Air Quality in Europe provides data for the roadside and background air quality situation in European cities in a comparable and easily understandable way as all detailed measurements are transformed into a single relative figure: the Common Air Quality Index (or CAQI).

Three different indices have been developed to enable the comparison of three different time scales:

1. An hourly index which describes the air quality today, based on hourly values, updated every hour;
2. A daily index which indicates the general air quality situation of the previous day, based on daily values, updated once a day; and
3. An annual index which represents the city's general air quality conditions throughout the year and compares this to European air quality norms.

This index is based on the pollutants' yearly average compared to annual limit values, updated once a year.