The Atmo index or multipollutant urban air quality index is a daily indicator intended to supply composite information on air quality in conurbations of over 100,000 inhabitants. It does not allow to bring to light localized phenomena: pollution in industrial, automobile nearness … It is representative of the global air quality on the whole town. It is calculated each day by accredited air quality monitoring associations (AASQA) on the basis of air concentrations in four pollutants: sulphur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3) and particles in suspension (PM10). For each of these pollutants, a sub-index is calculated on the basis of their concentrations and on a specific scale. It is based on a scale from 1 (very good) to 10 (very bad). The Atmo index of the day is equal to the highest of the 4 sub-indices.
Usually, the published data concerns the percentage or the number of days on which air quality was :
- very good, good (Atmo index: 1 to 4) ;
- average (index:5) ;
- mediocre, bad, very bad (index of 6 or higher).
Source: SOeS (site : http://www.statistiques.developpement-durable.gouv.fr)