Air pollution is a complex mixture of compounds. Two types of pollutants are distinguished. Primary pollutants are emitted directly from a source (cars, industries, etc.). Secondary pollutants, such as ozone, are formed in the atmosphere through chemical reactions. Pollutants classified as indicators provide an overall picture of air pollution. The most commonly monitored pollutants are fine particles with a diameter of less than 10 µm (PM10) or 2.5 µm (PM2.5), ozone, nitrogen dioxide (NO2), and sulfur dioxide (SO2). In the Rhône-Alpes region, monitoring is carried out by the Air Rhône-Alpes (R.A.) Association for Air Quality Monitoring (AASQA).