Wet bulb globe temperature variability and its implications on heat stress monitoring

The wet bulb globe temperature (WBGT) is widely used in athletics for assessing heat stress. Sports governing bodies provide a wide variety of recommendations on how often to take WBGT measurements during activities like practices and competitions. This can leave coaching staff and other sports medicine personnel in a quandary as to what is the "best practice." Our study is the first to use a large dataset to quantify the variability of WBGT within practice and competition sessions, and to identify the impact of using different measurement rates on the WBGT. The study leverages 6 years of data from thousands of American football practice and competition sessions from across Georgia, USA. We observed that, on average, WBGTs are coolest in the morning (26.67 & DEG;C) and evening (24.84 & DEG;C), and hottest in the midday (30.23 & DEG;C) and afternoon (27.21 & DEG;C). The variability within sessions tended to be greater for morning, midday, and afternoon than evenings, with session standard deviations of 0.96 & DEG;C to 1.27 & DEG;C and ranges of 2.67 & DEG;C to 3.55 & DEG;C when controlling for duration. WBGTs also tended to increase over time in morning sessions and decrease over time during afternoon and evening sessions. These changes are clinically important. We found that Georgia High School Association (GHSA) WBGT activity modification categories often changed during sessions, especially in the morning, midday, and afternoon where a change in GHSA category of & GE;1 occurred in 57% to 76% of sessions. Considering this variability, our results indicate that more frequent measurements better capture maximum WBGT values over a session and reduce the likelihood of misclassifying activity modification.