Critical Environmental Limits: Assessing the Limitations of Core Temperature Inflection Point (CTIP) and Biophysical Modeling

The Core Temperature Inflection Point (CTIP) method and biophysical modeling are widely used to determine critical environmental limits (CELs), yet their validity under prolonged heat exposure remains unverified. This study evaluated their predictive accuracy by exposing 36 healthy young adults (20 males, 16 females; age: 20.9–22.4 yr) to five counterbalanced 8-hour heat trials in a controlled chamber (36°C/74.5% RH, 40°C/55.0% RH, 44°C/29.2% RH, 47°C/35.6% RH, 50°C/24.5% RH). These conditions were selected based on prior CTIP and biophysical model predictions of CELs. Participants engaged in sedentary office tasks (1.29–1.67 METs), wore standardized summer clothing (0.39–0.40 clo), and had ad libitum access to an electrolyte drink, with a 500–kcal sandwich provided at midday. Rectal temperature (Trec) was continuously monitored. Contrary to model predictions, all five conditions remained compensable (Trec rise rate ≤ 0.1°C/h), with mean peak Trec well below heatstroke thresholds (38.2 ± 0.4°C). At 44°C/29.2% RH, females exhibited significantly lower Trec than males (p < 0.05), though steady-state Trec responses did not differ between sexes (all p > 0.10). Collectively, CTIP and biophysical models substantially underestimated CELs, leading to overpredicted heat risk across all trials. These findings challenge the reliability of current predictive methods, suggesting human tolerance may exceed existing estimates. Refining these models is essential for improving heat risk assessment and informing public and occupational health guidelines in a warming climate.