Agreement among common core temperature indices in defining critical environmental limits for human thermoregulation: A secondary analysis.

Authors

Olivia K Leach, Robert D Meade, Timo van den Bogaard, Fergus K O'Connor, Hein Am Daanen, W Larry Kenney, Glen P Kenny

Published in

American journal of physiology. Regulatory, integrative and comparative physiology. Jul 22, 2026. Epub Jul 22, 2026.

Abstract

Global intensification of heat extremes has heightened interest in defining critical environmental limits for human thermoregulation. These limits are commonly estimated from the inflection point in core temperature during progressive heat stress protocols. Esophageal temperature (T_es) is the criterion core temperature index, as it reflects right atrial blood temperature and responds rapidly to changes in the body's thermal state. However, measurements of rectal (T_re) and gastrointestinal temperature (T_gi) are more commonly used due to better participant tolerability. We evaluated agreement between inflection points of these core temperature indices using data from two previously published progressive heat stress protocols. Study 1 measured T_es and T_re in 12 resting young adults (3 female). Study 2 evaluated T_re and T_gi in 16 resting or cycling young adults (8 females). Ambient conditions at the identified inflection points were converted to wet-bulb temperature (T_wb) and analyzed with Bland-Altman analysis (±1.3°C T_wb acceptable agreement range). In Study 1, both the mean bias (0.3°C [95% CI: 0.0 to 0.6°C]) and the individual limits of agreement (LOA: -0.6 to 1.3°C) for the T_es-T_re comparison were within ±1.3 °C T_wb. In Study 2, the mean bias was also within the acceptable range (0.8°C [0.5 to 1.1°C]), whereas the limits of agreement (0.6 to 1.7°C T_wb) exceeded this threshold. Although larger confirmatory trials are needed, these findings suggest that T_re inflection identified during a progressive humidity protocol closely aligns with that identified using T_es. While T_re and T_gi demonstrated acceptable group-level agreement, caution is warranted when interpreting individual responses.

PMID:
42485052
Bibliographic data and abstract were imported from PubMed on 22 Jul 2026.

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