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ABSTRACT The accurate assessment of human heat tolerance in indoor environments is crucial for occupational safety, climate risk modeling, and public health, especially for young adults engaged in daily indoor activities, a key demographic for gauging population vulnerability. This review comprehensively compares classic prolonged fixed‐condition exposure studies with contemporary fast‐ramp protocols, such as those from the PSU HEAT project. Prolonged benchmarks, using hours‐long fixed conditions to allow thermal equilibration, support sustainable wet‐bulb temperatures (Stull‐recalculated) of 32.3°C–33.7°C for light‐to‐moderate work in acclimatized individuals, 35.7°C–36.3°C for rest in acclimatized individuals, 33.4°C–33.8°C for seated light indoor work in unacclimatized adults, and up to 37.9°C for highly acclimatized rest. Fast‐ramp protocols, by contrast, often yield lower estimates (∼26°C–31.2°C). Observed differences of up to ∼8°C appear linked to protocol design elements: (1) kinetic mismatch from short environmental steps relative to core temperature time constants (∼0.5–12 h), which shifts apparent inflection points in uncompensability criteria, (2) a low rise rate threshold (∼0.05°C/h) for defining uncompensability, which overlaps with normal physiological drift and sensor uncertainty (± 0.1°C), and (3) measurement variability. A compensability criterion of 0.10°C/h better aligns with physiological sustainability and sensor limits. To bridge methodological divides, we recommend prolonged exposures as the gold standard for absolute thresholds; when ramp methods are necessary, adaptive protocols that ensure stepwise equilibration are ideal, with slow ramps (e.g., 60‐min steps) as a practical alternative. These refinements may help ensure that heat tolerance estimates guiding policies are robust and reflect true physiological capacity.
Wang et al. (Thu,) studied this question.