Determining Location-Specific Thresholds for Heat Warning Systems to Mitigate Heatstroke Mortality in Japan

The current nationwide wet bulb globe temperature (WBGT) threshold (33 °C) of Japan's heat warning system (HWS) does not adequately account for regional variations in heat sensitivity and heatstroke mortality. In this study, we aimed to determine the critical WBGT threshold for effectively mitigating preventable heatstroke mortality across Japan. To this end, daily heatstroke mortality data (ICD-10: X30; 2010-2019) for all 47 prefectures of Japan were analyzed using a time-stratified case-crossover design based on a conditional quasi-Poisson regression combined with a distributed lag nonlinear model. Assuming that heatstroke mortality is preventable via interventions when prompted by Japan's HWS, the WBGT threshold required to reduce 50% of preventable heatstroke mortalities ("target50") was estimated; subgroup analyses by age, sex, and summer phase were also conducted. According to the results, 9702 heatstroke mortalities were recorded during the study period, with more cases observed in late summer and among older individuals. Further, the current HWS threshold (WBGT_max 33 °C) only accounted for 2%-3% of preventable heat-related deaths during summer months. However, a new critical threshold (WBGT_max approximately 31 °C), enabling the realization of target50 in most prefectures, was identified. Notably, northern regions required lower thresholds. Significant differences between summer phases (lower thresholds for early summer than those for late summer), as well as regional and demographic variations in heat sensitivity, were also observed. The application of the identified critical threshold, WBGT_max approximately 31 °C, aligned with the national target of reducing preventable heatstroke mortalities by half. Therefore, the findings of this study provide a scientific basis for revisiting Japan's HWS and improving mitigation measures.