The indicator measures the percentage of population in urban areas with PM10 concentrations exceeding the daily limit value (50 µg/m3). This daily limit should not be exceeded on more than 35 days in a calendar year. The exposure to PM10 pollution is estimated based upon PM10 measured at all urban monitoring stations, including urban and suburban traffic station types. The air quality data is collected by the European Environment Agency on an annual basis according to the Exchange of Information Decision (97/101/EC amended by the Commission Decision 2001/752/EC). All data is stored in AirBase, the European air quality database, which has data back to 1999. Information on cities (used to weight the data by population) is obtained from the Urban Audit data. Since 2008, the data has covered the EU 27 except Greece, Malta and Cyprus. The methodology for the calculation of the indicator is developed by the European Environment Agency.