Evaluating Japan's revised heat-health warning system in the face of recent escalating heat stress

In 2021, the Japanese government changed the exposure indicator of the national heat-health warning system (HHWS) from air temperature (T air) to Wet Bulb Globe Temperature (T WBG), reflecting the growing concerns about the escalating humid heat stress. However, a clear validation of the advantages of using heat stress indicators (HSIs) that combine T air and humidity in heat alerts and heat-health applications are still being explored. Here, by using the latest epidemiological data (2015-2019) before the COVID-19 pandemic, we examined the rationality of the revised HHWS for 47 prefectures in Japan. Specifically, we investigated the predictive power of different HSIs in modeling mortality and morbidity caused by different diseases (e.g. all causes, circulatory, respiratory, and heatstroke) and age groups. Our findings revealed substantial differences among the HSIs in identifying periods of intense heat stress, potentially leading to differences in the activation dates of the HHWS if various indicators were employed. While HSIs exhibited comparable performances in modeling daily mortality, our analysis demonstrates distinct advantages in using T WBG for daily morbidity predictions, and the quasi-Akaike Information Criterion of T WBG is much lower than the previously used T air. The merits of T WBG are consistent in modeling all causes, non-external, as well as heatstroke-related morbidity. Overall, this study underscores the practicality of incorporating HSIs in heat stress early warnings and provides critical insights for refining the HHWS to mitigate health impacts from heat stress under future climate change.