EPA's air quality monitoring network indicates that over 170 million people live in counties with unhealthy air due to one or more criteria air pollutants. In counties where pollutant concentrations are monitored, Scorecard presents a single uniform measure of air quality, called the Air Quality Index. The AQI indicates the percentage of days in a year when an area experiences good, moderate, or unhealthful air quality. The AQI also provides the basis for ranking areas by their air pollution levels.
An area that persistently fails to meet air quality standards can be designated as a nonattainment area for one or more pollutants. Presently, over 100 nonattainment areas have been designated by the U.S. Environmental Protection Agency. Most are located in metropolitan areas, where air quality problems are more severe. The Clean Air Act requires states that have nonattainment areas to submit a plan that will reduce pollutant emissions and concentrations and ensure compliance with national standards.