Defining human thermoregulation limits: a critical evaluation of predictive models using healthy young adults

Area of Science:

• Human physiology
• Environmental health
• Thermoregulation research

Background:

• The core temperature inflection point (CTIP) method and biophysical modeling are standard tools for assessing human thermoregulation limits.
• However, their accuracy under prolonged heat exposure has not been rigorously validated, raising concerns about their reliability in predicting heat strain.

Purpose of the Study:

• To evaluate the predictive accuracy of CTIP and biophysical models under prolonged indoor heat exposure.
• To determine if current models accurately predict human thermoregulation limits and heat risk in controlled environmental conditions.

Main Methods:

• Thirty-six healthy adults (20 males, 16 females) underwent five 8-hour heat trials at varying temperatures and relative humidity.
• Participants performed sedentary tasks while wearing standardized clothing, with continuous monitoring of rectal temperature (Trec).
• Environmental conditions were based on prior model predictions of human thermoregulation limits.

Main Results:

• All tested heat conditions were compensable, with rectal temperature rise rates below 0.1 °C/h and peak temperatures below heatstroke thresholds.
• Contrary to model predictions, human thermoregulation limits were not reached, indicating models substantially underestimated tolerance.
• While females showed lower Trec at 44°C/29.2%RH, no significant sex differences in steady-state responses were observed across other conditions.

Conclusions:

• CTIP and biophysical models significantly underestimate human thermoregulation limits and overestimate heat risk.
• Current predictive methods require substantial refinement to accurately assess human tolerance to prolonged heat exposure.
• Findings are crucial for improving heat risk assessments and informing public health guidelines in a warming climate.