EPA calculates an air quality index (AQI) for each air monitoring station using the following steps.
1. Calculating pollutant averages
Depending on the pollutant, the data from EPA’s monitoring stations is averaged over different time periods. These averages are calculated every hour and shown on the EPA AirWatch map.
A: For hourly updates shown on the map – uses the selected hourly average (defaults to the latest available reading).
For daily and weekly reports – uses the maximum of the preceding 24 one-hour averages.
B: For hourly updates shown on the map – uses the eight hours up to the selected hour (defaults to the latest eight-hour average).
For daily and weekly reports – uses the maximum of the preceding 16 rolling eight-hour averages.
C: For hourly updates shown on the map – the selected hourly average PM10 (defaults to the latest available reading), divided by a derived one-hour standard of 80 µg/m3.
For daily and weekly reports – uses an average of the preceding 24 hours of PM10 readings, divided by the 24-hour air quality standard of 50 µg/m3
D: Hourly updates for PM2.5 shown on the map are provided both as the latest rolling 24-hour average and 1-hour average. Currently PM2.5 data is not included in the daily and weekly reports.
2. Calculating a pollution index
Each average pollutant measurement is converted into a pollutant index.
A pollutant’s index is its concentration expressed as a percentage of the relevant air standard:
An index value of 100 means the pollutant is currently at a concentration equal to the corresponding air quality standard.
Air quality standards are designed to protect human health and the environment, and are taken from one of the following:
3. Calculating an AQI summary for an air monitoring station
Every hour, the different pollutant indexes at each monitoring station are compared. The highest pollutant index at each site is taken as the air quality index summary for that air monitoring station for that hour. For example, if the pollutant indexes at an air monitoring station was –
then the AQI summary for that station at that time would be 79, which corresponds to the ‘fair’ category. This would show as a black-and-white ‘F’ icon on the EPA AirWatch map.
|Very good (VG) air quality
|Good (G) air quality
|Fair (F) air quality
|Poor (P) air quality
|Very poor (VP) air quality
||150 or greater