Association of Wet-Bulb Globe Temperature with heat-related illness hospitalizations in Japan: a time-stratified, case-crossover study

BACKGROUND: Heat-related illnesses are a serious public health concern and are exacerbated by global warming. Wet-Bulb Globe Temperature (WBGT) is widely used as a heat stress indicator, but its clinical impact remains unclear. This study aimed to investigate the association between hourly variations in WBGT and the incidence of hospitalizations for heat-related illness in Japan using a nationwide database. By incorporating individual-level clinical data and performing stratified analyses, we sought to provide a more granular understanding of how heat exposure affects the risk of heat-related illness requiring hospitalization. METHODS: We conducted a time-stratified, case-crossover study using data collected from July to September in 2020 and 2021 in the Heatstroke STUDY registry. The inclusion criteria were patients registered in the Heatstroke STUDY registry, specifically hospitalized patients with heat-related illness who were transported to participating hospitals during the study period. Hourly WBGT values were assigned based on the nearest monitoring station to each hospital. Conditional logistic regression and distributed lag models were used to estimate associations between WBGT and the risk of hospitalization. RESULTS: A total of 1,653 heat-related illness hospitalizations were analyzed. The mean patient age was 67.9 years; 67.6% were male. Each 1 °C increase in WBGT at onset (hospital arrival) was associated with a significantly increased risk of hospitalization (OR 1.10, 95% CI: 1.05–1.15). The cumulative effect over the prior six hours was also significant (OR 1.56, 95% CI: 1.50–1.62). Compared with WBGT < 25 °C, adjusted ORs were 3.39 (25–27 °C), 8.81 (28–30 °C), and 22.10 (≥ 31 °C). Stratified analyses suggested stronger associations among several subgroups; however, only patients with mental disorders showed statistically significant effect modification, whereas elevated WBGT posed a risk across all groups. CONCLUSIONS: Higher WBGT levels were associated with an increased risk of heat-related hospitalization. Although the effect appeared greater in some subgroups, only patients with mental disorders demonstrated statistically significant effect modification, suggesting elevated WBGT confers risk broadly.