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Field-Based Thermal Physiology Assay: Cold Shock Recovery under Ambient Conditions
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Validating new limits for human thermoregulation.

Robert D Meade1,2, Fergus K O'Connor1, Brodie J Richards1

  • 1Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, ON K1N 6N5, Canada.

Proceedings of the National Academy of Sciences of the United States of America
|March 31, 2025
PubMed
Summary
This summary is machine-generated.

New research validates thermal-step protocols for assessing human survivability limits in extreme heat. These studies establish critical wet bulb temperature thresholds for thermoregulation, crucial for understanding heat stress risks.

Keywords:
critical environmental limitsextreme heatheat survivabilityheat wavethermoregulatory compensability

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Area of Science:

  • Environmental Physiology
  • Human Thermoregulation
  • Climate Change Adaptation

Background:

  • Global projections indicate increasing heat and humidity exceeding human thermoregulation limits.
  • Previous survivability limits were based on lab studies with a theoretical threshold of 35°C wet bulb temperature (Twb).
  • Newer empirical limits use core temperature inflection points from thermal-step protocols, but their validity is unestablished.

Purpose of the Study:

  • To validate the use of humidity-step protocols for determining the Twb threshold for core temperature inflection.
  • To assess if this threshold accurately demarcates the Twb above which human thermoregulation becomes impossible.
  • To evaluate the reliability of thermal-step protocols for assessing heat exposure survivability limits.

Main Methods:

  • A humidity-step protocol was employed to estimate the Twb threshold for core temperature inflection in 12 volunteers.
  • Participants were subsequently exposed to Twb above (~33.7°C) and below (~30.9°C) their individual inflection points (~32.3°C) for up to 9 hours.
  • Core body temperature changes were continuously monitored to assess thermoregulation capacity and project heat stroke onset.

Main Results:

  • Core body temperature continuously increased when exposed to Twb above the inflection point (Tabove).
  • Heat stroke (40.2°C core temperature) was projected to occur within 10 hours under Tabove conditions.
  • While Tbelow was also uncompensable, the rate of core temperature rise was significantly lower, projecting heat stroke onset in over 24 hours.

Conclusions:

  • The study validates thermal-step protocols as an effective method for evaluating heat exposure survivability limits.
  • The findings provide a direct assessment of the Twb threshold beyond which human thermoregulation is unsustainable.
  • This research refines our understanding of critical heat stress thresholds, with implications for public health and climate adaptation strategies.