Hourly Ambient Air Pollution Exposure and Acute Asthma Exacerbations from an 11-Year Time-Series Study: Characterizing Excess Risks, Hourly Hazard Windows, and Lag Structures

Daily air pollution metrics often obscure subdaily exposure peaks that trigger acute asthma exacerbations. This study quantified the short-term associations between hourly ambient PM2.5, NO2, and O3 concentrations and asthma emergency department visits (EDVs). We applied quasi-Poisson generalized additive models to evaluate age-stratified health risks across rolling-average exposures (1–24 h) and lag times up to 24 h using 2011–2021 data from Nantou, Taiwan. Hourly exposures to all three pollutants were significantly associated with asthma EDV risk, with NO2 showing robust and consistent effects. In single-pollutant models, a 10-ppb increase in 6-h average NO2 was associated with a 12.4% increase in EDVs at a 23-h lag overall. Age-stratified analyses indicated that children experienced distinct, rapid risk elevations at shorter lag times (0–5 h) following exposure to PM2.5, NO2, and O3, whereas adults and older adults exhibited more delayed associations (e.g., 13–23 h lags for NO2 and PM2.5. Excess risk emerged at detectable-risk concentrations, approximately 29.6 μg/m3 for PM2.5 (5 h exposure at 0 h lag), 15.0 ppb for NO2 (7 h exposure at 1 h lag), and 25.7 ppb for O3 (15 h exposure at 5 h lag) in the pediatric group. Acute asthma exacerbations are closely linked to transient, hourly pollution peaks. These findings indicate that statistically detectable increases in risk occur at concentrations well below conventional daily regulatory limits, providing empirical evidence that could inform future discussions on subdaily air quality guidelines to protect susceptible populations.