Revisiting Humidity Ramp Protocols for Assessing Human Heat Tolerance

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 ( T _cr ). 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 T _cr response to stepwise changes in equilibrium core temperature ( T _cr,eq ) determined by ambient humidity at a fixed dry-bulb temperature. Analytical solutions were derived for discrete humidity steps of duration Δ t , and sensitivity analyses were conducted across physiologically plausible time constants ( τ ). Fourteen healthy young males (23.5±1.8 yrs) completed two randomized trials in a 42 °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 Δt/τ ≪1, residual disequilibrium between T _cr,eq and T _cr accumulates, producing accelerated rises in T _cr and premature CELs. Longer dwell durations (≥ 1 hour per step) allowed near-equilibrium responses, yielding physiologically valid thresholds. Empirically, shorter ramp durations shifted apparent CELs downward by 3.4±1.9 °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.

Key points

• Humidity-ramp protocols are widely used to identify the transition from compensable to uncompensable heat stress, but their reliability is fundamentally constrained by the body’s thermal response time.

• A first-order dynamic model demonstrates that the ratio of step duration (Δ t ) to the physiological time constant for core temperature equilibration ( τ ) determines whether true steady-state conditions are achieved.

• When Δ t / τ ≪ 1 (e.g., 5–10 min steps), dynamic lags accumulate between environmental and physiological states, producing premature inflection points that systematically underestimate human heat tolerance.

• Empirical data confirm that conventional short-duration ramps fail to capture true critical limits, while longer ramps (Δ t ≥ 1 h; Δ t / τ ≈ 0.3–0.5) approach equilibrium but remain less robust than steady-state exposures.

• Accurate determination of critical environmental limits requires either prolonged steady-state exposure or validated dynamic correction models; uncorrected ramp protocols should not be used as stand-alone methods.