Figure 2.

Air pollution concentration in Chinese cities. Only the annual mean concentrations of the latest publication for each city were shown to avoid over-representation. The years of the study period were indicated after the city names. Dotted lines for PM₁₀ and NO₂ were WHO annual Air Quality Guidelines [23]. Dotted lines for SO₂ and O₃ were annual limits derived from WHO short-term Air Quality Guidelines [24].