To help protect the country’s air quality, the federal government established the Clean Air Act (originally in 1963, then major revisions in 1970 and 1990). Under the Act, both primary and secondary standards were created to help control the amount of harmful pollutants released into the air.
Primary standards set limits that protect the public’s health, including sensitive populations like asthmatics, children, and the elderly. Secondary standards set limits that help protect against damage to animals, crops, vegetation, buildings, and other general concerns.
The government sets National Ambient Air Quality Standards (NAAQS) for six principal pollutants (called "criteria" pollutants). You can learn more about them by downloading this NAAQS fact sheet.