http://airnow.gov/index.cfm?action=aqibroch.index
Office of Air and Radiation, U.S. Environmental Protection Agency
This booklet defines the AQI (Air Quality Index) and how it is used to obtain air quality information. Information about the possible health effects of major air pollutants at various levels are included as are suggestions for how consumers can protect their health when pollutants in their area reach unhealthy concentrations. Document may also be viewed in PDF format.