2025 Volume 74 Issue 2 Pages 119-129
[Objectives] This study was performed to elucidate the daily maximum WBGT values for each prefecture that correspond to the number of people transported to hospitals for heatstroke and to evaluate regional, seasonal, and age differences in heatstroke occurrence by defining heatstroke occurrence probability (Px). [Methods] Px was defined as the ratio of the number of days when the number of heatstroke emergency transport patients per million people was x or more to the total number of days in the relevant category, which was divided into categories of 1ºC for the daily maximum WBGT. The daily maximum WBGT when Px is 50% (Wx) can be considered as the threshold value for the number of emergency heatstroke transport patients per million people. Px was calculated for each month and age group under the following four conditions: x = 1, 2, 5, and 10. Furthermore, we created three types of regression models for Px using binary logistic regression analysis (an all-ages, all-period model; an all-ages, monthly model; and an age-specific, monthly model) and verified the validity of the regression models using indices of negative predictive value, positive predictive value, and area under the curve. Finally, we evaluated regional differences in heatstroke risk using Wx by prefecture. [Results] When Px for Tokyo was calculated by month or age group, and the daily WBGT maxima were analyzed when they were 50% or higher, the highest maxima were found in the following orders: September>June to August>May; and adults>children>the elderly. When a regression model was created, the positive predictive value of the monthly and age-specific models was sometimes 0% owing to data imbalance; however, the Wx obtained from the all-age, all-period model and the all-age, monthly model was considered valid. Finally, when Wx was evaluated by prefecture, Wx was clearly lower in relatively cold regions. [Conclusion] Px and Wx values revealed that regional differences and seasonal variations in daily maximum WBGT should be taken into account during heatstroke emergency transport, and age should also be taken into account if analysis is limited to the elderly. In recent years, some local governments have built systems that automatically issue warnings based on WBGT measurements. The results of this study may be useful for local governments when setting warning criteria.
