Under the Clean Air Act, EPA was required to establish a nationally uniform air quality index for the reporting of air quality. In 1976, EPA established this index, then called the Pollutant Standards Index ( Later renamed the Air Quality Index), for use by state and local communities across the country.

The Index provides information on pollutant concentrations for ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The Index is "normalized" across pollutants so that an Index value of 100 represents the level of health protection associated with the health-based standard for each pollutant and an Index value of 500 represents the significant harm level.

This Index has been adopted internationally and is used around the world to provide the public with information on air pollutants.

On July 18, 1997, EPA revised the ozone and particulate matter standards, in light of a comprehensive review of new scientific evidence. EPA replaced the 1-hour ozone standard with an 8-hour ozone standard, and supplemented the particulate matter standard with 24-hour and annual standards for fine particulate matter. The revised Index was developed through extensive coordination with public information, health, and air quality experts from state and local agencies, as well as input from the general public through EPA-sponsored focus groups.

For example, EPA sponsored eight focus groups in major U.S. cities to help evaluate how to most effectively communicate air quality and health effects information. Numerous state and local agencies and associations also participated through workshops to provide comment on the Index revisions.
Source: EPA