The role of ozone as a mediator in the relation between heat waves and mortality in 15 French urban agglomerations.

Heat and tropospheric ozone have acute impact on premature deaths. Warm temperature affects the photochemical processes in ozone formation suggesting ozone as a mediator to the acute health effect of heat on mortality. We assembled a summertime daily time-series dataset of 15 French agglomerations during 2000 and 2015 to decompose the acute total effect of heat wave on mortality into natural direct and indirect effects using regression-based product method under the potential outcomes framework. For each agglomeration, we estimated the effect of heat wave on mortality using a quasi-Poisson model with adjustment for covariates like lagged NO2 concentration, and modeled ozone with a linear regression of heat wave and the same set of covariates. We pooled estimates across agglomerations using random-effect models. We also provide R syntax to reproduce or replicate our analysis. Most agglomerations demonstrated evidence of mediation by ozone, with the pooled natural indirect effect being 1.03(95% confidence interval (CI): 1.02 to 1.05), 1.03(95% CI: 1.01 to 1.04), and 1.02(95% CI: 1.00 to 1.07) for non-accidental, cardiovascular, and respiratory mortality, respectively. We found evidence of mediation effect by ozone in the association between heat wave and mortality in France, which varied by geographical location and cause of mortality.