Extremes of human heat tolerance: life at the precipice of thermoregulatory failure

1. Human life is sustainable only below an internal temperature of roughly 42–44 °C. Yet our ability to survive at severe environmental extremes is testimony to the marvels of integrative human physiology. 2. One approach to understanding human thermoregulatory capacity is to examine the upper limits of thermal balance between man and the air environment, i.e. the maximal environmental conditions under which humans can maintain a steady-state core temperature. Heat acclimation expands the zone of thermal balance. 3. Human beings can and do, often willingly, tolerate extreme heat stresses well above these thermal balance limits. Survival in all such cases is limited to abbreviated exposure times, which in turn are limited by the robustness of the thermoregulatory response. 4. Figures are provided that relate tolerance time and the rate of change in core temperature to environmental characteristics based on data compiled from the literature.