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

Article https://doi.org/10.1038/s41467-023-43121-5 A physiological approach for assessing human survivability and liveability to heat in a changing climate Received: 14 March 2023 Check for updates 1234567890():,; 1234567890():,; Accepted: 1 November 2023 Jennifer Vanos Coen Bongers 1 , Gisel Guzman-Echavarria2, Jane W. Baldwin , Kristie L. Ebi 7 & Ollie Jay 6 3,4 , 5,6 Most studies projecting human survivability limits to extreme heat with climate change use a 35 °C wet-bulb temperature (Tw) threshold without integrating variations in human physiology. This study applies physiological and biophysical principles for young and older adults, in sun or shade, to improve current estimates of survivability and introduce liveability (maximum safe, sustained activity) under current and future climates. Our physiology-based survival limits show a vast underestimation of risks by the 35 °C Tw model in hot-dry conditions. Updated survivability limits correspond to Tw~25.8–34.1 °C (young) and ~21.9–33.7 °C (old)—0.9–13.1 °C lower than Tw = 35 °C. For older female adults, estimates are ~7.2–13.1 °C lower than 35 °C in dry conditions. Liveability declines with sun exposure and humidity, yet most dramatically with age (2.5–3.0 METs lower for older adults). Reductions in safe activity for younger and older adults between the present and future indicate a stronger impact from aging than warming. Adverse health impacts of extreme heat exposure are expected to rise globally due to a warming climate, urban-induced warming, and a growing and aging population1,2. The concerns for human health, productivity, and well-being are greater in humid climates and for vulnerable populations3–5, such as older adults, unhoused, and/or those with chronic diseases. Therefore, robust models to assess current heat-health impacts and project future risks must incorporate specific vulnerabilities and diverse environmental contexts6. Methods to project future heat stress risk can be broadly categorized into epidemiology/econometric and physiology-based approaches, which have contrasting benefits and limitations. Epidemiology/econometric approaches are empirical in nature, analyzing time series of historical temperature paired with particular health consequences (e.g., morbidity or mortality) across populations to determine heat-health relationships. These studies often find higher rates of cardiovascular and respiratory deaths associated with high ambient temperatures. Future health burdens from heat can be estimated by applying these relationships to climate model outputs (i.e., daily temperature) under different warming scenarios7,8. Empirical approaches are based on real-life outcomes and the range of realistic living conditions, and they can explore the cumulative effects of exposures over multiple days. However, two limitations for climate change projections include 1) assumptions needed to extrapolate results to warmer temperatures than observed in the historical sample9 and 2) ambiguity regarding the role of humidity in heat-health outcomes10. While some epidemiological studies find a relationship between mortality in the heat and humidity11, most find minimal associations between humidity and heat-health outcomes12. Given that specific humidity is robustly expected to increase with global warming, this uncertainty is a key research gap for epidemiology-based projections of future heat stress. Physiology-based studies of future heat stress risk employ relationships between the thermal environment and health outcomes based on human energy balance considerations, with parameters 1 School of Sustainability, Arizona State University, Tempe, AZ, USA. 2School of Geographical Sciences and Urban Planning, Arizona State University, Tempe, AZ, USA. 3Department of Earth System Science, University of California Irvine, Irvine, CA, USA. 4Lamont-Doherty Earth Observatory, Palisades, NY, USA. 5 Department of Medical Sciences, Radboud university medical center, Nijmegen, The Netherlands. 6Heat and Health Research Incubator, University of e-mail: Sydney, Sydney, NSW, Australia. 7Center for Health and the Global Environment, University of Washington, Seattle, WA, USA. Nature Communications | (2023)14:7653 1 Article constrained by studies of physiologic processes. In contrast to epidemiology/econometric approaches, physiology-based studies of heathealth outcomes consistently find a robust role of atmospheric humidity in heat stress via its modulation of evaporative cooling from sweat10. However, physiology studies are limited in not directly observing health outcomes, such as hospitalization or death, and employ idealized conditions from thermal chamber studies. A range of physiology-based metrics has been applied to project future heat stress. Sherwood & Huber12 introduced a 35 °C wet bulb temperature (Tw) threshold that would result in death after 6 h of exposure and applied this threshold to project future adaptability limits under varying levels of warming. Since then, numerous studies have used this approach, wherein a psychometric Tw of 35 °C assumes death13–17. The Tw of 35 °C represents a thermodynamic limit to heat exchange, whereby the human body becomes an adiabatic system, assuming the person is indoors or shaded, unclothed, completely sedentary, fully heat acclimatized, and of average size without thermoregulatory impairments12. As an example of a different metric, Dunne et al.18 estimated future reductions in labor capacity under different warming scenarios using established guidelines for physical labor under different wet bulb globe temperature levels. While these studies incorporate valuable information about humidity and physiology more realistically than epidemiological studies, their thermal physiology theory remains relatively unsophisticated. These approaches cannot capture complexities and personal characteristics affecting human thermoregulation (e.g., body size, activity levels, clothing, or physiological restrictions—such as sweating—to thermoregulation6,19), which may cause substantial errors. To be useful, heat-health projections should realistically account for factors that increase health risks, such as individual physical characteristics and physiological impairments, as well as interventions that modify or decrease impacts (e.g., lowering metabolic rate; behaviors to reduce exposures). Moreover, models should incorporate ranges in environmental parameters that, together with temperature, result in specific thermoregulatory effects (e.g., dry, humid, sun/shade, windspeed)19. Physiological and biophysical models offer new opportunities to assess how humans might live and work in a warmer future rather than merely determining the prospects for life and death. Here, we demonstrate a unique approach using physiological principles that align with human thermal responses to heat (e.g., heat strain) to overcome simplified approaches that miss essential physiologic and behavioral fa (...truncated)