The annual mean limit value is 40 µg NO2/m³. Over the years 1997-2008 the total population, for which exposure estimates are made, increased from 55 to 118 million people due to an increasing number of monitoring stations reporting air quality data under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Bosnia and Herzegovina, Iceland, Liechtenstein, Luxembourg, Malta, the former Yugoslav Republic of Macedonia, Serbia, and Turkey are not included due to the geographical coverage of the Urban Audit and/or lack of air quality data.
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