Mapping Human Survivability at Extreme Wet-Bulb Temperatures 32–35°C

Climate change-driven extreme heat events increasingly threaten human health. Here, we provide the first empirical map of human survivability across four extreme wet-bulb temperatures (Tw=32–35°C) in controlled trials. Thirty-six healthy young adults (20 males, 16 females) were exposed to twelve dry-bulb temperature and relative humidity combinations in shaded indoor settings under full hydration. Participants remained seated and performed light office tasks until core temperature (Tcore) reached 39°C or exposures lasted 8 hours. Clinical heatstroke threshold (Tcore=40.5°C) were projected by extrapolating from our direct Tcore measurements during controlled exposure trials. At Tw=35°C, males and females reached heatstroke in approximately 7.1–7.7 and 8.3–8.6 hours, respectively–partly supporting the 6-hour theoretical limit while revealing enhanced short-duration resilience when hydrated. At Tw=34°C, projected times are 12.5–12.9 hours (males) and 15.5–16.2 hours (females). Conditions at Tw=32°C and 33°C remained (quasi-)compensable, with projected heatstroke onset after 33–35 hours. Females consistently tolerated heat stress longer than males. These physiological benchmarks are critical for improving heat‑health models and embedding sex‑specific vulnerability in equitable adaptation strategies amid accelerating global warming.