The Air Quality Index is a measure of five major air pollutants calculated by the Environmental Protection Agency. The Clean Air Act regulates ground-level ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide and particle pollution - also called particulate matter. The AQI measures these pollutants in a range from zero to 500. Anything above 100 on the scale (to the right of the yellow MODERATE level) represents a health concern, according to the EPA. Learn more at the AIRNow Web site.

SOURCE: AIRNow, a cross-agency U.S. Government Web site