bioRxiv2025
O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=146 SRC="FIGDIR/small/677706v1_ufig1.gif" ALT="Figure 1"> View larger version (52K): org.highwire.dtl.DTLVardef@70b72dorg.highwire.dtl.DTLVardef@9fbfeorg.highwire.dtl.DTLVardef@17f05a5org.highwire.dtl.DTLVardef@4b977b_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical abstractC_FLOATNO C_FIG HighlightsWe report the first empirical human survival map across wet-bulb temperatures of 32-35{degrees}C. HydraAon and behavior cooling extend survival beyond the 6-hour theoreAcal limit. Females consistently tolerate extreme humid heat longer than males in controlled trials. Our findings refine heat-health models and inform equitable climate adaptaAon strategies. In briefExtreme humid heat, measured by wet-bulb temperature (Tw), is nearing the threshold of human survivability under climate change. In controlled trials with 36 adults, we provide the first empirical survival map across Tw=32-35{degrees}C, showing that hydration and behavioral adaptations extend endurance beyond the theoretical 6-hour limit, with women tolerating heat longer than men. These findings refine human heat tolerance thresholds and inform equitable climate adaptation and heat-health strategies. SCIENCE FOR SOCIETYAs climate change accelerates, extreme humid heat events--measured by wet-bulb temperature (Tw)--pose a growing threat to human survival. This study provides the first empirical benchmarks of human survivability across Tw=32-35{degrees}C, clarifying how long healthy adults can endure these conditions under full hydration and minimal activity. We find that while the theoretical 6-hour survival limit at Tw=35{degrees}C is partly supported, both men and women can withstand these conditions for 7-8 hours when hydrated, with women demonstrating greater tolerance. At Tw=34{degrees}C, projected heatstroke onset extends beyond 12-16 hours, while conditions at Tw=32-33{degrees}C remain largely compensable for more than 30 hours. These results challenge prevailing heat tolerance models, underscore the role of behavioral and physiological adaptations, and provide critical data for designing equitable heat-health policies and early warning systems that account for sex differences and vulnerable populations. 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{degrees}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{degrees}C or exposures lasted 8 hours. Clinical heatstroke threshold (Tcore=40.5{degrees}C) were projected by extrapolating from our direct Tcore measurements during controlled exposure trials. At Tw=35{degrees}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{degrees}C, projected times are 12.5-12.9 hours (males) and 15.5-16.2 hours (females). Conditions at Tw=32{degrees}C and 33{degrees}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.