Human heat tolerance limits under prolonged exposure to extreme heat

Extreme heat poses an escalating threat to human health, yet current estimates of human heat tolerance across environmental conditions rely largely on shortcut or predictive approaches rather than direct evidence from prolonged experimental exposure. Here, we exposed 36 unacclimatized healthy young adults to four humid wet-bulb temperature (T_w) conditions (32°C, 33°C, 34°C and 35°C) for up to 8 h while performing seated light office work under unrestricted hydration. Core temperature (T_core) responses were consistent across different dry-bulb temperature-humidity combinations at the same T_w, confirming the wet-bulb temperature as an effective integrated heat stress index in shaded extreme heat conditions without appreciable radiative sources. Conditions at T_w = 32-33°C were compensable, whereas T_w = 34-35°C were uncompensable, as evidenced by a clear progressive rise in T_core over time. At T_w = 35°C, the widely cited upper survival limit, projected tolerance times to life-threatening hyperthermia were 7.1-7.7 h in males and 8.3-8.6 h in females, with females consistently exhibiting slower rates of T_core increase. Collectively, these findings provide the first prolonged, stable-exposure experimental characterization of human heat tolerance across T_w = 32-35°C and enable psychrometric mapping of time-resolved, sex-specific tolerance limits. These empirical benchmarks constitute a critical empirical resource for refining climate-health projections of extreme humid heat and for validating biophysical models, while also complementing operational heat action plans and early-warning systems that focus on lower, sub-extreme heat exposures where the majority of heat-related morbidity occurs.