Clean Air Act (CAA)

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The Clean Air Act (CAA), codified in U.S.C. Title 42, Chapter 85, is a central piece of legislation in the field of environmental law which promulgates uniform national standards for a wide range of air pollutants and sources. The CAA utilizes a two-prong approach - in addition to regulating the air quality levels, it also regulates sources of pollution

The CAA requires the Environmental Protection Agency (EPA) to establish a set of “criteria” pollutants, or pollutants that endanger the public welfare and are emitted from numerous/diverse sources. Currently, there are 6 criteria pollutants

  • ozone
  • carbon monoxide
  • sulfur dioxide
  • particulate matter
  • nitrogen dioxide 
  • lead

Additionally, under CAA §108, the EPA is required to regularly consider adding pollutants to the criteria list. 

Criteria pollutants are subject to National Ambient Air Quality Standards (NAAQS). NAAQS regulations require any given criteria pollutant to stay below a level to "protect the public health" within an "adequate margin of safety". While the EPA is mandated by the CAA to set and regulate criteria pollutants, courts have concluded that the EPA has discretion in determining what is considered an adequate margin of safety (See: Lead Industries Ass'n v. EPA). 

There are two types of NAAQS: Primary standards (which protect human health) and secondary standards (which protect the public welfare, which includes effects on animals, wildlife, water and visibility).

Areas that successfully comply with all NAAQS criteria pollutant standards are known as Attainment Areas. Places that fail to comply with at least one criteria pollutant standard are known as Non-Attainment Areas. Attainment Areas and Non-Attainment Areas are subject to different standards and regulations. 

The CAA requires all states to create a State Implementation Plan (SIPs) detailing how they intend to force their Non-Attainment areas to achieve the NAAQS standards. States have a wide range of freedom in which regulatory measures/non-regulatory measures they include in their SIPs to meet the required standards. States have the discretion to require a higher level of air quality in the SIPs but may not go below the federal standard. If the EPA feels a proposed SIP will not achieve the NAAQS requirements, can instead establish a Federal Implementation Plan (FIP) and mandate the state follow it

Furthermore, pollution regulation under the CAA depends on whether the source emitting the pollution is stationary or mobile. 

For stationary sources, the level of pollution control technology required depends on if the area is an attainment area or a non-attainment area.

  • Attainment areas are focused on Prevention of Significant Deterioration (PSD). In other words, the standards must be high enough to maintain their high quality of air. These areas require Best Available Control Technology (BACT).
  • Non-Attainment areas are focused on cleaning up the air by requiring higher standards. New / modified major sources of air pollution must employ control technology with the lowest achievable emission rate (LAER), and existing sources must use reasonably available control technology (RACT). 

For mobile sources of air pollution, the EPA focuses on both the fuel going into the vehicle, and the emissions that are coming out. Mobile sources of pollution are unique because they tend to emit a large quantity of carbon dioxide, a non-criteria pollutant. Nonetheless, the Supreme Court held in Mass v. EPA that not only could the EPA regulate carbon dioxide emissions from mobile sources, but they are required to do so.   

For more information, see the EPA Website:

[Last updated in July of 2022 by the Wex Definitions Team]

See EPA Website: