Pollutants that can injure health, harm the environment and cause damage to property are called criteria air pollutants by the EPA. They are regulated by first developing health-based criteria that are science-based guidelines as the basis for setting permissible levels. One set of limits called primary standards protects health and another set of limits called secondary standards are meant to prevent environmental and property damage. A geographic area that meets or does better than the primary standards is called an attainment area and areas that do not meet these standards are called nonattainment areas.

Even though the EPA has been regulating the criteria air pollutants, there are many urban areas classified as nonattainment areas for at least one criteria air pollutant.

The Clean Air Act gives the EPA the power to regulate the amount of pollution a certain place may have. The EPA and state governors cooperate to identify nonattainment areas for each criteria air pollutant. These areas are classified according to how badly they are polluted. There are five classes of nonattainment areas for smog, ranging from relatively easy to clean up quickly called marginal to B>extreme which means it will take a lot of work and time to clean up.

The Clean Air Act uses this classification system to tailor clean-up requirements to the severity of the pollution and to set realistic deadlines for reaching clean-up goals. If a deadline is missed, the law allows more time to clean up but usually this new deadline will have to meet stricter clean-up requirements set for more polluted areas.

In 1970, the Clean Air Act gave EPA the authority to list air toxics for regulation and then to regulate the chemicals. The agency listed and regulated seven chemicals through 1990. The 1990 Act includes a list of 189 hazardous air pollutants selected by the Congress on the basis of potential health and/or environmental hazards. The EPA must regulate these listed air toxics and is also allowed to add any new chemicals to the list if necessary. To regulate the air pollutants, the EPA must identify categories of sources that release the 189 chemicals listed in the 1990 Act. Once these categories are listed, the EPA issues regulations. Regulations for major sources must be issued first and then for the smaller sources. These are just a few of the many ways the EPA works with the government and the 1990 Clean Air Act to reduce air pollution.