Proposing the flexible application of heat safety thresholds for junior high and high school sports clubs in Japan

Abstract

Abstract Objectives To verify the appropriateness of applying uniform heat safety thresholds (HSTs) in sports club activities at Japanese junior high and high schools, and to propose the flexible application of HSTs according to the conditions of the activity. Methods A case-crossover study was conducted using 2011–2019 heat illness incident data from Japanese junior high and high school sports club activities and hourly wet-bulb globe temperature (WBGT) data from nearby locations. Controls were matched for each case by year, month, day of the week (DOW), and hour for conditional logistic regression analysis, and explanatory variables were selected by the Group Lasso method. An analysis stratified by the selected explanatory variables was also conducted. Results Ten explanatory variables were selected: the WBGT at the time of the incident (WBGT-Incident), average WBGT of the previous day (WBGT-Prev), average WBGT during the summer season (WBGT-Summer), type of school, club, location, year, month, DOW, and hour. There was a significant relationship between heat illness incidence and both the WBGT-Incident and WBGT-Prev, regardless of whether stratification was used. Additionally, when the data were stratified by club, location, year, month, and DOW, the strata showed significant differences. Conclusion Heat illness risk varies with factors such as heat acclimatization, location, and duration, and applying uniform HSTs is inappropriate. We recommend using condition-tailored thresholds up to 3 ~ 4°C lower than the current HSTs, particularly during cooler summer months of April to June, on days with an unusually high WBGT, outdoor activities, and longer activity durations.