Authors
Wang, F., Xu, Y., Wang, H., Cui, M., Hou, X., Wei, B.
Abstract
Background: Humidity ramp protocols are widely used to determine human heat tolerance, yet it remains unclear how the rate and duration of environmental changes affect the apparent inflection in core temperature (Tcr). This study integrates theoretical modeling and empirical trials to examine how the temporal structure of humidity-ramp protocols affects the accuracy of estimated critical environmental limits (CELs). Methods: A first-order model was developed to describe Tcr response to stepwise changes in equilibrium core temperature (Tcr,eq) determined by ambient humidity at a fixed dry-bulb temperature. Analytical solutions were derived for discrete humidity steps of duration {Delta}t, and sensitivity analyses were conducted across physiologically plausible time constants ({tau}). Fourteen healthy young males (23.5{+/-}1.8 yrs) completed two randomized trials in a 42 {degrees}C heat chamber: 1) Slow-ramp: 4-hour equilibration at 40% RH followed by +6% RH/hour for 2 h, then +3% RH/hour (40-61% RH); and 2) Aggressive-ramp: 30 min equilibration followed by +2% RH every 5 min (28-88% RH). Rectal and skin temperatures, heart rate, and perceptual ratings were recorded continuously. Results: When {Delta}t/{tau}<<1, residual disequilibrium between Tcr,eq and Tcr accumulates, producing accelerated rises in Tcr and premature CELs. Longer dwell durations ([≥]1 hour) allowed near-equilibrium responses, yielding physiologically valid thresholds. Empirically, shorter ramp durations shifted apparent CELs downward by by 3.4{+/-}1.9 {degrees}C. Conclusion: Dynamic lags from short dwell intervals lead to the systematic underestimation of heat tolerance. Reliable determination of CELs requires either prolonged steady-state exposures or dynamic correction models validated against such conditions.
Preprint server:
bioRxiv
The authors list and abstract were imported from bioRxiv on 12 Nov 2025.
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