Hourly differences in air pollution on the risk of asthma exacerbation.

We investigated the association between hourly differences in air pollution and asthma exacerbation in Korea using asthma-related emergency department data and verified seasonality and demographic modifiers with an hourly temporal resolution. We applied time-stratified case-crossover adjusted for weather and influenza; the lag was stratified as 1-6, 7-12, 13-18, 19-24, 25-48, and 49-72 h. Odds ratios (95% confidence interval) per interquartile range increase were 1.05 (1.00-1.11) after 1-6 h for PM10-2.5 and 1.10 (1.04-1.16) after 19-24 h for O3. Effect size was 1.14 (1.06-1.22) at a 1-6 h lag in spring for PM10-2.5, and 1.25 (1.03-1.51) at a 25-48 h lag in winter for O3. O3 effects were age- and low socio-economic status-modified at a 7-12 h lag [1.25 (1.04-1.51)]. Increased PM10-2.5 and O3 increased the risk of asthma exacerbation; the effect of PM10-2.5 was most immediate.