Impact Of Regional Climate On Exertional Heat Illness Rates In Japanese High School Sports

Due to global warming and urban heat island effects, school-organized sports face higher exertional heat illness (EHI) risks during the summer months. Previous studies also suggest the influence of regional climate variations on the EHI incidence. PURPOSE: To investigate the incidence and environmental characteristics of EHI events in school-organized sports across various climate regions in Japan. METHODS: The regional heat risk category was calculated from the 90th percentile of the daily maximum wet bulb globe temperature (WBGT) from 841 weather stations across Japan from April to October of 2010-2021were used to determine locally extreme WBGT classifications (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]). Daily activity was logged by the coaches of two high school teams (baseball, n = 1; American football, n = 1) from July to September 2024. The log included: (1) activity date, (2) number of participants, (3) activity location (indoor or outdoor), (4) wet bulb globe temperature (WBGT) at the start of activity and peak WBGT, when available, (5) temperature at the start of activity, (6) humidity at the start of activity, and (7) number of EHI cases. WBGT was measured on-site using JIS B7922-compliant equipment. EHI incidence per 1,000 athlete-exposure (AE) was calculated, followed by a comparison of z-scores for (1) WBGT at the start of activity and (2) peak WBGT on days with and without EHI. RESULTS: Both teams were classified in Category 4. They observed 3,208 AE, with 43 EHI cases, resulting in an incidence of 13.4 per 1,000 AE. The average and z-score of WBGT at the start of activity on days with and without EHI were 29.2 ± 1.2 °C (z = -1.09) and 26.9 ± 1.8 °C (z = -1.84), respectively. The z-score was statistically greater on days with EHI (t[40.5] = -2.69, p = 0.01). The average and z-score of peak WBGT on days with and without EHI was 30.2 ± 1.9 °C (Z = -0.7) and 28.1 ± 1.8 °C (Z = -0.9), respectively, with no statistical difference in z-scores (t[39] = -0.42, p = 0.68). CONCLUSIONS: A notably high EHI incidence was observed in Category 4 teams. The negative z-scores suggest that teams within this region should consistently implement precautionary measures for EHI, as even relatively moderate temperatures for the area may already pose a risk for outdoor sports.