Air Quality – ATMO Index – Pays de la Loire – 2017/2020

The air quality index is a daily indicator graduated from 1 (very good) to 10 (very bad) that makes it possible to characterise in a simple and comprehensive way the air quality of an urban agglomeration. It consists of 4 sub-indexes, each representative of an air pollutant: \- fine particles (PM10) \- ozone (O3) \- Nitrogen dioxide (NO2) \- sulphur dioxide (SO2) The highest sub-index determines the day’s index. It is calculated from measurements of stations representative of background pollution. It does not take into account proximity phenomena (automotive or industrial). The Atmo index is defined at national level for agglomerations with more than 100000 inhabitants. For more information, see the Order of 22 July 2004 on air quality indices. For smaller agglomerations, the calculation of this indicator remains possible but it is then the simplified Air Quality Index (AQI) calculated according to the same rules but with the possibility of reducing the number of pollutants monitored. In Pays de la Loire, the air quality index is calculated for 7 agglomerations: \- the Atmo index in Angers, Le Mans, Nantes and Saint-Nazaire (agglomerations with more than 100000 inhabitants), \- the IQA index in Cholet, La Roche-sur-Yon and Laval (agglomerations of less than 100000 inhabitants). Air Pays de la Loire daily makes a daily forecast of the Atmo index of the same day and the next day based on air quality forecasting models, weather data, measurement station results and local expertise. Each day, the day-to-day index is recalculated from the measured concentrations of pollutants taken into account in the calculation of the index over the whole day. There may therefore be differences between the expected index and the index calculated for the same date.