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 (CEL), yet their validity under prolonged heat exposure remains untested. 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{degrees}C/74.5% RH, 40{degrees}C/55.0% RH, 44{degrees}C/29.2% RH, 47{degrees}C/35.6% RH, 50{degrees}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 CTIP and biophysical model predictions, all five conditions remained compensable (Trec rise rate [&le;] 0.1{degrees}C/h), with mean peak Trec staying well below heatstroke thresholds (38.2 {+/-} 0.4{degrees}C). At 44{degrees}C/29.2% RH, females had significantly lower Trec than males (p < 0.05), but steady-state Trec responses were similar 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 during real-world heatwaves and informing public and occupational health guidelines in a warming climate. NEW & NOTEWORTHYThis study demonstrates that healthy young adults maintained heat balance over 8 hours across five extreme heat conditions (36-50{degrees}C), despite the Core Temperature Inflection Point (CTIP) method and biophysical modeling predicting uncompensability. These findings challenge the reliability of these tools for assessing prolonged heat exposure, highlighting the need for refined models to accurately predict critical environmental limits in real-world scenarios, such as extreme heatwaves.