Exposure to hourly ambient air pollutants and risk of emergency department visits for asthma: A multicenter time-stratified case-crossover analysis

Background: Exposure to daily ambient air pollutants has been linked with increased risk of asthma morbidity and mortality. However, the association between exposure to air pollution and risk of emergency department (ED) visits for asthma on an hourly level remains unclear.Methods: We conducted a time-stratified case-crossover design and included a total of 121,112 asthma ED visits from 11 hospitals in three cities located in Zhejiang province, China, between January 1, 2016 and September 30, 2021. We used conditional logistic regressions combined with a distributed lag linear model to estimate the association between risk of asthma ED visits and hourly exposure to fine particulate matter (PM2.5), respirable particulate matter (PM10), nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), and carbon monoxide (CO) after adjusted for meteorological factors and public holiday. 'Results: Hourly exposure to PM2.5, PM10, NO2, SO2, and CO was associated with an increased risk of asthma ED visits. This association was most pronounced in the concurrent hour of exposure (lag 0 h), rapidly attenuated over a period of 3-12 h, and diminished thereafter. Each interquartile range increase in air pollutant over 0-72 h was associated with increased risk of asthma ED visits by 7.0% (95% confidence interval [CI]: 5.4%, 8.6%) for PM2.5, 6.7% (95% CI: 5.1%, 8.2%) for PM10, 18.1% (95% CI: 15.4%, 21.0%) for NO2, 6.5% (95% CI: 4.8%, 8.2%) for SO2, and 7.4% (95% CI: 5.6%, 9.1%) for CO.Conclusion: Our findings suggest that hourly exposure to PM2.5, PM10, NO2, SO2, and CO, but not O3, is associated with increased risk of ED visits for asthma shortly after exposure.