Air Quality Index (AQI)

How is Air Quality Index (AQI) computed?

In Europe since 2017 (and what CAMS use for Euronews AQI products), the following source of AQI is used: 

The computation of AQI is explained under "About European Air Quality Index" link (see in the Country and Stations selection box on the right-hand side at the top of the page).

In the United States, the U.S. Environmental Protection Agency (EPA) has published a document on the reporting of daily air quality:

but there are almost as many computation formulas as countries!