A more thorough explanation:

The Clean Air Act (CAA) is a law that sets national standards for air pollution and its sources. It has two parts: regulating air quality levels and regulating sources of pollution. The Environmental Protection Agency (EPA) is responsible for enforcing the CAA.

The CAA requires the EPA to identify and regulate pollutants that endanger public health and are emitted from diverse sources. These are called "criteria pollutants." Currently, there are six criteria pollutants:

• Ozone
• Carbon monoxide
• Sulfur dioxide
• Particulate matter
• Nitrogen dioxide
• Lead

The EPA regularly considers 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 that protects public health with an adequate margin of safety.

Areas that comply with all NAAQS criteria pollutant standards are called Attainment Areas. Places that fail to comply with at least one criteria pollutant standard are called 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 (SIP) 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. If the EPA feels a proposed SIP will not achieve the NAAQS requirements, they can establish a Federal Implementation Plan (FIP) and mandate the state follow it.

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 require Best Available Control Technology (BACT). Non-Attainment areas require 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 tend to emit a large quantity of carbon dioxide, a non-criteria pollutant. Nonetheless, the Supreme Court held in Mass v. EPA that the EPA can regulate carbon dioxide emissions from mobile sources and is required to do so.

For example, if a factory in a Non-Attainment Area emits too much sulfur dioxide, the state must create a plan to reduce the emissions to meet the NAAQS standards. The state can require the factory to install new pollution control technology to reduce emissions. If the state's plan is not effective, the EPA can create a plan and require the state to follow it.

Another example is regulating the emissions from cars. The EPA sets standards for the amount of pollutants that can come out of a car's tailpipe. The EPA also sets standards for the fuel that goes into the car. This helps reduce the amount of pollution that cars emit into the air.