Interaction between extreme temperature events and ozone on mortality: Evidence from a time-stratified case-crossover study

Mounting evidence links ozone (O₃) and extreme temperature events (ETEs) to increased mortality, yet the interaction between these factors remains poorly understood. This study investigates the relationship between ambient maximum 8-h moving average ozone concentrations (O₃-8h) and exposure to heatwaves and cold spells, and their impact on non-accidental, cardiovascular, and respiratory mortality. Using a case-crossover design, we analyzed 248,060 non-accidental deaths in Qingpu, Shanghai, China, from 2012 to 2019. Heatwaves and cold spells were defined as periods of sustained extreme temperatures lasting more than two consecutive days. Various temperature thresholds and durations were used to define these events at the grid level. Conditional logistic regression models were employed to assess exposure-response relationships and additive interactions. The odds ratio (OR) for non-accidental mortality associated with low O₃-8h concentrations during heatwaves (defined as P92.5_3d) was 1.096 (95 % CI: 0.954, 1.260), compared to low O₃-8h without heatwave exposure. During heatwaves, the OR was 1.046 (95 % CI: 1.007, 1.086). Significant synergistic effects were observed between heatwaves and ozone exposure on respiratory and non-accidental mortality (RERI >0, AP > 0, S > 1). No significant interaction was found between cold spells and ozone exposure. Ozone and heatwaves accounted for approximately 1.9 %, 1.3 %, and 3.1 % of non-accidental, cardiovascular, and respiratory deaths, respectively. These findings suggest that heatwaves and ozone exposure significantly increase mortality risk, with a synergistic effect exacerbating the risk.