Novel Regional Heat Guidelines Based On Historical Wet Bulb Globe Temperature Data In Japan

Japan is long and narrow in the north and south, and the climatic characteristics of each region are quite different. Due to the differences in climate and geographic variations in heat exposure and acclimatization, it is hypothesized that there are regional differences in the incidence rate (IR) of heat illness in Japanese high schools. PURPOSE: To develop a regional heat categorization and analyze exertional heat illness (EHI) data among high school students using the new categorization. METHODS: We used the 90th percentile warm season (May through October) daily maximum value for 841 weather stations in Japan to determine locally extreme WBGTs. Jenks natural breaks classification method was used to develop the new WBGT-based heat category system. EHI data were collected separately in a different study between 2019 and 2022 by high school teams. IR of EHI was calculated using 1,000 athlete-exposure (AE). 95% confidence interval (CI) was calculated, and statistical significance was set at p < 0.05 a priori. RESULTS: Four heat categories were identified based on local extremes: Category 1 (22.9-26.7 °C), Category 2 (26.8-28.8 °C), Category 3 (28.9-30.7 °C), and Category 4 (30.8-32.9 °C) (Figure 1). EHI data from Category 3 and 4 high schools were found in a separate study. The IR of EHI was 1.86/1,000 AE (95%CI, 0.85―3.54) and 2.62/1,000 AE (95%CI, 1.73―3.81), respectively. CONCLUSIONS: While the IR of EHI was greater in Category 4 than 3, there was no statistical difference observed in the current dataset. This suggests that the category system appropriately adjusted the environmental extremes to capture EHI risk in Category 3. Additional clinical data from Category 1 and 2 are needed to test the external validity of the new categorization.