Short-Term Interaction Effects of PM 2.5 and O 3 on Daily Mortality: A Time-Series Study of Multiple Cities in China.

In recent years, PM 2.5 and O 3 have been the two main pollutants affecting public health in China, but the interaction of the two pollutants on human health remains unclear. A two-stage analytical approach was used to investigate the relationships of PM 2.5 -O 3 co-pollution with nonaccidental, cardiovascular, and respiratory mortality levels across 14 cities in China. We first utilized a generalized additive model (GAM) to determine the city-specific associations of PM 2.5 and O 3 with daily mortality. The associations were then combined at the national and regional levels using meta-analysis. To investigate the potential interactions between the two pollutants and cause-specific mortality, we performed stratified analyses by co-pollutant exposure levels and the synergy index ( SI ) ( SI > 1 indicates a synergistic interaction). The effect of changes in the two pollutants' concentrations (in 10 μg/m 3 increases) on mortality was assessed. The stratification analysis results suggested that each 10 μg/m 3 increase in PM 2.5 at lag0-1 (lag01) in the low, moderate, and high strata of the O 3 concentrations increased nonaccidental mortality by 0.07% (95% confidence interval: -0.03%, 0.17%), 0.33% (0.13%, 0.53%), and 0.68% (0.30%, 1.06%), respectively, with significant between-group differences ( p < 0.001). Moreover, each 10 μg/m 3 increase in O 3 (lag01) in the low, moderate, and high strata of the PM 2.5 concentrations increased nonaccidental mortality by 0.15% (-0.06%, 0.36%), 0.53% (0.19%, 0.87%), and 0.75% (0.14%, 1.36%), respectively, with significant between-group differences ( p < 0.001). We also found substantial synergistic interactions between the two pollutants and nonaccidental, cardiovascular, and respiratory mortality levels, with SI values of 1.48, 1.51, and 1.33, respectively. Additionally, a subgroup analysis revealed that the interaction of these two pollutants on nonaccidental mortality were greater in South China compared to elsewhere, and during the warm season compared to during the cold season. Our findings suggested that the simultaneous control of PM 2.5 and O 3 within the context of combined air pollution could significantly decrease the disease risk, especially in southern China and during the warm season.