EPA uses an air quality index (AQI) summary to give an overall measurement of air quality at each air monitoring site. The lower the index, the better the air quality is.
There are five AQI categories:
| Category |
Index range |
| Very good (VG) air quality |
0–33 |
| Good (G) air quality |
34–66 |
| Fair (F) air quality |
67–99 |
| Poor (P) air quality |
100–149 |
| Very poor (VP) air quality |
150 or greater |
Black-and-white icons with letters on the EPA AirWatch map show the overall AQI category for each air monitoring station, from ‘very good’ (VG) to ‘very poor’ (VP). The AQI summary (as a number) is also indicated at the bottom of the popup box. This makes it easy to compare air quality at different locations.
See
Calculating a station air quality index for more information.