Mortality Risk from Chronic Ischemic Heart Disease Associated with Short-Term Co-Exposure to Summer Heatwave and...

Summer heat and air pollution may pose a great threat to public health in the context of climate warming, however, their interactive effects on deaths from chronic ischemic heart disease (CIHD) have not been evaluated in existing investigations. This study aims to assess the associations of concurrent heatwave and ozone pollution with CIHD mortality in Chinese population. We carried out a province-wide, individual-level case-crossover study by analyzing 33,770 CIHD deaths occurring in warm season (May-September) across Jiangsu, eastern China, between 2016 and 2019. Spatially resolved estimates of maximum 8-h average ozone concentrations and air temperatures were estimated on case days and control days at the residential address. Heatwave events were defined using multiple temperature thresholds (percentiles 90-97.5) and durations (2-4 days). Conditional logistic regression model was employed to assess the odds ratio (OR) and its 95% confidence interval (CI) of CIHD death associated with heatwave and ozone exposure. Stratified analyses were performed to compare ozone-related risks on heatwave days versus non-heatwave days, and heatwave-related risks on low-ozone days versus high-ozone days. Additive interactive effects were testified using multiple metrics including relative excess odds due to interaction (REOI), proportion attributable to interaction (AP), and synergy index (SI). In the overall population, the odds of CIHD mortality were 1.010 (95% CI, 1.007-1.014) for a 10-µg/m³ rise in warm-season ozone at lag-01 day and ranged from 1.20 (95% CI, 1.15-1.25) to 1.64 (95% CI, 1.54-1.73) for heatwaves under various definitions. Stratified analyses showed intensified ozone-related CIHD risks on heatwave days compared to non-heatwave days and higher heatwave-related risks on high-ozone days than low-ozone days. We observed significant synergistic interactive effects of heatwave and high-level ozone on CIHD mortality (SI > 1), where the excess risk was elevated by 10-19% (REOI > 0) under co-exposure scenarios, and 8-11% (AP > 0) of the odds could be attributed to the additive interaction. Excess fractions of CIHD deaths attributable to heatwave and high-level ozone among overall populations ranged from 4.73% (95% CI, 3.21-6.25%) to 6.43% (95% CI, 4.33-8.45%) under different heatwave definitions. In stratified analyses, we observed similar synergistic effects across age groups and among female subgroups only in multiple heatwave definitions. This study provided novel evidence for the synergistical effects of short-term exposure to heatwave and ozone in elevating CIHD death risk and burden. Our findings highlighted the public health urgency of collective response to climate warming crisis and ambient ozone pollution.