In cities which are covered by both European Environment Agency (EEA) and the AV database, data from each database is compared. AirVisual compared 2 factors: data availability (calendar & avg. daily) as the first priority, together with number of stations in the city. AirVisual then chose to use the city data from either EEA or AV database depending on which gave better coverage. AV data was used in preference to EEA data for approx. 219 cities within Europe. This is because in some cities (e.g. Paris, London), the EEA data had far fewer stations than the AV database, and in many cities the EEA data had lower availability overall.