Variations in Athlete Heat Loss Potential between Hot-Dry and Warm-Humid Environments at Equivalent WBGT Thresholds

Context: Many organizations associated with sports medicine recommend wet bulb globe temperature (WBGT)-based activity modification guidelines that are uniform across the country. However, there is no consideration about whether the WBGT thresholds are appropriate for different weather conditions, such as warm-humid (WH) and hot-dry (HD), given known differences in physiological responses to these environments.

Objective: To identify if regions with drier conditions and greater evaporative cooling potential should consider different WBGT activity modification thresholds than those with more humid weather.

Setting & Design: Modeling study utilizing weather stations across the contiguous U.S. and physiological models.

Main Outcome Measures: A 15-year hourly WBGT dataset from 217 weather stations across the contiguous U.S. was used to identify particular combinations of globe temperature, wet bulb temperature, and air temperature that would produce WBGTs of 27.9°C, 30.1°C, and 32.3°C. Thousands of observations were clustered into HD and WH weather conditions. From these clusters, maximum heat loss potential (H­_total) and energy budget values were modeled at equivalent WBGT thresholds (i.e., 32.3^oC) with varying activity levels and clothing/equipment configurations.

Results:We identified strong geographic patterns, with HD predominant in the western and WH in the eastern halves of the country, respectively. Heat loss was systematically greater within HD versus WH conditions, indicating an overall less stressful environment, even at equivalent WBGT values. At a WBGT of 32.3°C, this difference was 11 Wm^-2at an activity speed of 0.3ms^-1, which doubled when the speed increased to 0.7 ms^-1. The HD-WH difference increased with the WBGT level, demonstrating that evaporative cooling in HD is comparatively even more important at a higher, rather than lower, WBGT.

Conclusions: The H_totalwas consistently greater in HD than WH environments despite the equal WBGTs. These findings support the need for further clinical studies of appropriate WBGT thresholds between dry or humid environments to maximize safety yet avoid unnecessary limitations.