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The French Ministry of the Environment and the AASQAs have developed an index, called "ATMO", to qualify air quality in urban areas with more than 100,000 inhabitants. For cities with fewer than 100,000 inhabitants, the index is a simplified air quality index (AQI). The ATMO index has regulatory status, and its calculation methods are specified in the decree of July 22, 2004 on air quality indices. It is determined from four sub-indices calculated individually from the levels reached by four typical pollutants, measured at selected stations reflecting background pollution: SO ₂ , NO ₂ , O ₃ and PM _...