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Significance

Research blending climate models with physiological data has projected that large geographical areas may soon experience heat stress exceeding limits for human thermoregulation. Modeled thermoregulatory limits were derived from laboratory research using thermal-step protocols. Despite the growing popularity of this technique, its principal assumption—that core temperature inflection during stepped increases in heat or humidity demarcates thermoregulatory upper limits—has not been validated. By exposing participants for 9 h to conditions just above or below the core temperature inflection point, we found that thermal-step protocols effectively identify the conditions above which thermoregulation is impossible. Our findings provide critical support for heat stress projections incorporating empirical tolerance limits. We also provide data characterizing physiological strain during prolonged, uncompensable heat exposure.

Abstract

Recent projections suggest that large geographical areas will soon experience heat and humidity exceeding limits for human thermoregulation. The survivability limits modeled in that research were based on laboratory studies suggesting that humans cannot effectively thermoregulate in wet bulb temperatures (Twb) above 26 to 31 °C, values considerably lower than the widely publicized theoretical threshold of 35 °C. The newly proposed empirical limits were derived from the Twb corresponding to the core temperature inflection point in participants exposed to stepped increases in air temperature or relative humidity in a climate-controlled chamber. Despite the increasing use of these thermal-step protocols, their validity has not been established. We used a humidity-step protocol to estimate the Twb threshold for core temperature inflection in 12 volunteers. To determine whether this threshold truly demarcates the Twb above which thermoregulation is impossible, each participant was subsequently exposed to Twb above (~33.7 °C, Tabove) and below (~30.9 °C, Tbelow) their respective inflection point (~32.3 °C, Twb) for up to 9 h (in random order). Core temperature rose continuously in Tabove. It was projected that core temperatures associated with heat stroke (40.2 °C) would occur within 10 h. While Tbelow was also uncompensable, the core temperature rate of rise was considerably lower than in Tabove such that it would take >24 h to reach 40.2 °C. Our study supports thermal-step protocols as an effective technique for evaluating survivability limits for heat exposure and provides a direct assessment of the limits of human thermoregulation.