Exploring biases in heat tolerance testing with a conceptual heat balance approach

Global climate change increases the number and intensity of heat days, which increases the risk of exertional heat illness. The heat tolerance test (HTT) is performed to classify individuals who have suffered exertional heat stroke as heat tolerant or intolerant, prior to their return to activity/duty. The current analysis aimed to i. examine potential biases within the HTT conditions or measures; ii. enhance the interpretation of the HTT results through the prediction of sweat rate required to reach thermal balance (S_req). We calculated all the elements of the heat balance equation including metabolic heat production that was predicted using the Pandolf equation. The evaporative requirement for heat balance, the maximum evaporative rate possible and S_req were calculated for a range of body masses (50–100 kg), body mass indices (18.5–30 kg⋅m⁻²) and wind speeds (0–1.5 m⋅s⁻¹). The calculations suggest that, based on metabolic heat production, the change in rectal temperature appears to be a valid criterion to judge heat tolerance. However, the analysis of skin wettedness reveals that the HTT conditions are not physiologically compensable for more than 75% of the scenarios considered in the present study. Physiological strain will likely differ depending on the morphology of individuals. Thus, it may be necessary to ensure that the HTT conditions are compensable to improve HTT specificity. Finally, the prediction of S_req appears to be a promising tool for interpreting heat intolerance and improving medical care, in cases of significant deviation between measured and estimated S_req.