AQI Defined

US EPA’s ­Air Quality Index (AQI) is the method used by the EPA to report air quality on a real-time basis. The EPA’s AQI is also referred to as the “NowCast” and represents data collected overtime periods of varying length in order to reflect present conditions as accurately as possible.  The EPA has assigned a specific color to each AQI category to enable a quick assessment of air quality in the community. 

There are many apps and sites that offer AQI levels, including the "BreezoMeter" associated with the iPhone weather app; however, for the most accurate and official AQI information, visit the EPA's AirNow.gov website.