A relative scale developed by the U. S. Environmental Protection Agency that applies to specified air pollutants (ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and particulates), designed to clearly and simply inform the public of the current air quality. The scale is normalized such that an index value of 100 corresponds to the U. S. Federal air- quality standard for each specified pollutant. The minimum and maximum values are 0 and 500, corresponding to perfectly clean and extremely dirty air. Instead of presenting to the public different numbers for different pollutants, usually only the worst value is presented. The relationship between the pollutant standards index and health advisory level are: 0–50 Good; 50–100 Moderate; 100– 200 Unhealthful; 200–300 Very unhealthful; >300 Hazardous. See air pollution episode.