Modification and mediation effects of ozone on heatwave-mortality association: A time series study in five provinces...

Epidemiological studies have demonstrated that heatwaves are associated with increased mortality risk. However, knowledge about modifying and mediating effects of O₃ on this relationship is still insufficient. Daily mortality data for the summer of 2013-2017 were obtained from 278 locations across China. Daily meteorological data for the same period were sourced from the fifth generation of the European reanalysis (ERA5), while daily O₃ data were obtained from the China High Air Pollutants (CHAP) dataset. The modification effects of O₃ on the heatwave-mortality associations were estimated using multiplicative interaction and additive interaction analyses. A mediation model was employed to assess the mediating effect of O₃ on the association. The excess risk (ER) of heatwave on mortality was 9.17 % (95 % confidence interval [CI]: 7.91-10.45 %). Heatwaves were associated with a higher mortality risk during periods of high O₃ pollution (ER = 9.68 %, 95 % CI: 8.35-11.05 %) than periods of low O₃ pollution (ER = 5.96 %, 95 % CI: 4.23-7.74 %). Additionally, we observed a 2.70 % (95 % CI: 0.20-5.20 %) additive mortality interaction between O₃ and heatwaves for the total population, with much higher values among individuals aged ≥85 years (6.80 %, 95 % CI: 2.50-10.90 %) and those with cardiovascular diseases (4.20 %, 95 % CI: 0.60-7.70 %). Moreover, we found that 9.33 % (95 % CI: 7.17-11.76 %) of the heatwave effect on mortality was mediated by O₃, with a higher value observed in individuals aged ≥85 years (11.48 %, 95 % CI:8.94-14.33 %). O₃ modified and mediated the heatwave-mortality association. Our results provide evidence that O₃ may function in a dual capacity: partly mediating the effect of heatwave on mortality and partly modifying the strength of this association. This dual role reflects the complex interplay between meteorological conditions, air quality, and health outcomes.