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Table 1 Examples of air quality standards for common air pollutants as given in the European Ambient Air Quality Directive (Directive 2008/50/EC) and World Health Organization Air Quality Guidelines (WHO AQG) for the protection of human health

From: Urban population exposure to air pollution in Europe over the last decades

Air pollutant EU limit and target value (threshold in µg m−3) WHO AQG (threshold in µg m−3)
PM10 a Annual mean (40) Annual mean (20)
PM10 a Number of exceedance of 24-h mean (50) Number of exceedance of 24-h mean (50)
PM2.5 b Annual mean (25) Annual mean (10)
PM2.5 b Number of exceedance of 24-h mean (25)
O3 c Number of exceedance of maximum daily 8-h mean (120) Number of exceedance of maximum daily 8-h mean (100)
NO2 d Annual mean (40) Annual mean (40)
NO2 d Number of exceedance of 1-h mean (200) Number of exceedance of 1-h mean (200)
SO2 e Number of exceedance of 24-h mean (125) Number of exceedance of 24-h mean (20)
COf Maximum daily 8-h (10,000) Maximum daily 8-h (10,000)
  1. aAnnual mean PM10 concentration and number of days with 24-h PM10 concentration over 50 µg m–3 for the protection of human health. The annual mean PM10 concentration does not to exceed 40 µg m–3 (Directive 2008/50/EC) or 20 µg m–3 (WHO AQG). The 24-h PM10 mean concentration does not to exceed 50 µg m–3 (WHO AQG) or more than 35 times a year (EC)
  2. bAnnual mean PM2.5 concentration and numbers of days with 24-h PM2.5 mean concentration over 25 µg m−3 (WHO AQG). The annual mean PM2.5 concentration does not to exceed 25 µg m–3 (EC) or 10 µg m–3 (WHO AQG)
  3. cFor the protection of human health, the Directive 2008/50/EC has introduced a threshold of 120 µg m–3 for the daily maximum 8-h average. The threshold level should not be exceeded on more than 25 times a year. Number of days with daily maximum 8-h O3 concentrations over 100 µg m–3 as limit value for the protection of human health (WHO AQG)
  4. dAnnual mean NO2 concentration and number of hours with NO2 concentrations above 200 µg m−3. The annual mean NO2 concentration does not to exceed 40 µg m–3 (EC and WHO AQG) while the hourly threshold should not be exceeded more than 18 times a year (EC)
  5. eThe 24-h SO2 mean concentration does not to exceed 125 µg m–3 more than 3 times a year (EC) and does not to exceed 20 µg m–3 (WHO AQG). f The Directives have introduced a threshold of 10 mg m–3 for the maximum daily 8-h mean concentration