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Determining Location-Specific Thresholds for Heat Warning Systems to Mitigate Heatstroke Mortality in Japan
Vera Ling Hui Phung1, Yasushi Honda2, Kazutaka Oka2
1Department of Global Health Policy, Graduate School of Medicine, The University of Tokyo, Tokyo 113-0033, Japan.
Environment & Health (Washington, D.C.)
|January 21, 2026
Summary
Japan's current heat warning system threshold is insufficient. A revised threshold of approximately 31°C WBGT could halve preventable heatstroke deaths nationwide, considering regional differences.
Area of Science:
- Environmental Health
- Public Health
- Climatology
Background:
- Japan's current nationwide heat warning system (HWS) uses a Wet Bulb Globe Temperature (WBGT) threshold of 33°C.
- This threshold inadequately addresses regional variations in heat sensitivity and heatstroke mortality.
- Existing HWS may not effectively mitigate preventable heatstroke deaths across diverse prefectures.
Purpose of the Study:
- To determine a critical WBGT threshold for reducing preventable heatstroke mortality across Japan.
- To assess the efficacy of the current HWS threshold in mitigating heat-related deaths.
- To identify regional and demographic variations in heat sensitivity and optimal warning thresholds.
Main Methods:
- Analysis of daily heatstroke mortality data (ICD-10: X30; 2010-2019) for all 47 Japanese prefectures.
- Application of a time-stratified case-crossover design with conditional quasi-Poisson regression and a distributed lag nonlinear model.
- Estimation of WBGT thresholds required to reduce 50% of preventable heatstroke mortalities (target50), including subgroup analyses.
Main Results:
- A total of 9702 heatstroke mortalities were recorded, with higher incidence in late summer and among older individuals.
- The current HWS threshold (WBGTmax 33°C) accounted for only 2%-3% of preventable heat-related deaths.
- A new critical threshold (WBGTmax approximately 31°C) was identified, capable of achieving target50 in most prefectures, with lower thresholds needed for northern regions and early summer phases.
Conclusions:
- The current HWS WBGT threshold of 33°C is insufficient for mitigating heatstroke mortality nationwide.
- A revised critical threshold of approximately 31°C WBGT is proposed to effectively reduce preventable heatstroke deaths by half.
- Findings support revising Japan's HWS to incorporate regional and temporal variations for improved public health protection against heatwaves.
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