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Background: Few studies have evaluated the impact of air pollution levels on the severity of exacerbations. Thus, we compared the relative risks posed by air pollutant levels on moderate and severe exacerbations. Methods: Exacerbation episodes of 618 from 143 adult asthmatics were retrospectively collected between 2005 and 2015 in a tertiary hospital of Korea. Air pollution GPS data for the location closest to each patient's home were obtained from the national ambient monitoring station. The relative impacts of air pollutants on asthma exacerbations were evaluated via a time-trend controlled symmetrical, bidirectional, case-crossover design using conditional logistic regression models on the day of the exacerbation (T-0) and up to 3 days before the exacerbation (T-1–T-3). Results: Overall asthma exacerbation were associated with O3 levels in summer and winter (OR: 1.012[1.003–1.02] and 1.009[1.003–1.016]), SO2 levels in spring and summer (OR: 1.009[1–1.018] and 1.02[1.006–1.035]) and NO2 levels in winter (OR: 1.007[1.003–1.011]). Analyses of the temporal relationship between O3 concentrations and exacerbations demonstrated that 63.2% of episodes in the summer occurred when the O3 concentrations on T-1 were significantly higher than those on control days, while 51% of exacerbation episodes in the winter occurred. Severe and moderate exacerbations were similarly associated with O3 levels in winter (OR: 1.012 [1.003–1.02] vs. 1.01 [0.999–1.021], p > 0.05) and in summer (OR: 1.006 [1.002–1.009] vs. 1.009 [1.003–1.016], p > 0.05). Conclusions: Asthma exacerbations may be associated with the seasonal elevation of O3, SO2 and NO2 levels in summer and winter with the similar relative risk between moderate and severe exacerbations. [ABSTRACT FROM AUTHOR]