Assessing the impact of compound heatwave-ozone event with varied definitions on mortality risk in Hong Kong

High temperatures and ozone pollution are recognized as significant risk factors for human health, but their combined effects have been less studied, particularly in densely populated Hong Kong. In this study, daily mortality, meteorological, and air pollution data in Hong Kong were collected from March to November for the years 2000 to 2021. Compound heat-ozone events were identified and categorized into same-day events, overlapping events, and successive events. The effects of compound heat-ozone exposure and the additive interactions between heat and ozone on mortality risk were estimated. Compared to non-event days, same-day events were associated with an increased risk of non-external mortality (relative risk [RR]: 1.05, 95 % confidence interval [CI]: 1.01-1.10), respiratory mortality (RR: 1.13, 95 % CI: 1.03-1.23), and pneumonia-related mortality (RR: 1.12, 95 % CI: 1.01-1.24). Both overlapping and successive events were linked to heightened mortality risks as well, except for cardiovascular mortality. The impact of compound events, particularly same-day events, on mortality risk was more pronounced among females (RR: 1.07, 95 % CI: 1.01-1.13) and the elderly (RR: 1.11, 95 % CI: 1.06-1.17). Furthermore, an additive interaction effect between heat and ozone exposure was notably observed among the elderly, especially in overlapping event scenarios. In summary, same-day and overlapping compound heat-ozone events were associated with increased mortality risks, whereas the effect of successive compound event was limited to specific causes of death. These findings underscore the importance of clarifying the definition of compound event in order to develop targeted public health strategies and mitigate the effects of combined exposure more efficiently.