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

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 11 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°C to 0.6°C)] and the individual limits of agreement (LOA: -0.6°C 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°C to 1.1°C)], whereas the limits of agreement (0.6°C 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. T_re and T_gi demonstrated acceptable group-level agreement but caution is warranted when interpreting individual responses.NEW & NOTEWORTHY We assessed agreement in inflection points for esophageal (T_es), rectal (T_re), and gastrointestinal (T_gi) temperatures in young adults during progressive humidity protocols. T_es and T_re exhibited good agreement on both group (study) and individual levels. Although group-level agreement between T_gi and T_re was acceptable, individual agreement was poor. These findings indicate that all three techniques are suitable for studies of critical environmental limits, though T_es or T_re are preferred when assessing individual thresholds.