Effects of air pollution on moderate and severe asthma exacerbations.

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 O ₃ levels in summer and winter (OR: 1.012[1.003-1.02] and 1.009[1.003-1.016]), SO ₂ levels in spring and summer (OR: 1.009[1-1.018] and 1.02[1.006-1.035]) and NO ₂ levels in winter (OR: 1.007[1.003-1.011]). Analyses of the temporal relationship between O ₃ concentrations and exacerbations demonstrated that 63.2% of episodes in the summer occurred when the O ₃ 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 O ₃ 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 O ₃ , SO ₂ and NO ₂ levels in summer and winter with the similar relative risk between moderate and severe exacerbations.