Short-term exposure to multiple air pollutants and synergistic effects on asthma hospitalization in children: A nationwide case-crossover study in China.

Air pollution has been recognized as a risk factor for asthma in children. However, evidence on multiple air pollutants on children's asthma hospitalization and the potential synergistic effects remains insufficient. This individual-level, time-stratified case-crossover study analyzed 5045 asthma hospitalizations from 153 hospitals across 20 Chinese provinces (2013-2021). Daily air pollution (particulate matter with a diameter ≤ 2.5μm (PM2.5) or ≤ 10μm (PM10), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), and carbon monoxide (CO)) was estimated using satellite-based spatiotemporal models. Associations with asthma hospitalization were assessed using conditional logistic regression combined with distributed lag nonlinear models. The synergistic effects between pollutants were assessed. In the single-pollutant model for lag 0-1, each 10 µg/m³ increase in PM2.5, PM10, NO2, SO2, and each 0.1 mg/m³ increase in CO were significantly associated with increased risks of asthma hospitalization of 1.71 % (95 % confidence interval (CI): 0.65, 2.78), 2.26 % (95 %CI: 1.02, 3.49), 4.95 % (95 %CI: 2.93, 6.97), 2.18 % (95 %CI: 0.75, 3.61), and 3.16 % (95 %CI: 1.05, 5.27), respectively. In the two-pollutant model, NO2 exhibited the strongest and most robust association with asthma hospitalization. The exposure-response relationship for all pollutants showed an approximate linear relationship. There was a synergistic effect between PM2.5 and NO2 on childhood asthma hospitalization, with the synergy index of 1.88 (95 %CI 1.21, 1.96). These findings provide crucial scientific evidence for reducing air pollution and highlight that comprehensive prevention strategies incorporating reductions in both PM2.5 and NO2 exposure could synergistically enhance primary prevention of childhood asthma hospitalizations.