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

ABSTRACT 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 ( T rec ) was continuously monitored. Contrary to model predictions, all five conditions remained compensable ( T rec rise rate ≤ 0.1°C/h), with mean peak T rec well below heatstroke thresholds (38.2 ± 0.4°C). At 44°C/29.2% RH, females exhibited significantly lower T rec than males ( 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. NEW & NOTEWORTHY This study demonstrates that healthy young adults maintained heat balance for 8 hours in five extreme heat conditions (36–50°C), despite the Core Temperature Inflection Point (CTIP) method and biophysical model predictions indicating uncompensable heat stress. These findings challenge the reliability of current assessment tools for prolonged heat exposure and underscore the need for refined models to more accurately predict critical environmental limits, particularly during extreme heatwaves.