Air Quality Index (AQI)  -  A nationally calculated index for reporting and forecasting daily air quality presented as a color-coded scale that ranges from Green or Good (0 to 50) through to Maroon or Hazardous (301 to 500). As the AQI value increases, the percentage of the population increases of those who are likely to experience increasingly severe adverse health effects. The AQI is based on the four most common air pollutants regulated under the Clean Air Act: ground-level ozone (O3), particle pollution (PM10 and PM2.5), carbon monoxide (CO), sulfur dioxide (SO2) and nitrogen dioxide (NO2).