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Cited by 208Thermoregulation and physiological responsesClimate Change and Health ImpactsUrban Heat Island Mitigation

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

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Vanos, Jennifer, Guzman-Echavarria, Gisel, Baldwin, Jane W., Bongers, Coen, Ebi, Kristie L., Jay, Ollie·2023·Nature Communications
DOI 10.1038/s41467-023-43121-5PMC PMC10687011
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Abstract

Most studies projecting human survivability limits to extreme heat with climate change use a 35 °C wet-bulb temperature (T w ) 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 T w model in hot-dry conditions. Updated survivability limits correspond to T w ~25.8–34.1 °C (young) and ~21.9–33.7 °C (old)—0.9–13.1 °C lower than T w = 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.

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Fig. 1 Overview of environmental conditions, populations, assumptions, and assessment types examined in the present study. Full model presented in Supplementary Information. All variables in the model remain constant during the exposure (e.g., wind, clothing, sweat rate, skin wettedness). Tcore Core temperature, Tair Air temperature, RH Relative humidity.
Fig. 1 Overview of environmental conditions, populations, assumptions, and assessment types examined in the present study. Full model presented in Supplementary Information. All variables in the model remain constant during the exposure (e.g., wind, clothing, sweat rate, skin wettedness). Tcore Core temperature, Tair Air temperature, RH Relative humidity.CC BY · PubMed Central
Fig. 2 New Tw survivability limits for younger and older adults under shaded/indoor or sun-exposed conditions. Physiological heat stroke Tw survivability limits (thick black line), modeled across 6 h of constant exposure for young adults (a, c) and older adults (b, d) in shaded/indoor (a, b) and sun-exposed/outdoor (c, d) conditions. Zones 1 and 2 (yellow shades) represent areas of survivability, whereas zones 3–5 are non-survivable areas due to evaporative restrictions from the environment (zone 3), sweating limits (zone 4), or both (zone 5) (see also Supplementary Fig. S1). Tw = 35 °C line is shown by the thin gray line. These physiological survivability limits illustrate the environmental conditions in which the body would reach a deadly Tcore (i.e., after reaching uncompensable heat stress and accumulating enough heat to increase base Tcore by 6 °C, from 37 °C (normothermia) to 43 °C (heat stroke)). (See Supplementary Figs. S2 and 3 for vapor pressure and specific humidity on y-axis, respectively). Source data are provided as a Source Data file.
Fig. 2 New Tw survivability limits for younger and older adults under shaded/indoor or sun-exposed conditions. Physiological heat stroke Tw survivability limits (thick black line), modeled across 6 h of constant exposure for young adults (a, c) and older adults (b, d) in shaded/indoor (a, b) and sun-exposed/outdoor (c, d) conditions. Zones 1 and 2 (yellow shades) represent areas of survivability, whereas zones 3–5 are non-survivable areas due to evaporative restrictions from the environment (zone 3), sweating limits (zone 4), or both (zone 5) (see also Supplementary Fig. S1). Tw = 35 °C line is shown by the thin gray line. These physiological survivability limits illustrate the environmental conditions in which the body would reach a deadly Tcore (i.e., after reaching uncompensable heat stress and accumulating enough heat to increase base Tcore by 6 °C, from 37 °C (normothermia) to 43 °C (heat stroke)). (See Supplementary Figs. S2 and 3 for vapor pressure and specific humidity on y-axis, respectively). Source data are provided as a Source Data file.CC BY · PubMed Central
Fig. 3 Updated survival limits (thick blue line) in extreme heat versus the Tw of 35ºC assumption (black thick line). Comparison of the Tw = 35 °C assumption (black) with our physiological survivability limits (blue), based on 3-hour (dashed lines) and 6-hour (solid lines) of constant exposure at given air temperature and relative humidity combinations within shaded/indoors conditions. Graphs indicate limits to survivability for (a) young and (b) older adults, wherein the limit is based on reaching uncompensable heat stress and accumulating enough body heat for a fixed time of constant exposure to increase Tcore by 6 °C (from 37 °C (normothermia) to 43 °C (heat stroke)). Black lines show Tw values up to 37 °C to avoid unrealistic conditions (thick line shows Tw of 35 °C), and horizontal dark blue lines indicate the ΔTw values at RH levels of 10, 25, 50, 75, and 90, and 100% in Table 2. See Supplementary Figs. S4 and 5 for vapor pressure and specific humidity on y-axis, respectively. Source data are provided as a Source Data file.
Fig. 3 Updated survival limits (thick blue line) in extreme heat versus the Tw of 35ºC assumption (black thick line). Comparison of the Tw = 35 °C assumption (black) with our physiological survivability limits (blue), based on 3-hour (dashed lines) and 6-hour (solid lines) of constant exposure at given air temperature and relative humidity combinations within shaded/indoors conditions. Graphs indicate limits to survivability for (a) young and (b) older adults, wherein the limit is based on reaching uncompensable heat stress and accumulating enough body heat for a fixed time of constant exposure to increase Tcore by 6 °C (from 37 °C (normothermia) to 43 °C (heat stroke)). Black lines show Tw values up to 37 °C to avoid unrealistic conditions (thick line shows Tw of 35 °C), and horizontal dark blue lines indicate the ΔTw values at RH levels of 10, 25, 50, 75, and 90, and 100% in Table 2. See Supplementary Figs. S4 and 5 for vapor pressure and specific humidity on y-axis, respectively. Source data are provided as a Source Data file.CC BY · PubMed Central
Fig. 4 Estimates of liveability at varying combinations of air temperature and relative humidity. Liveability estimates based on maximum safe metabolic rate (Mmax) that a person can generate without a sustained positive rate of heat storage even with a maximal thermoregulatory response. Results are presented across a range of air temperature and relative humidity for younger (a, c) and older adults (b, d) in shaded (top) or sun-exposed (bottom) steady-state environments. The new 3-hour survivability line is shown in purple; constant Tw values are shown by the solid black lines until 37 °C to avoid unrealistic conditions, with Tw = 35 °C shown by the thick black line. Activities by MET level range from no activity (sitting ~1.5 METs), to housework (~3.0 METs), dancing (~5.0 METs), and heavy lifting (~7.0 METs). The hatched area indicates conditions that are survivable but not livable (i.e., people cannot increase their activity without continuously storing heat inside the body, which will lead to a continuous rise in core temperature, but heat stroke death after a 3-hour exposure would not occur). Icons indicate MET-equivalent activities according to Ainsworth et al.56. Circles indicate critical Tw limits reported by Wolf et al.62 for minimal (~1.8 METs–filled circles) and light physical activity (~3.2 METs–open circles). Note: 1 MET corresponds to complete rest. See Supplementary Figs. S6 and 7 for vapor pressure and specific humidity on y-axis, respectively. Icons provided by Icons8 (https://icons8.com). Source data are provided as a Source Data file.
Fig. 4 Estimates of liveability at varying combinations of air temperature and relative humidity. Liveability estimates based on maximum safe metabolic rate (Mmax) that a person can generate without a sustained positive rate of heat storage even with a maximal thermoregulatory response. Results are presented across a range of air temperature and relative humidity for younger (a, c) and older adults (b, d) in shaded (top) or sun-exposed (bottom) steady-state environments. The new 3-hour survivability line is shown in purple; constant Tw values are shown by the solid black lines until 37 °C to avoid unrealistic conditions, with Tw = 35 °C shown by the thick black line. Activities by MET level range from no activity (sitting ~1.5 METs), to housework (~3.0 METs), dancing (~5.0 METs), and heavy lifting (~7.0 METs). The hatched area indicates conditions that are survivable but not livable (i.e., people cannot increase their activity without continuously storing heat inside the body, which will lead to a continuous rise in core temperature, but heat stroke death after a 3-hour exposure would not occur). Icons indicate MET-equivalent activities according to Ainsworth et al.56. Circles indicate critical Tw limits reported by Wolf et al.62 for minimal (~1.8 METs–filled circles) and light physical activity (~3.2 METs–open circles). Note: 1 MET corresponds to complete rest. See Supplementary Figs. S6 and 7 for vapor pressure and specific humidity on y-axis, respectively. Icons provided by Icons8 (https://icons8.com). Source data are provided as a Source Data file.CC BY · PubMed Central
Fig. 5 Difference in liveability between older and younger adults for shaded or sun-exposed conditions. The difference in liveability—or maximum safe metabolic rate (Mmax)—between young and older adults in sun-exposed (a) and shaded/indoor (b) conditions. The dotted area indicates air temperature and relative humidity combinations that, with aging, shift the situation from liveable to only survivable (i.e., people cannot increase their activity without continuously storing heat inside the body, which will lead to a continuous in core temperature, but heat stroke death after a 3-hour exposure would not occur). For example, in the shade, a young adult can perform 1 METs more work that an older adult within the green-shaded area. Note: The ΔMET values can also be seen by directly comparing Fig. 4a, b (for 5a) or Fig. 4c, d (for 5b). Source data are provided as a Source Data file.
Fig. 5 Difference in liveability between older and younger adults for shaded or sun-exposed conditions. The difference in liveability—or maximum safe metabolic rate (Mmax)—between young and older adults in sun-exposed (a) and shaded/indoor (b) conditions. The dotted area indicates air temperature and relative humidity combinations that, with aging, shift the situation from liveable to only survivable (i.e., people cannot increase their activity without continuously storing heat inside the body, which will lead to a continuous in core temperature, but heat stroke death after a 3-hour exposure would not occur). For example, in the shade, a young adult can perform 1 METs more work that an older adult within the green-shaded area. Note: The ΔMET values can also be seen by directly comparing Fig. 4a, b (for 5a) or Fig. 4c, d (for 5b). Source data are provided as a Source Data file.CC BY · PubMed Central
Fig. 6 Global maps of liveability estimates. a, d Differences in median safe Mmax (maximum safe metabolic rate) (ΔMET) between the present (2016–2026) and end-of-century (2091–2100) for SSP2-4.5 and SSP5-8.5, respectively (negative values indicate less activity possible), with dotted areas indicating locations where median difference is significantly different (P < 0.05), (b, e) Median Mmax for current decade (2016–2026) for SSP2-4.5 and SSP5-8.5, respectively, where lower values (e.g., in Bangladesh) indicate the most oppressive conditions and least ability to perform activity, (c, f) the percentage of time for 2091–2100 decade with Tair > 25 °C for SSP2-4.5 and SSP5-8.5, respectively. All analyses are based on warm conditions (Tair >25 °C) for young adults. 3-hourly CMIP6 data are from GFDL ESM4 (~1° × 1.25° atmosphere/land grid) following SSP2-4.5. Areas with no data indicate locations that do not reach Tair > 25 °C in the given decade. Note: 1 MET corresponds to complete rest. CMIP6 Coupled Model Intercomparison Project phase 6, GFDL ESM4 Geophysical Fluid Dynamics Laboratory Earth Systems Model 4. SSP Shared socioeconomic pathway. Made with Natural Earth - free vector and raster map data at naturalearthdata.com. Source data are provided as a Source Data file.
Fig. 6 Global maps of liveability estimates. a, d Differences in median safe Mmax (maximum safe metabolic rate) (ΔMET) between the present (2016–2026) and end-of-century (2091–2100) for SSP2-4.5 and SSP5-8.5, respectively (negative values indicate less activity possible), with dotted areas indicating locations where median difference is significantly different (P < 0.05), (b, e) Median Mmax for current decade (2016–2026) for SSP2-4.5 and SSP5-8.5, respectively, where lower values (e.g., in Bangladesh) indicate the most oppressive conditions and least ability to perform activity, (c, f) the percentage of time for 2091–2100 decade with Tair > 25 °C for SSP2-4.5 and SSP5-8.5, respectively. All analyses are based on warm conditions (Tair >25 °C) for young adults. 3-hourly CMIP6 data are from GFDL ESM4 (~1° × 1.25° atmosphere/land grid) following SSP2-4.5. Areas with no data indicate locations that do not reach Tair > 25 °C in the given decade. Note: 1 MET corresponds to complete rest. CMIP6 Coupled Model Intercomparison Project phase 6, GFDL ESM4 Geophysical Fluid Dynamics Laboratory Earth Systems Model 4. SSP Shared socioeconomic pathway. Made with Natural Earth - free vector and raster map data at naturalearthdata.com. Source data are provided as a Source Data file.CC BY · PubMed Central
Fig. 7 Histograms of safe activity in six global cities comparing young and older adults as well as present and future climates. Maximum safe metabolic rate (Mmax) histograms for six selected locations (a) Roma, Italy; (b) Riyadh, Saudi Arabia; (c) Cartagena, Colombia; (d) Kumasi, Ghana; (e) New Delhi, India; (f) Dakha, Bangladesh using data from the present (2016–2026) and end-of-century (2090–2100) for young adults (teal and red) and over 65 years old (gold and yellow). 3-hourly CMIP6 data from GFDL ESM4 following SSP2-4.5 (left column) and SSP5-8.5 (right column) is used selecting the grid point closest to the city location. Note: the Mmax range truncates around a minimum value of 1.5 METs as the lower limit of activity intensity to survive but not live. Based on distinct moisture regimes, the six locations were chosen within areas that will experience the top 5% Mmax decline for young adults and overlap with the main cities in those regions. CMIP6: Coupled Model Intercomparison Project phase 6; GFDL ESM4: Geophysical Fluid Dynamics Laboratory Earth Systems Model 4. SSP: Shared socioeconomic pathway. Source data are provided as a Source Data file.
Fig. 7 Histograms of safe activity in six global cities comparing young and older adults as well as present and future climates. Maximum safe metabolic rate (Mmax) histograms for six selected locations (a) Roma, Italy; (b) Riyadh, Saudi Arabia; (c) Cartagena, Colombia; (d) Kumasi, Ghana; (e) New Delhi, India; (f) Dakha, Bangladesh using data from the present (2016–2026) and end-of-century (2090–2100) for young adults (teal and red) and over 65 years old (gold and yellow). 3-hourly CMIP6 data from GFDL ESM4 following SSP2-4.5 (left column) and SSP5-8.5 (right column) is used selecting the grid point closest to the city location. Note: the Mmax range truncates around a minimum value of 1.5 METs as the lower limit of activity intensity to survive but not live. Based on distinct moisture regimes, the six locations were chosen within areas that will experience the top 5% Mmax decline for young adults and overlap with the main cities in those regions. CMIP6: Coupled Model Intercomparison Project phase 6; GFDL ESM4: Geophysical Fluid Dynamics Laboratory Earth Systems Model 4. SSP: Shared socioeconomic pathway. Source data are provided as a Source Data file.CC BY · PubMed Central

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Metadata

Authors
Vanos, J., Guzman-Echavarria, G., Baldwin, J. W., Bongers, C., Ebi, K. L., & Jay, O.
Affiliations
  • Arizona State University (US)
  • Lamont-Doherty Earth Observatory (US)
  • The University of Sydney (AU)
  • University of Washington (US)
Corresponding author
Vanos, Jennifer
ORCID
Venue
Nature Communications
Publisher
Springer Science and Business Media LLC
Published
2023-11-29
Volume
14
Issue
1
Pages
7653-7653
ISSN
2041-1723 (ISSN-L)
DOI
10.1038/s41467-023-43121-5
PMC
PMC10687011
OpenAlex
W4389141827
Cited by
206
References
80
Citations by year
2026: 72 · 2025: 83 · 2024: 50 · 2023: 1
FWCI
36.76
Type
journal article
Language
en
Open access
Licence
Locations
  • Nature Communications (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • eScholarship (California Digital Library) (submittedVersion, cc-by)
  • RePEc: Research Papers in Economics (submittedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
  • National Science Foundation (CMMI-2045663)
  • Department of Health | National Health and Medical Research Council (2021/GNT2009507)
Topics
  • Health Sciences › Medicine › Physiology (medical) › Thermoregulation and physiological responses
  • Physical Sciences › Environmental Science › Health, Toxicology and Mutagenesis › Climate Change and Health Impacts
  • Physical Sciences › Environmental Science › Environmental Engineering › Urban Heat Island Mitigation
Keywords
wet-bulb temperature, human thermoregulation, climate change, extreme heat, older adults, aging, sun exposure, humidity, physiological limits, heat risk assessment
MeSH terms
Aged, Female, Hot Temperature, Humans, Humidity, Temperature, Extreme Heat, Climate Change

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