Primary standards are numerical limits of allowable air and water pollutants designed to protect human health but not necessarily other parts of the environment. Primary standards differ from secondary standards, which are those that protect against all adverse effects on the environment, such as those on animals and vegetation. While the term primary standards usually relates to air pollution or air quality, there are federal primary standards for the drinking water supply.

The Clean Air Act is the major piece of legislation that protects and enhances the nation's air quality. As part of the act, Congress required the Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) for air pollutants, which were characterized by wide dispersal and emission from many sources.

The NAAQS, or primary air quality standards, define the level of air quality to be achieved and maintained nationwide for six criteria...