Interactive effects of ambient fine particulate matter and ozone on daily mortality in 372 cities: two stage time series analysis

ObjectiveTo investigate potential interactive effects of fine particulate matter (PM2.5) and ozone (O3) on daily mortality at global level.DesignTwo stage time series analysis.Setting372 cities across 19 countries and regions.PopulationDaily counts of deaths from all causes, cardiovascular disease, and respiratory disease.Main outcome measureDaily mortality data during 1994-2020. Stratified analyses by co-pollutant exposures and synergy index (>1 denotes the combined effect of pollutants is greater than individual effects) were applied to explore the interaction between PM2.5and O3in association with mortality.ResultsDuring the study period across the 372 cities, 19.3 million deaths were attributable to all causes, 5.3 million to cardiovascular disease, and 1.9 million to respiratory disease. The risk of total mortality for a 10 μg/m3increment in PM2.5(lag 0-1 days) ranged from 0.47% (95% confidence interval 0.26% to 0.67%) to 1.25% (1.02% to 1.48%) from the lowest to highest fourths of O3concentration; and for a 10 μg/m3increase in O3ranged from 0.04% (−0.09% to 0.16%) to 0.29% (0.18% to 0.39%) from the lowest to highest fourths of PM2.5concentration, with significant differences between strata (P for interaction <0.001). A significant synergistic interaction was also identified between PM2.5and O3for total mortality, with a synergy index of 1.93 (95% confidence interval 1.47 to 3.34). Subgroup analyses showed that interactions between PM2.5and O3on all three mortality endpoints were more prominent in high latitude regions and during cold seasons.ConclusionThe findings of this study suggest a synergistic effect of PM2.5and O3on total, cardiovascular, and respiratory mortality, indicating the benefit of coordinated control strategies for both pollutants.