Revisiting the Humidity Ramp Protocol for Assessing Human Heat Tolerance Limits

Abstract

BackgroundHumidity ramp protocols are widely used to estimate human heat tolerance limits, yet reported critical environmental limits (CELs) vary markedly across studies. Whether this variability reflects true physiological differences or systematic methodological artifacts related to ramp design remains unresolved. MethodsWe combined first-order thermal modeling with controlled physiological trials to examine how ramp temporal structure (step dwell time) influences apparent rectal temperature (Tcr) inflection points. Twenty-six healthy young adults completed randomized trials at 42 {degrees}C under two protocols: an aggressive-ramp (30-min equilibration followed by humidity increases every 5 min) and a slow-ramp (4-h equilibration followed by hourly humidity increments). CELs derived from ramp protocols were further evaluated using prolonged fixed-condition exposures in independent cohorts (n=14 per sex). ResultsShort-step dwell times ({Delta}t/{tau} <<1) prevented rectal temperature from approaching its step-specific equilibrium, resulting in kinetically constrained, non-equilibrium dynamics and earlier Tcr inflection points. Consequently, aggressive-ramp protocols yielded substantially lower CELs than slow-ramp protocols in both sexes ({approx}3.5 {degrees}C difference). CELs derived from the slow ramp closely matched those obtained from prolonged fixed-condition exposures, whereas aggressive ramps misclassified physiologically compensable conditions as uncompensable. ConclusionRapid humidity increments systematically underestimate heat tolerance because environmental forcing outpaces physiological response kinetics. Accurate CEL determination requires either prolonged fixed-condition exposures (benchmark approach) or sufficiently slow ramps that allow near-equilibrium responses ({Delta}t/{tau} {gtrsim}0.3-0.5).