The U.S. Environmental Protection Agency (EPA), pursuant to the 1977 Amendment of the Clean Air Act, established National Ambient Air Quality Standards (NAAQS) for six "criteria pollutants" considered harmful to public health and welfare:

Lead is currently the only criteria pollutant that is not monitored and reported with the Air Quality Index (AQI).

Two types of NAAQS were defined -- primary and secondary:

• "Primary" standards are designed to establish limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly.
• "Secondary" standards set limits to protect public welfare, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.

The AQI helps you you understand what local air quality means to your health. To make the AQI as easy to understand as possible, the EPA has divided the AQI scale into six categories. Each category corresponds to a different level of health concern.

• View the EPA's Air Quality Index standards guide