We examined the robustness of the main results to: (1) adjustment in turn by co-pollutants using two-pollutant models and (2) lag selection. We did not adjust for BC in the two-pollutant models because the time period of BC measurement overlapping with other pollutants was less than 50%. Consequently, simultaneously entering BC into the models might introduce large uncertainty when the pollutants involved represent a different time period of data. We also examined the effect of air pollutants with different lag structures including both single-day lag from (Lag0 to Lag5) and two-day moving average lag (Lag 01). Lag 01 corresponds to 2-day moving average of pollutant concentration of the current and previous day. Lag 01 models were used for our main analysis, given that single-day lag models may underestimate the cumulative effect of pollutants on hospital admissions