Fig. 2From: Coal burning-derived SO2 and traffic-derived NO2 are associated with persistent cough and current wheezing symptoms among schoolchildren in Ulaanbaatar, MongoliaComparison of the outdoor SO2 concentrations in each district in each season. District A: box with stripes, district B: mesh box, district C: black box, district D: gray box; Asterisk indicates statistically significant (p < 0.05); air quality standard for the mean annual SO2 concentration: 10 μg/m3 (3.8 ppb)Back to article page