Over the years 1996-2005 the total population, for which exposure estimates are made, increased from 60 to 136 million people due to an increasing number of monitoring stations reporting air quality data
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Data prior to 1996 with a coverage of less than 50 million people are not representative for the European situation. Year-to-year variations in exposure classes are partly caused by changes in spatial coverage. Only urban background stations have been included in the calculations.