 Clean Air Act Amendments grouped most nonattainment areas into classifications based on the extent to which the NAAQS was exceeded, and established specific pollution controls and attainment dates for each classification. These requirements are described here as spelled out in Sections 181-193 of the act. [EPA modified the ozone standard, specified in the statute as 0.12 parts per million averaged over a 1-hour period, to 0.08 parts per million averaged over an 8-hour period, through regulations promulgated in July 1997. (The standard was revised again in March 2008, to 0.075 ppm averaged over 8 hours) In April 2004, the agency promulgated an implementation rule for the 1997 8-hour standard. Under this rule, the 1-hour standard was revoked as of June 15, 2005, and areas that had not yet attained it were converted to new classifications depending on their 8-hour concentration of ozone. As a result of court challenges, the ramifications of this conversion to the 8-hour standard are still unfolding, but in general the former 1-hour nonattainment areas remain subject to the controls specified for their 1- hour category. New nonattainment areas that did not exceed the 1-hour standard, but do violate the 8-hour standard, in general are subject to more flexible controls under Subpart 1 (Sections 171-179B) of the act.]

Nonattainment areas are classified on the basis of a “design value,” which is derived from the pollutant concentration (in parts per million or micrograms per cubic meter) recorded by air quality monitoring devices. The design value for the one-hour ozone standard was the fourth highest hourly reading measured during the most recent three-year period. Using these design values, the act created five classes of ozone nonattainment, as shown in Table 2. Only Los Angeles fell into the “extreme” class, but 97 other areas were classified in one of the other four ozone categories. A simpler classification system established moderate and serious nonattainment areas for carbon monoxide and particulate matter with correspondingly more stringent control requirements for the more polluted class.

As shown in the table, the deadlines for attainment for ozone nonattainment areas stretched from 1993 to 2010, depending on the severity of the problem. (Under the 8-hour ozone standard, which replaced the 1-hour standard in 2004, these deadlines are changed to 2007 to 2021.) For carbon monoxide, the attainment date for moderate areas was December 31, 1995, and for serious areas, December 31, 2000. For particulate matter, the deadline for areas designated moderate nonattainment as of 1990 was December 31, 1994; for those areas subsequently designated as moderate, the deadline is six years after designation. For serious areas, the respective deadlines are December 31, 2001, or 10 years after designation.

Table 2. Ozone Nonattainment Classifications
Class Marginal Moderate Serious Severe Extreme
Deadline 3 years 6 years 9 years 15-17 yearsa 20 years
Areasb 42 areas 32 areas 14 areas 9 areas 1 area
Design 0.121 ppm- 0.138 ppm- 0.160 ppm- 0.180 ppm- >0.280 ppm-
Value 0.138 ppm- 0.160 ppm- 0.180 ppm- 0.280 ppm- >0.280 ppm-

a. Areas with a 1988 design value between 0.190 and 0.280 ppm were given 17 years to attain; others had 15 years.

b. Number of areas in each category as of the date of enactment.

Requirements for Ozone Nonattainment Areas

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

Marginal Areas

  • Inventory emissions sources (to be updated every three years).
  • Require 1.1 to 1 offsets (i.e., new major sources of volatile organic compounds (VOCs) must reduce emissions from an existing facility or facilities in the area by 10% more than the emissions of the new source.
  • Impose reasonably available control technology (RACT) on all major sources emitting more than 100 tons per year for the nine industrial categories where EPA had already issued control technique guidelines describing RACT prior to 1990. Moderate Areas
  • Meet all requirements for marginal areas.
  • Impose a 15% reduction in volatile organic compounds (VOCs) in six years.
  • Adopt a basic vehicle inspection and maintenance program.
  • Impose RACT on all major sources emitting more than 100 tons per year for all additional industrial categories where EPA will issue control technique guidelines describing RACT.
  • Require vapor recovery at gas stations selling more than 10,000 gallons per month.
  • Require 1.15 to 1 offsets.

Serious Areas

  • Meet all requirements for moderate areas.
  • Reduce definition of a major source of VOCs from emissions of 100 tons per year to 50 tons per year for the purpose of imposing RACT.
  • Reduce VOCs 3% annually for years 7 to 9 after the 15% reduction already required by year 6.
  • Improve monitoring.
  • Adopt an enhanced vehicle inspection and maintenance program.
  • Require fleet vehicles to use clean alternative fuels.
  • Adopt transportation control measures if the number of vehicle miles traveled in the area is greater than expected.
  • Require 1.2 to 1 offsets. • Adopt contingency measures if the area does not meet required VOC reductions. Severe Areas
  • Meet all requirements for serious areas. • Reduce definition of a major source of VOCs from emissions of 50 tons per year to 25 tons per year for the purpose of imposing RACT.
  • Adopt specified transportation control measures.
  • Implement a reformulated gasoline program.
  • Require 1.3 to 1 offsets.
  • Impose $5,000 per ton penalties on major sources if the area does not meet required reductions.

Extreme Areas

  • Meet all requirements for severe areas.
  • Reduce definition of a major source of VOCs from emissions of 25 tons per year to 10 tons per year for the purpose of imposing RACT.
  • Require clean fuels or advanced control technology for boilers emitting more than 25 tons per year of NO.
  • Require 1.5 to 1 offsets.

As noted, EPA promulgated a new, 8-hour ozone standard in July 1997. Following extensive court challenges, the agency designated nonattainment areas for the new standard on April 30, 2004. State Implementation Plans were required to be submitted in 2007.

Requirements for Carbon Monoxide Nonattainment Areas

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

Moderate Areas

  • Conduct an inventory of emissions sources.
  • Forecast total vehicle miles traveled in the area.
  • Adopt an enhanced vehicle inspection and maintenance program.
  • Demonstrate annual improvements sufficient to attain the standard.

Serious Areas

  • Adopt specified transportation control measures.
  • Implement an oxygenated fuels program for all vehicles in the area.
  • Reduce definition of a major source of CO from emissions of 100 tons per year to 50 tons per year if stationary sources contribute significantly to the CO problem.

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

Requirements for Particulate Nonattainment Areas

Particulate (PM10) nonattainment areas are also subject to specified control requirements. These are:

Moderate Areas

  • Require permits for new and modified major stationary sources of PM10.
  • Impose reasonably available control measures (RACM).

Serious Areas

  • Impose best available control measures (BACM).
  • Reduce definition of a major source of PM10 from 100 tons per year to 70 tons per year.

In July 1997, EPA promulgated new standards for fine particulates (PM2.5). The PM2.5 standards were also subject to court challenges. The absence of a monitoring network capable of measuring the pollutant also delayed implementation. Nonattainment areas for PM2.5 were designated on April 14, 2005. States had three years subsequent to designation to submit State Implementation Plans. Revisions to the NAAQS promulgated in October 2006 strengthened the PM2.5 standard.

New Source Performance Standards

Title I also stablishes New Source Performance Standards (NSPSs), which are nationally uniform emission standards for new stationary sources falling within particular industrial categories. NSPSs are based on the pollution control technology available to that category of industrial source, but allow the affected industries the flexibility to devise a cost-effective means of reducing emissions.

New Source Performance Standards

Section 111 of the act requires EPA to establish nationally uniform, technology-based standards (called New Source Performance Standards, or NSPS) for categories of new industrial facilities. These standards accomplish two goals: first, they establish a consistent baseline for pollution control that competing firms must meet, and thereby remove any incentive for states or communities to weaken air pollution standards in order to attract polluting industry; and second, they preserve clean air to accommodate future growth, as well as for its own benefits.

NSPS establish maximum emission levels for new major stationary sources—powerplants, steel mills, and smelters, for example—with the emission levels determined by the best “adequately demonstrated” continuous control technology available, taking costs into account. EPA must regularly revise and update NSPS applicable to designated sources as new technology becomes available, since the goal is to prevent new pollution problems from developing and to force the installation of new control technology.

The standards also apply to modifications of existing facilities, through a process called New Source Review (NSR). The law’s ambiguity regarding what constitutes a modification (subject to NSR) as opposed to routine maintenance of a facility has led to litigation, with EPA proposing in recent years to modify its interpretation of the requirements of this section.

National Emission Standards for Hazardous Air Pollutants

Under Title I, EPA establishes and enforces National Emission Standards for Hazardous Air Pollutants (NESHAPs), nationally uniform standards oriented towards controlling particular hazardous air pollutants (HAPs)

Completely rewritten by the Clean Air Act Amendments of 1990, Section 112 of the act establishes programs for protecting public health and the environment from exposure to toxic air pollutants. As revised by the 1990 amendments, the section contains four major provisions: Maximum Achievable Control Technology (MACT) requirements; health-based standards; standards for stationary “area sources” (small, but numerous sources, such as gas stations or dry cleaners, that collectively emit significant quantities of hazardous pollutants); and requirements for the prevention of catastrophic releases.

First, EPA is to establish technology-based emission standards, called MACT standards, for sources of 188 pollutants listed in the legislation, and to specify categories of sources subject to the emission standards. (The 1990 amendments specified 189 pollutants, but P.L. 102-187, enacted on December 4, 1991, deleted hydrogen sulfide from the list of toxic pollutants, leaving 188.) EPA is to revise the standards periodically (at least every eight years). EPA can, on its initiative or in response to a petition, add or delete substances or source categories from the lists.

Section 112 establishes a presumption in favor of regulation for the designated pollutants; it requires regulation of the pollutants unless EPA or a petitioner is able to show “that there is adequate data on the health and environmental effects of the substance to determine that emissions, ambient concentrations, bioaccumulation or deposition of the substance may not reasonably be anticipated to cause any adverse effects to human health or adverse environmental effects.”

EPA is required to set standards for sources of the listed pollutants that achieve “the maximum degree of reduction in emissions” taking into account cost and other non-air-quality factors. The standards for new sources “shall not be less stringent than the most stringent emissions level that is achieved in practice by the best controlled similar source.” The standards for existing sources may be less stringent than those for new sources, but must be no less stringent than the emission limitations achieved by either the best performing 12% of existing sources (if there are more than 30 such sources in the category or subcategory) or the best performing five similar sources (if there are fewer than 30). Existing sources are given three years following promulgation of standards to achieve compliance, with a possible one-year extension; additional extensions may be available for special circumstances or for certain categories of sources. Existing sources that achieve voluntary early emissions reductions will receive a six-year extension for compliance with MACT.

The second major provision of Section 112 directs EPA to set health-based standards to address situations in which a significant residual risk of adverse health effects or a threat of adverse environmental effects remains after installation of MACT. This provision requires that EPA, after consultation with the Surgeon General of the United States, submit a report to Congress on the public health significance of residual risks, and recommend legislation regarding such risks. If Congress does not legislate in response to EPA’s recommendations, then EPA is required to issue standards for categories of sources of hazardous air pollutants as necessary to protect the public health with an ample margin of safety or to prevent an adverse environmental effect. A residual risk standard is required for any source emitting a cancer-causing pollutant that poses an added risk to the most exposed person of more than one in a million. Residual risk standards are due eight years after promulgation of MACT for the affected source category. Existing sources have 90 days to comply with a residual risk standard, with a possible two-year extension. In general, residual risk standards do not apply to area sources.

The law directed EPA to contract with the National Academy of Sciences (NAS) for a study of risk assessment methodology, and created a Risk Assessment and Management Commission to investigate and report on policy implications and appropriate uses of risk assessment and risk management. In 1994 NAS published its report, Science and Judgment in Risk Assessment. The commission study, Framework for Environmental Health Risk Management, was released in 1997.

Third, in addition to the technology-based and health-based programs for major sources of hazardous air pollution, EPA is to establish standards for stationary “area sources” determined to present a threat of adverse effects to human health or the environment. The provision requires EPA to regulate the stationary area sources responsible for 90% of the emissions of the 30 hazardous air pollutants that present the greatest risk to public health in the largest number of urban areas. In setting the standard, EPA can impose less stringent “generally available” control technologies, rather than MACT.

Finally, Section 112 addresses prevention of sudden, catastrophic releases of air toxics by establishing an independent Chemical Safety and Hazard Investigation Board. The board is responsible for investigating accidents involving releases of hazardous substances, conducting studies, and preparing reports on the handling of toxic materials and measures to reduce the risk of accidents.

EPA is also directed to issue prevention, detection, and correction requirements for catastrophic releases of air toxics by major sources. Section 112(r) requires owners and operators to prepare risk management plans including hazard assessments, measures to prevent releases, and a response program.

Solid Waste Incinerators

Prior to 1990, solid waste incinerators, which emit a wide range of pollutants, were subject to varying degrees of state and federal regulation depending on their size, age, and the type of waste burned. In a new Section 129, the 1990 amendments established more consistent federal requirements specifying that emissions of 10 categories of pollutants be regulated at new and existing incinerators burning municipal solid waste, medical waste, and commercial and industrial waste. The amendments also established emissions monitoring and operator training requirements.

Prevention of Significant Deterioration/Regional Haze

Sections 160-169 of the act establish requirements for the prevention of significant deterioration of air quality (PSD). The PSD program reflects the principle that areas where air quality is better than that required by NAAQS should be protected from significant new air pollution even if NAAQS would not be violated.

The act divides clean air areas into three classes, and specifies the increments of SO2 and particulate pollution allowed in each. Class I areas include international and national parks, wilderness and other pristine areas; allowable increments of new pollution in these areas are very small. Class II areas include all attainment and not classifiable areas, not designated as Class I; allowable increments of new pollution in these areas are modest. Class III represents selected areas that states may designate for development; allowable increments of new pollution are large (but not exceeding NAAQS). Through an elaborate hearing and review process, a state can have regions redesignated from Class II to Class III (although none have yet been so redesignated).

While the 1977 amendments only stipulated PSD standards for two pollutants, SO2 and particulates, EPA is supposed to establish standards for other criteria pollutants. Thus far, only one of the other four has been addressed: the agency promulgated standards for NO2 in 1988. Newly constructed polluting sources in PSD areas must install best available control technology (BACT) that may be more strict than that required by NSPS. The justifications of the policy are that it protects air quality, provides an added margin of health protection, preserves clean air for future development, and prevents firms from gaining a competitive edge by “shopping” for clean air to pollute.

In Sections 169A and B, the act also sets a national goal of preventing and remedying impairment of visibility in national parks and wilderness areas, and requires EPA to promulgate regulations to assure reasonable progress toward that goal. In the 1990 amendments, Congress strengthened these provisions, which had not been implemented.

The amendments required EPA to establish a Grand Canyon Visibility Transport Commission, composed of Governors from each state in the affected region, an EPA designee, and a representative of each of the national parks or wilderness areas in the region. Other visibility transport commissions can be established upon EPA’s discretion or upon petition from at least two states. Within 18 months of receiving a report from one of these commissions, EPA is required to promulgate regulations to assure reasonable progress toward the visibility goal, including requirements that states update their State Implementation Plans to contain emission limits, schedules of compliance, and other measures necessary to make reasonable progress. Specifically mentioned is a requirement that states impose Best Available Retrofit Technology on existing sources of emissions impairing visibility.

The Grand Canyon Commission delivered a set of recommendations to EPA in June 1996, and the agency subsequently promulgated a “regional haze” program applicable to all 50 states under this authority.


Section 113 of the act, which was also strengthened by the 1990 amendments, covers enforcement. The section establishes federal authority to issue agency and court orders requiring compliance and to impose penalties for violations of Act requirements. Section 114 authorizes EPA to require sources to submit reports; to monitor emissions; and to certify compliance with the act’s requirements, and authorizes EPA personnel to conduct inspections.

Like most federal environmental statutes, the Clean Air Act is enforced primarily by states or local governments; they issue most permits, monitor compliance, and conduct the majority of inspections. The federal government functions as a backstop, with authority to review state actions. The agency may act independently or may file its own enforcement action in cases where it concludes that a state’s response was inadequate.

The act also provides for citizen suits both against persons (including corporations or government agencies) alleged to have violated emissions standards or permit requirements, and against EPA in cases where the Administrator has failed to perform an action that is not discretionary under the act. Citizen groups have often used the latter provision to compel the Administrator to promulgate regulations required by the statute.

The 1990 amendments elevated penalties for some knowing violations from misdemeanors to felonies; removed the ability of a source to avoid an enforcement order or civil penalty by ceasing a violation within 60 days of notice; gave authority to EPA to assess administrative penalties; and authorized $10,000 awards to persons supplying information leading to convictions under the act.

Title II - Emission Standards for Moving Sources

Under Title II, EPA regulates most mobile sources of air pollution (e.g., automobiles and airplanes). Performance standards issued by EPA limit the emissions of certain pollutants from these sources. Fuel-related requirements under Title II are designed to further reduce emissions from mobile sources.

Title II has required emission standards for automobiles since 1968. The 1990 amendments significantly tightened these standards: for cars, the hydrocarbon standard was reduced by 40% and the nitrogen oxides (NOx) standard by 50%. The standards—referred to as “Tier 1” standards—were phased in over the 1994-1996 model years.

The amendments envisioned a further set of reductions (“Tier 2” standards), but not before model year 2004. For Tier 2 standards to be promulgated, the agency was first required to report to Congress concerning the need for further emission reductions, the availability of technology to achieve such reductions, and the cost-effectiveness of such controls compared to other means of attaining air quality standards. EPA submitted this report to Congress in August 1998, concluding that further emission reductions were needed and that technology to achieve such reductions was available and cost-effective. Tier 2 standards, requiring emission reductions of 77% to 95% from cars and light trucks were promulgated in February 2000, and were phased in over the 2004-2009 model years. To facilitate the use of more effective emission controls, the standards also required a more than 90% reduction in the sulfur content of gasoline, beginning in 2004.

The 1990 amendments also stipulated that oxygenated gasoline, designed to reduce emissions of carbon monoxide, be sold in the worst CO nonattainment areas and that “reformulated” gasoline (RFG), designed to reduce emissions of volatile organic compounds and toxic air pollutants, be sold in the nine worst ozone nonattainment areas (Los Angeles, San Diego, Houston, Baltimore, Philadelphia, New York, Hartford, Chicago, and Milwaukee); metropolitan Washington, DC, and four areas in California were added to the mandatory list later. Other ozone nonattainment areas can opt in to the RFG program; as of 2006, additional areas in 11 states had done so.

The fuels provisions were modified by the Energy Policy Act of 2005, removing the requirement that RFG contain oxygenates. Instead, the 2005 law required the use of increasing amounts of renewable fuel, most likely to be ethanol, not just in RFG but in all motor fuels, beginning in 2006. The Energy Independence and Security Act of 2007 further strengthened this renewable fuel requirement.

In addition to the above program, California’s Zero Emission Vehicle (ZEV) program also is intended to promote the development of alternative fuels and vehicles. Section 209(b) of the Clean Air Act grants California the authority to develop its own vehicle emissions standards if those standards are at least as stringent as the federal standards. In addition to setting more stringent standards for all vehicles, California used this authority to establish a program requiring auto manufacturers to sell ZEVs (electric or hydrogen fuel cell vehicles) in the state beginning in 2003. This program has been substantially modified since it was enacted, and now allows credit for hybrid and partial ZEV vehicles in addition to true ZEVs, but it has served as an incubator for lower emission technologies since its adoption. Section 177 of the act allows other states to adopt California’s stricter standards: 11 states (Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington) and the District of Columbia have already adopted them or are in the process of doing so.

The 1990 amendments also imposed tighter requirements on certification (an auto’s useful life is defined as 100,000 miles instead of the earlier 50,000 miles), on emissions allowed during refueling, on low temperature CO emissions, on in-use performance over time, and on warranties for the most expensive emission control components (eight years/80,000 miles for the catalytic converter, electronic emissions control unit, and onboard emissions diagnostic unit). Regulations were also extended to include nonroad fuels and engines.

Standards for trucks and buses using diesel engines were also strengthened. The 1990 amendments required new urban buses to reduce emissions of diesel particulates 92% by 1996, and all other heavy-duty diesel engines to achieve an 83% reduction by the same year. NOx emissions must also be reduced, 33% by 1998. Authority to further strengthen these standards led to promulgation in January 2001 of new emission standards requiring a further 90%-95% reduction in emissions phased in over the 2007-2010 model years, and a reduction of 97% in the allowable amount of sulfur in highway diesel fuel. These regulations were followed in May 2004 by similar requirements for nonroad diesel equipment, which will be phased in between 2007 and 2015.

Title III - General

Title III of the CAA further directed EPA to develop a list of sources that emit any of 188 HAPs, and to develop regulations for these categories of sources. To date, EPA has listed 174 categories and developed a schedule for the establishment of emission standards. The emission standards will be developed for both new and existing sources based on "maximum achievable control technology" (MACT). The MACT is defined as the control technology achieving the maximum degree of reduction in the emission of the HAPs, taking into account cost and other factors.

The 1990 CAA Amendments significantly changed the pre-existing system for control of hazardous air pollutants. The pre-1990 CAA approach required EPA to establish a list of hazardous air pollutants and impose health-based emission standards for each pollutant. The amendment identifies 188 Air Toxics (hazardous air pollutants), directs EPA to identify sources of the 188 pollutants, and establishes a ten-year time period for EPA to issue technology-based emissions standards for each source category. Title III provides for a second phase under which EPA is to assess residual risk after the implementation of the first phase of standards and impose new standards, when appropriate, to protect public health.

Title IV - Acid Deposition Control

The Clean Air Act Amendments of 1990 added an acid deposition control program (Title IV) to the act to address acid rain caused by sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions from fossil-fueled electric power plants and, to a lesser extent, from other industrial and transportation sources. Traditionally, environmental regulation has been achieved by the "command and control" approach, in which the regulator specifies how to reduce pollution, by what amount, and what technology to use. Title IV, however, took a different approach by giving utilities flexibility in choosing how to achieve the required reductions. Utilities could reduce emissions by switching to low-sulfur coal, installing pollution-control devices (scrubbers), or shutting down plants.

CAA set goals for the year 2000 of reducing annual SO2 emissions by 10 million tons from 1980 levels and reducing annual NOx emissions by 2 million tons, also from 1980 levels.

The SO2 reductions were imposed in two steps. Under Phase 1, owners/operators of 111 electric generating facilities listed in the law that are larger than 100 megawatts had to meet tonnage emission limitations by January 1, 1995. This would reduce SO2 emission by about 3.5 million tons. Phase 2 included facilities larger than 75 megawatts, with a deadline of January 1, 2000. Compliance has been 100%.

To introduce some flexibility in the distribution and timing of reductions, the act creates a comprehensive permit and emissions allowance/trading system. An allowance is a limited authorization to emit a ton of SO2. Issued by EPA, the allowances would be allocated to Phase 1 and Phase 2 units in accordance with baseline emissions estimates. Powerplants which commence operation after November 15, 1990, would not receive any allowances. These new units would have to obtain allowances (offsets) from holders of existing allowances. Allowances may be traded nationally during either phase. The law also permits industrial sources and powerplants to sell allowances to utility systems under regulations to be developed by EPA. Allowances may be banked by a utility for future use or sale.

The act provided for two types of sales to improve the liquidity of the allowance system and to ensure the availability of allowances for utilities and independent power producers who need them. First, a special reserve fund consisting of 2.8% of Phase 1 and Phase 2 allowance allocations has been set aside for sale. Allowances from this fund (25,000 annually from 1993 to 1999 and 50,000 thereafter) are sold at a fixed price of $1,500 an allowance. Independent power producers have guaranteed rights to these allowances under certain conditions. Second, an annual, open auction sold allowances (150,000 from 1993 to 1995, and 250,000 from 1996 to 1999) with no minimum price. Utilities with excess allowances may have them auctioned off at this auction, and any person may buy allowances.

The act essentially caps SO2 emissions at individual existing sources through a tonnage limitation, and at future plants through the allowance system. First, emissions from most existing sources are capped at a specified emission rate multiplied by an historic baseline level. Second, for plants commencing operation after November 15, 1990, emissions must be completely offset with additional reductions at existing facilities beginning after Phase 2 compliance. However, as noted above, the law provides some allowances to future powerplants which meet certain criteria. The utility SO2 emission cap is set at 8.9 million tons, with some exceptions.

The act provides that if an affected unit does not have sufficient allowances to cover its emissions, it is subject to an excess emission penalty of $2,000 per ton of SO2 and required to reduce an additional ton of SO2 the next year for each ton of excess pollutant emitted.

The act also requires EPA to inventory industrial emissions of SO2 and to report every five years, beginning in 1995. If the inventory shows that industrial emissions may reach levels above 5.60 million tons per year, then EPA is to take action under the act to ensure that the 5.60 million ton cap is not exceeded.

The act requires EPA to set specific NOx emission rate limitations—0.45 lb. per million Btu for tangentially-fired boilers and 0.50 lb. per million Btu for wall-fired boilers—unless those rates can not be achieved by low-NOx burner technology. Tangentially and wall-fired boilers affected by Phase 1 SO2 controls must also meet NOx requirements. EPA was to set emission limitations for other types of boilers by 1997 based on low-NOx burner costs, which EPA did. In addition, EPA was to propose and promulgate a revised new source performance standard for NOx from fossil fuel steam generating units, which EPA also did, in 1998.

Utilities have taken advantage of the options in Title IV by choosing less costly ways to reduce emissions, such as switching from high-to low-sulfur coal, and as a result they have been achieving sizable reductions in their SO2 emissions. Fifty-five percent of Phase 1 plants opted to switch to low-sulfur coal, 16 percent chose to install scrubbers, and only 3 percent initially planned to purchase allowances (which allow plants to emit extra SO2)..

Title V – Permits

The Clean Air Act Amendments of 1990 added a Title V to the act which requires states to administer a comprehensive permit program for the operation of sources emitting air pollutants. These requirements are modeled after similar provisions in the Clean Water Act. Previously, the Clean Air Act contained limited provision for permits, requiring only new or modified major stationary sources to obtain construction permits (under Section 165 of the act)..

Sources subject to the permit requirements generally include major sources that emit or have the potential to emit 100 tons per year of any regulated pollutant, plus stationary and area sources that emit or have potential to emit lesser specified amounts of hazardous air pollutants. However, in nonattainment areas, the permit requirements also include sources which emit as little as 50, 25, or 10 tons per year of VOCs, depending on the severity of the region’s nonattainment status (serious, severe, or extreme)..

States were required to develop permit programs and to submit those programs for EPA approval by November 15, 1993. EPA had one year to approve or disapprove a state’s submission in whole or in part. After the effective date of a state plan, sources had 12 months to submit an actual permit application..

States are to collect annual fees from sources sufficient to cover the “reasonable costs” of administering the permit program, with revenues to be used to support the agency’s air pollution control program. The fee must be at least $25 per ton of regulated pollutants, excluding carbon monoxide (adjusted for inflation). Permitting authorities have discretion not to collect fees on emissions in excess of 4,000 tons per year and may collect other fee amounts, if appropriate..

The permit states how much of which air pollutants a source is allowed to emit. As a part of the permit process, a source must prepare a compliance plan and certify compliance. The term of permits is limited to no more than five years; sources are required to renew permits at that time. State permit authorities must notify contiguous states of permit applications that may affect them; the application and any comments of contiguous states must be forwarded to EPA for review. EPA can veto a permit; however, this authority is essentially limited to major permit changes. EPA review need not include permits which simply codify elements of a state’s overall clean air plan, and EPA has discretion to not review permits for small sources. Holding a permit to some extent shields a source from enforcement actions: the act provides that a source cannot be held in violation if it is complying with explicit requirements addressed in a permit, or if the state finds that certain provisions do not apply to that source.

Title VI - Stratospheric Ozone Protection

Title VI of the 1990 CAA Amendments addresses stratospheric ozone depletion. It provides for the phase-out of certain ozone depleting substances and reflects the Montreal Protocol on Substances that Deplete the Ozone Layer. Title VI identifies certain class I (e.g., chlorofluorocarbons) and class II (e.g., hydrochlorofluorocarbons) ozone depleting substances. The Protocol and CAA impose limits on the production and consumption of class I and II substances according to specified schedules.

Title VI and regulations issued there under also call for reduction in the use and emission of class I and II substances to the lowest achievable level; ban nonessential products containing class I substances or containing or manufactured with class II substances; contain requirements for the servicing of motor vehicle air conditioners; require warning statements on containers of, and products containing or manufactured with, class I or II substances; and promote the use of substitutes for ozone-depleting substances that reduce overall risks to human health and the environment.

Section 606(a)(3) provides that the Environmental Protection Agency shall adjust phase-out schedules for ozone-depleting substances in accordance with any future changes in Montreal Protocol schedules. As a result, the phase-out schedules contained in Title VI for various ozone-depleting compounds have now been superseded by subsequent amendments to the Montreal Protocol.

Since passage of Title VI, ozone-depleting substances such as CFCs, methyl chloroform, carbon tetrachloride, and halons (referred to as Class 1 substances) have been phased out by industrial countries, including the United States. New uses of hydrochlorofluorocarbons (HCFCs) (called Class 2 substances under Title VI) are banned beginning January 1, 2015, unless the HCFCs are recycled, used as a feedstock, or used as a refrigerant for appliances manufactured prior to January 1, 2020. Production of HCFCs is to be frozen January 1, 2015, and phased out by January 1, 2030. Exemptions consistent with the Montreal Protocol are allowed.

The EPA is required to add any substance with an ozone depletion potential (ODP) of 0.2 or greater to the list of Class 1 substances and set a phase-out schedule of no more than seven years. For example, methyl bromide (ODP estimated by EPA at 0.7) was added to the list in December 1993, requiring its phaseout by January 1, 2001; this decision was altered by Congress in 1998 to harmonize the U.S. methyl bromide phase-out schedule with the 2005 deadline set by the parties to the Montreal Protocol in 1997. Also, EPA is required to add any substance that is known or may be reasonably anticipated to harm the stratosphere to the list of Class 2 substances and set a phase-out schedule of no more than ten years.

Title VI contains several implementing strategies to avoid releases of ozone-depleting chemicals to the atmosphere, including (1) for Class 1 substances used as refrigerants—lowest achievable level of use and emissions, maximum recycling, and safe disposal required by July 1, 1992; (2) for servicing or disposing refrigeration equipment containing Class 1 and 2 substances—venting banned as of July 1, 1992; (3) for motor vehicle air conditioners containing Class 1 or 2 substances—recycling required by January 1, 1992 (smaller shops by January 1, 1993); (4) sale of small containers of class 1 and 2 substances—banned within two years of enactment; and (5) nonessential products—banned within two years of enactment.

Title VI also has some provisions covering global warming, which focus mostly on conducting studies.

Note: Significant potions of this article were extracted from from the Congressional Research Service Report RL30798, Environmental Laws: Summaries of Major Statutes Administered by the Environmental Protection Agency by David M. Bearden, Claudia Copeland, Linda Luther, James E. McCarthy, Linda-Jo Schierow, and Mary Tiemann (October 8, 2010).

Further Reading



(2012). Clean Air Act, United States. Retrieved from


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