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) which makes it possible to characterise in a simple and comprehensive way the air quality of an urban agglomeration. It consists of 4 sub-indices, each representative of an air pollutant: \- fine particles (PM10) \- ozone (O3) \-Nitrogen dioxide (NO2) \- sulphur dioxide (SO2) The highest sub-index determines the index of the day. It shall be calculated on the basis of 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 100,000 inhabitants. For more information, see the Air Quality Index Order of July 22, 2004. For smaller agglomerations, the calculation of this indicator remains possible, but this is 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 100,000 inhabitants), \- the IQA index in Cholet, La Roche-sur-Yon and Laval (agglomerations of less than 100,000 inhabitants). Air Pays de la Loire makes daily a forecast of the Atmo index for the same day and the next day based on air quality forecast models, meteorological data, measurement station results and local expertise. Each day, the index of the day before is recalculated from the measured concentrations of the 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.