Validation of livability environmental limits to heat and humidity

Rising global temperatures, driven by climate change, pose a threat to human health and regional livability. Empirical data and biophysical model-derived estimates suggest that the critical environmental limits (CELs) for livability are dependent on ambient temperature and humidity. We use a well-validated, physiology-based, six-cylinder thermoregulatory model (SCTM) to independently derive CELs during sustained minimal, light, and moderate activity across a broad range of ambient temperatures and humidity levels and compare with published data. The activity and environments were considered livable if predicted core temperatures did not reach 38 ± 0.25°C within 6 h. The outcomes for minimal activity revealed CELs ranging from 34°C/95% relative humidity (RH) to 50°C/5% RH. Corresponding dry heat losses ranged from 14 to -72 W·m^-2 (negative = heat gain) and evaporative heat losses ranged from 39 to 104 W·m^-2. The wet-bulb temperature (<i>T</i>_wb) at the CELs ranged from 33.3°C to 20.9°C. Activity shifted CELs toward lower temperatures and humidities. Importantly, our predicted CELs largely agree with observed livability CELs from physiology and those from a biophysical model. The physiology-grounded SCTM has utility for assessing the impact of climate change on regional livability.<b>NEW & NOTEWORTHY</b> This study is the first to use a physiology-grounded thermoregulatory model to predict critical environmental limits (CELs) above which human thermoregulatory capacity is exceeded. The model outcomes closely approximate empirically derived CELs, showing it is a strong model for estimating and preparing for the impact of climate warming on local, regional, and world human population livability and migration.