A physiological approach for assessing human survivability and liveability to heat in a changing climate

The study addresses the limitations of using a fixed 35 °C wet-bulb temperature (Tw) threshold to project human survivability under extreme heat by not accounting for physiological variability and environmental diversity. It highlights growing global risks from extreme heat due to climate warming, urban heat, and population aging, with particular vulnerability in humid climates and among older adults, the unhoused, and those with chronic illness. Two major methodological traditions—epidemiology/econometric and physiology-based approaches—are compared: epidemiological studies provide empirical associations with health outcomes but face challenges extrapolating to unprecedented temperatures and clarifying humidity’s role; physiology-based approaches capture humidity’s effect on evaporative cooling but often rely on idealized conditions and simplified theory. The paper proposes a physiology- and biophysics-based modeling framework to estimate survivability (heat-stroke death risk from hyperthermia in 3–6 h exposures) and to introduce liveability (maximum safe, sustained metabolic activity without continuous heat storage), accounting for age-related thermoregulatory differences, sun versus shade, wind, clothing, and the full spectrum of temperature-humidity combinations. The goal is to improve projections of survival limits and to assess how people can safely live and work under present and future climates.

Prior work frequently adopts a universal Tw = 35 °C threshold (Sherwood & Huber) as an adaptability limit implying death within ~6 h under idealized shaded, nude, sedentary, fully acclimatized conditions. Subsequent studies have used this threshold to identify future survivability risks. Other metrics include labor capacity reductions under wet-bulb globe temperature (WBGT) (e.g., Dunne et al.). Epidemiological studies often report higher cardiovascular and respiratory mortality with high temperature, but the role of humidity is inconsistent across studies, posing challenges for future projections as specific humidity is expected to rise with warming. Physiology-based research consistently underscores humidity’s role in limiting evaporative cooling via sweat but traditionally lacks complexity to capture interindividual differences (body size, clothing, activity, sweating capacity). Recent chamber studies (e.g., Vecellio et al.; Wolf et al.) suggest lower moist heat tolerance limits than the canonical 35 °C Tw, especially in hot-dry environments, supporting the need for more nuanced, physiology-informed thresholds.

The authors apply a whole-body human heat exchange model grounded in partitional calorimetry to simulate heat transfer between the body and environment across a matrix of warm conditions (air temperature 25–60 °C; relative humidity 0–100%; wind speed 1 m/s). Two radiation settings are modeled: shaded/indoors (mean radiant temperature Tr ≈ Tair) and sun-exposed/outdoors (Tr = Tair + 15 °C, representing partly sunny midday conditions). Two subpopulations are assessed: young healthy adults (18–40 years) and older adults (>65 years), with age-specific body size, sweat rate limits, and skin wettedness assumptions. For survivability, the model defines death by heat stroke as reaching core temperature Tcore = 43 °C after 3 or 6 hours of constant exposure. From an initial Tcore = 37 °C, the allowable cumulative heat storage is 17.88 kJ kg−1, yielding critical storage rates Ssurv = 0.82 W kg−1 (6 h) and 1.65 W kg−1 (3 h). Assuming resting metabolic rate = 1.5 METs (1.8 W kg−1), the required net heat loss from skin is 0.98 W kg−1 (6 h) or 0.15 W kg−1 (3 h). Nudity (zero clothing resistance) and adequate hydration are assumed for survivability. Evaporative heat loss constraints are explicitly modeled: (1) environmental limit (Emax) due to humidity; (2) physiological limit via maximum skin wettedness; and (3) maximum sweat rate (Smax). An algorithm classifies outcomes across five zones: surviving within sweating limits; surviving while exceeding sweating limits; non-survival due to environmental evaporative restriction; non-survival due to required sweat rate exceeding Smax; or both environmental and sweating limits exceeded. Liveability is defined as the maximum metabolic rate (Mmax) achievable with sustained compensable heat stress (S = 0) given steady-state thermoregulation, assuming light clothing (0.36 clo), adequate hydration, and constant sweating/skin temperature; Mmax is reported in METs. The liveability model is applied to CMIP6 climate data (3-hourly near-surface air temperature, specific humidity, surface pressure) from GFDL ESM4 and MPI ESM1.2 for present (2016–2025) and end-of-century (2091–2100) under SSP2-4.5 and SSP5-8.5. Analyses focus on shaded conditions for Tair > 25 °C. Median changes in Mmax are evaluated using two-sided Mann–Whitney U tests (p < 0.05). Model code is in Python with open-source libraries; full equations and parameters are provided in the Supplementary Materials and Zenodo repository.

- Physiological survivability limits are substantially lower than the canonical Tw = 35 °C, especially in hot, dry air and for older adults. In very humid shaded conditions (RH > 90%), physiological limits are ~0.7–1.3 °C lower than 35 °C for 6 h exposures, but in dry heat they are much lower. - Shaded 6-hour survivability (Table 2): • Young adults: Tw limits range 25.8–34.1 °C (i.e., 0.9–9.2 °C below 35 °C). Example: at 50% RH, survival up to Tair 43.3 °C (Tw 33.6 °C); at 25% RH, survival up to Tair 49.9 °C (Tw 31.3 °C). • Older adults: Tw limits range 21.9–33.7 °C (i.e., 1.0–13.1 °C below 35 °C). Example: at 25% RH, survival up to Tair 45.4 °C (Tw 27.8 °C), which is 7.2 °C below 35 °C; at 10% RH, would not survive 6 h beyond Tw 21.9 °C (Tair 46.4 °C). - Sun exposure further reduces survivability compared to shade (not captured by fixed 35 °C Tw approaches). - Liveability (Mmax): • Mmax is highest in cooler, drier conditions and declines with higher Tair, humidity, sun exposure, and age. • In shade at Tair = 25 °C, young adults can safely sustain ~5.0 METs in humid air and up to ~8.4 METs in dry air; in sun at the same Tair, limits drop to ~3.9–7.4 METs. • In very humid air (RH > 75%), Mmax falls sharply with warming; young adults cannot safely perform activity >1.5 METs at ~Tair 35.5 °C (older adults at ~34.0 °C). The hatched region indicates conditions that are survivable but not liveable (no activity increase without continuous heat storage). • Age effect: in hot-dry conditions, young adults can perform ~2.5–3.0 METs more than older adults; differences narrow to ~0.6–0.8 METs at lower Tair/higher RH. Sun exposure reduces Mmax by ~1.0 MET (young) and ~0.86 METs (older) relative to shade at the same Tair and RH. - Global projections (GFDL ESM4; shaded; Tair > 25 °C): median Mmax declines by about −0.25 METs by 2100 under SSP2-4.5, and by roughly −0.64 METs under SSP5-8.5. Many warm, humid, and coastal regions show >5% reductions in median Mmax and significant increases in time that is survivable but not liveable (up to ~5–7% of the decade under SSP5-8.5 in regions such as the Arabian Peninsula, Northern India, and Bangladesh). - Across six exemplar cities, aging imposes a stronger reduction in safe activity than projected warming alone; in some locations, many older adults already cannot safely exceed ~2.5 METs during warm conditions.

Findings demonstrate that a fixed 35 °C wet-bulb threshold overestimates human heat tolerance, particularly in hot-dry environments and for older adults, because it ignores physiological constraints on sweating and evaporative heat loss and neglects solar radiation impacts. By explicitly modeling evaporative limits (environmental Emax, skin wettedness, and maximum sweat rate), the physiological approach captures nonlinear survivability behavior, yielding much lower Tw limits in dry air and widening age-related differences. Solar load further erodes both survivability and liveability. Comparisons with chamber studies (e.g., Wolf et al., Vecellio et al.) broadly support the modeled shaded liveability limits for young adults. The liveability framework extends beyond survival by estimating the maximum sustainable activity intensity; results indicate substantial current and future constraints on safe activity in many regions, with aging exerting a larger impact than climate warming on Mmax distributions. These insights refine projections of heat-health risks and identify regions where survivable conditions may increasingly become unliveable without adaptation, informing public health preparedness and adaptation strategies.

The study introduces a physiology-based modeling framework that improves estimates of survivability and defines liveability (maximum safe, sustained activity) under extreme heat for diverse climates and populations. It reveals large underestimations of risk by the canonical Tw = 35 °C approach, especially in hot-dry conditions and for older adults, with physiological survivability limits up to ~13 °C Tw lower than 35 °C. Liveability declines are projected globally by end-of-century, particularly in already hot and humid, populous regions. Aging reduces Mmax more than projected warming, emphasizing the importance of demographic change in future heat-health risks. The flexible model enables integration with climate projections and supports more robust assessments of global survivability and liveability under increasing heat stress. Future work should expand subpopulation characteristics (co-morbidities, medications), behaviors, personal cooling strategies, clothing, wind and activity effects, and utilize multi-model climate ensembles and downscaling to refine projections and guide heat risk management.

- Outcomes focus on heat stroke death via hyperthermia (Tcore ≥ 43 °C); cardiovascular collapse and renal failure are not modeled, likely underestimating total heat-related mortality. - Assumes constant conditions during 3–6 h exposures (steady-state variables such as sweating rate, skin temperature, wind, and clothing) and adequate hydration. - Population scope limited to younger and older female adults with specific body sizes and thermophysiological parameters; does not incorporate co-morbidities, medications, fitness, acclimatization variability, or behavioral adaptations beyond sun/shade scenarios. - Survivability assumes nudity; liveability assumes light clothing (0.36 clo); clothing diversity and metabolic transients are not explored. - Climate application uses only two GCMs (GFDL ESM4, MPI ESM1.2), without bias correction or urban downscaling; urban microclimates and model biases are not fully represented. - Sensitivity to wind speed, activity velocity, personal cooling strategies (e.g., fans, dousing), and ensemble climate uncertainties are not comprehensively analyzed (identified as future work).