Effect modification by temperature on the association between O3 and emergency ambulance dispatches in Japan: A multi-city study

Numerous epidemiological studies have reported that ozone (O3) and temperature are independently associated with health outcomes, but modification of the effects of O3 on health outcomes by temperature, and vice versa, has not been fully described. This study aimed to investigate effect modification by temperature on the association between O3 and emergency ambulance dispatches (EADs) in Japan. Data on daily air pollutants, ambient temperature, and EADs were obtained from eight Japanese cities from 2007 to 2015. A distributed lag non-linear model combined with Poisson regression was performed with temperature as a confounding factor and effect modifier to estimate the effects of O3 on EADs at low (75th percentile) temperature for each city. The estimates obtained from each city were pooled by random-effects meta-analysis. When temperature was entered as a confounder, the estimated effects of O3 on EADs for all acute, cardiovascular, and respiratory illnesses were largest at lag 0 (current-day lag). Therefore, this lag was used to further estimate the effects of O3 on EADs in each temperature category. The estimated effects of O3 on EADs for all acute, cardiovascular, and respiratory illnesses in all eight Japanese cities increased with increasing temperature. Specifically, a 10 ppb increase in O3 was associated with 0.80 % (95 % CI: 0.25 to 1.35), 0.19 % (95 % CI: −0.85 to 1.25), and 1.14 % (95 % CI: −0.01 to 2.31) increases in the risk of EADs for all acute, cardiovascular, and respiratory illnesses, respectively, when city-specific daily temperature exceeded the 75th percentile. Our findings suggest that the association between O3 and EADs for all acute, cardiovascular, and respiratory illnesses is the highest during high temperature. Finding of this study can be used to develop potential mitigation measures against O3 exposure in high temperature environment to reduce its associated adverse health effects. [Display omitted] •Effect modification by temperature on O3-EADs association was examined.•EADs visits for all-cause, cardiovascular, and respiratory illnesses were explored.•Ambient temperature can modify the association between O3 and EADs.•Effect of O3 on EADs was greater when temperature was higher (>75th percentile).•Significant effect of O3 on EADs for all-acute illnesses during hot days was observed.