Quantifying localized heatwave impact on mortality: a multi-country modeling study in the Asia-Pacific region.

Background: Heatwave is a global health threat. However, existing heatwave definitions often rely on fixed temperature thresholds without incorporating region-specific health outcomes or population acclimatization, limiting their applicability across diverse climatic and demographic contexts. We aimed to assess the impact of localized heatwave on mortality based on a proposed framework of health-based heatwave definition. Methods: Based on daily data on death and weather from 25 cities in Australia, China, South Korea, and Thailand, we proposed a health-based excess heat factor (HEHF) to define the localized heatwave. First, a tiered health risk-based (THR) approach was used to fit the heatwave-mortality association to detect the region-specific temperature threshold for heatwave. Then, the HEHF was derived from a three-day-averaged temperature exceeding the region-specific heatwave threshold multiplied by the acclimatization index, reflecting the difference between recent (past three-day) and preceding (past thirty-day) average temperatures. Finally, the mortality burden attributable to heatwaves was estimated to compare the performance of distinct heatwave definitions including the HEHF, percentile-based definition, and country-specific official definition. Findings: A total of 2,255,634 deaths from four countries were analyzed. Heatwave of all definitions was associated with an increased mortality risk in four countries, with a higher risk estimated by HEHF. The HEHF not only detected a health-based, localized, and time-varying temperature threshold for the heatwave but also captured a continuous pattern of mortality risk associated with changes in heatwave intensity. Compared with percentile-based and country-specific official definitions, using the HEHF also yielded a larger proportion of deaths attributable to heatwaves, accounting for 8.68% (95% CI: 7.19%, 10.50%) in China, 4.50% (95% CI: 3.33%, 5.58%) in Thailand, 2.99% (95% CI: 1.54%, 4.33%) in Australia, and 1.98% (95% CI: 1.24%, 2.71%) in South Korea. The subtropical zone exhibited a higher attributable fraction than temperate and tropical zones. Interpretation: This multi-country study has developed a generalizable and health-based framework for defining the localized heatwave, assisting in assessing and comparing health impact of heatwaves across regions and climates. Funding: National Natural Science Foundation of China.