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Air quality in the United States is largely ensured through the federal Clean Air Act, which establishes a regulatory framework designed to prevent and curtail air pollution. The Act divides duties between the federal and state governments, and while the federal government has the ultimate authority, most of the Act is implemented by state agencies.

This regulatory framework largely revolves around EPA setting ambient air quality standards that states must achieve. States create implementation plans that direct efforts to maintain or attain these standards. To help reduce pollution, the Act also provides pre-construction review programs and categorical (for categories of sources like power plants, refineries, etc.) emissions limitations for new and modified stationary sources and also provides for a single, federal air permit. Finally, the Act regulates vehicles, acid rain, and stratospheric ozone separately. Taken as a whole, the regulatory framework seeks to maximize reliance on state regulation of air pollutants that harm human health and the environment only from major sources of pollution.

Watch or listen to and download materials from the ELI Summer School Seminar Clean Air. For a detailed discussion of the Clean Air Act and how it works, see Arnold Reitze, Air Pollution Control and Climate Change Mitigation Law 2d. ed.

ELI members can listen to and download materials from a recent ELI seminar on the Clean Air Act, Recent Air Regulations: What Picture Will the Jigsaw Pieces Create?

Air Quality Standards

The Clean Air Act charges the Environmental Protection Agency (EPA) with setting “national ambient air quality standards” or NAAQS for air pollutants that are widespread, emitted by numerous sources, and harmful to human health or welfare.

A NAAQS is the average concentration of a pollutant in the outdoor air over a specified time period sufficient to ensure an absence of adverse effects on human health or welfare. As the name implies, NAAQS are national standards: they are uniform nationwide and do not vary by region or state. Courts have read the statute to require EPA only to consider health effects and not costs or technological feasibility of achieving limits when setting a NAAQS. Once EPA has determined that a pollutant is widespread, emitted by numerous sources, and harmful to human health or welfare, it has a nondiscretionary duty to regulate it.

ELI members can listen to and download materials from the ELI seminar Setting Secondary NAAQS to Protect the Environment.

EPA has determined that six pollutants or classes of pollutants are so-called “criteria pollutants” for which NAAQS should be set:

EPA must update NAAQS every five years, but political and regulatory delays and litigation often delay these mandated updates.

For articles on NAAQS, see Ami Grace, The Clean Air Act 2005 Severe Ozone Nonattainment Deadline: A Prime Opportunity to Realize the Goals of the Clean Air Act and Craig Oren, The Supreme Court Forces a U-Turn: The Fate of American Trucking

State Implementation Plans

The Clean Air Act requires all areas in the United States to meet the national ambient air quality standards. Once EPA sets a NAAQS, states then designate areas within each state, on roughly a county basis, as either in attainment or not in attainment of the standard. Areas not in attainment are called nonattainment areas, while areas that meet the standard are called attainment areas. States must submit a plan to the EPA, called a state implementation plan or SIP, describing how nonattainment areas will come into attainment and how attainment areas will remain in compliance as the area grows and changes. The deadline for NAAQS attainment is generally five years, but this can vary and is frequently not met.

SIP development is a complicated process. States must determine not only how much pollution is in the air at the time, but also project the impact on emissions of economic growth in the future. It then has to use air quality models to prove that the proposed SIP actions will result in attainment and maintenance of NAAQS, taking into account future growth. SIPs can contain a variety of control and policy measures (see the EPA's SIP Status and Information and Menu of Control Measures) to reduce and prevent air pollution, ranging from requiring new or modified facilities to meet certain emission standards to transportation planning.

Every time EPA issues a new or updated NAAQS, states have to submit a new SIP for EPA approval. Failure to do so can lead to sanctions, such as a cut in federal funding. Further, EPA must also step in and develop a federal implementation program if a state fails to write an acceptable SIP.

For a discussion of the contentious interaction between a state and EPA on air permitting issues, listen to and download materials from the ELI seminar Texas Commission on Environmental Quality and EPA Air Permitting: A Way Forward.

Many of the criteria pollutants easily move from state to state, yet states cannot regulate out-of-state pollution sources. For example, some of the chronic pollution in Baltimore results from emissions in Virginia, Washington, DC, and even Midwestern states. Therefore, the Clean Air Act has so-called transport provisions that allow state agencies and EPA to address such pollution transport.

ELI members can listen to and download materials from a recent ELI seminar on the Clean Air Act’s transport provisions, From Clean Air Transport to Cross-State Air Pollution: EPA’s New Rule.

State Implementation Plans are discussed in detail in the Law of Environmental Protection, §§12:8-12:58.

Pre-Construction Review Programs

In order to help states control air pollution through their SIPs, pre-construction review programs require new or modified sources of air pollutants to receive permission from the state before commencing construction.

The most prevalent of these programs is called new source review or NSR. NSR applies to new or modified sources that will create significant emissions of criteria pollutants. States must implement a procedure requiring anyone who plans to build a new major source to show prior to construction that the project complies with the SIP emissions limits and to give the public advance notice of the project.

New source review helps with both maintenance in attainment areas and progress towards achievement of NAAQS in polluted areas. New or modified major sources in NAAQS attainment areas must comply with the applicable emission limits designed to prevent deterioration in air quality—hence the program is known as prevention of significant deterioration or PSD. Such sources in nonattainment areas, on the other hand, must satisfy “offset” requirements. The specific requirements vary greatly by state, but the core requirement is the same: any proposed new or modified major source must be able to show that total emissions from all sources in the region will be less than they were prior to application for a permit. The notion is that it forces a non-attainment area’s total stock of pollution producing facilities to become progressively cleaner over time, thus helping to bring the area into attainment with the NAAQS.

New Source Review has been discussed and explained widely in these ELR articles: Rolf von Oppenfeld, A Primer on New Source Review and Strategies for Success, Arnold Reitze, New Source Review: Should it Survive? and Shi-Ling Hsu, The Real Problem with New Source Review.

Categorical Emission Standards

In order to help control air pollution, EPA and the states also implement categorical emission standards to ensure that new facilities and modifications of existing facilities result in lower or acceptable levels of air pollution.

The new source performance standards (NSPS) require certain newly built or modified facilities to meet specified emissions limitations. Section 111 of the Clean Air Act directs EPA to list categories of industries that create pollution, such as petroleum refining, textile mills, or power plants, including all “major sources.” EPA determines the appropriate NSPS by first identifying the best adequately demonstrated technologies for cutting pollution, accounting for cost, energy use, and other environmental side effects. Then EPA determines the amount of pollution reduction that can be achieved by using that technology and then uses that amount as the basis for the NSPS limit. Although an NSPS is calculated by considering pollution reduction technology, the Act itself does not require new sources to adopt any particular technology; compliance is still determined by measuring emissions. States may adopt the NSPS calculated by EPA or adopt their own NSPS standards, provided they are equivalent to or more stringent than the EPA standard.

With the 1990 amendments to the Clean Air Act, Congress added a second categorical emissions limitation—the national emission standards for hazardous air pollutants or NESHAPs. The NESHAPs require major and area sources to install maximum achievable control technology (MACT) on certain sources of hazardous air pollutants. The MACT program is not tied to ambient air quality standards because hazardous air pollutants are not criteria pollutants and there are no NAAQS for them. For the 187 hazardous pollutants designated by the Act, EPA surveys existing pollution control technologies and sets emission limits for each pollutant at the lowest achievable level, also taking into consideration costs and other health and environmental impacts.

NSPS is discussed and critiqued in these ELR articles: Arnold Reitze, EPA’s Proposed New Source Performance Standards to Control Greenhouse Gas Emissions From Electric Utility-Generating Units and Scott Segal, New Source Performance Standards for Global Greenhouse Gas Emissions From the Power and Refining Sectors: Wrong Mechanism at the Wrong Time.

Acid Rain Program

The acid rain program is one of the most well-known successes of the Clean Air Act. This program applies only to fossil-fuel fired utility power plants, which produce the bulk of the two classes of chemicals that cause acid rain—sulfur dioxide (SO2) and nitrogen oxides (NOx). The 1990 amendments to the Act implemented an emissions trading approach to reduce these compounds. Although this is a “market-based” approach, it is important to understand that the market here, like many markets, is created by and made possible by government regulation. EPA sets a nationwide limit on the total pollutant emissions allowable, and then assigns transferrable emissions allowances to power plants so that the allowances add up to the national cap. The plants are only permitted to emit up to their allowance, but are allowed to purchase additional allowances from other plants if they cannot reduce emissions. In this way power plants are that can more easily mitigate their SO2 and NOx emissions benefit financially from the reduction by being able to sell their allowances. The innovative program has achieved large reductions in SO2 and NOx emissions and helped combat acid rain at an economically efficient cost.

For a free report on how the acid rain program informs greenhouse gas controls, see Implementing an Emissions Cap and Allowance Trading System for Greenhouse Gases: Lessons from the Acid Rain Program. For a review of EPA’s acid rain program, see Dallas Burtraw, A New Standard of Performance: An Analysis of the Clean Air Act's Acid Rain Program and Curtis Moore, The 1990 Clean Air Act Amendments: Failing the Acid Test.

Stratospheric Ozone

Unlike ground-level ozone, which can harm human health and the environment, ozone in the stratosphere helps deflect some ultraviolet light from the sun and offers a protective effect. When it was learned that some chemicals were destroying stratospheric ozone, the United States joined other nations by ratifying the Montreal Protocol on Substances that Deplete the Ozone Layer. This obligated the nation to reduce production of various classes of ozone-destroying chemicals.

The Clean Air Act contains provisions to implement this treaty and control ozone-depleting substances. EPA has implemented a regulatory program to identify ozone-depleting substances, track them, and reduce or eliminate their use. The statute sets timetables for the total phaseout of all nonessential uses of listed compounds and encourages finding replacement substances. EPA, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration also monitor stratospheric ozone and report to Congress on the effectiveness of the Clean Air Act in stopping depletion.

For a discussion of stratospheric ozone depletion, the chemicals that cause it, and climate change, see chapter 10 of Reporting on Climate Change: Understanding the Science. For ELR articles on stratospheric ozone, see Theodore Garrett, Stratospheric Ozone Protection” and Cass Sunstein, Of Montreal and Kyoto: A Tale of Two Protocols.


The 1990 Clean Air Act amendments provided for a single, comprehensive operating permit under the Act called a Title V permit. The permitting requirements apply to a wide variety of major, area and other sources depending on the type of source, whether the source is in an attainment area, and the locally-applicable regulations. Congress incorporated operating permits in the Act following the model of the Clean Water Act as a streamlining measure to make it easier for polluters, regulators, and the public to determine which emissions limits apply to any given source and to eliminate duplicative or conflicting requirements under the various programs. As with most of the Clean Air Act’s provisions, operating permits are usually issued by states, not federal EPA.

Title V is discussion in the Law of Environmental Protection, §§ 12:86-:87. For an ELR article summarizing EPA’s implementation of Title V, see David Novello, The New Clean Air Act Operating Permit Program: EPA’s Final Rules.

Mobile Sources

Title II of the Clean Air Act deals with air pollution caused by mobile sources: cars, trucks, and other vehicles. Like those for stationary sources, the Act creates areas for federal and for state control aimed at achieving NAAQS and other air pollution objectives. Mobile sources are a major source of criteria pollutants in many nonattainment areas. EPA is authorized to set emissions standards for newly manufactured vehicles. The standards are based on available technology, environmental factors, cost-effectiveness, and other factors.

California has traditionally been a leader in controlling motor vehicle pollution. In recognition of this, Congress allows California to set its own mobile source standards, provided they are http://epa.gov/oms/climate/ca-waiver.htm ">approved by EPA. Other states must adopt either the federal or California mobile source standards—they may not adopt their own.

Listen to an ELI seminar Ports, Shipping and Air Emissions.

Mobile source regulation is discussed in detail in Law of Environmental Protection, §§ 12:120-:163. For ELR articles on mobile sources and fuels, see Jocelyn D’Ambrosio, Alternative Fuels: An Evaluation of Corn Ethanol, Cellulosic Ethanol, and Gasoline and Jonathan Martel, The Explosion of Clean Air Act Regulation of Fuel.

The Act also provides for federal regulation of fuels and fuel additives because they can significantly affect vehicle emissions. EPA requires registration of a fuel before it can be marketed and may prohibit or limit those that cause or contribute to air pollution or health problems.

With respect to state regulation of mobile sources, the Act requires that each state implementation plan include a transportation control plan (TCP). TCPs can include programs like the creation of dedicated bus or carpool lanes, toll roads, or other schemes designed to reduce the amount of pollution caused by driving. Heavily polluted areas are also required to implement vehicle inspection and maintenance programs. Inspection and maintenance programs identify existing, older vehicles that might need repairs to their emissions control components.