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 (CEL) 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 to published data. The activity and environments were considered livable if predicted core temperatures did not reach 38±0.25°C within six hours. The outcomes for minimal activity revealed CELs ranging from 34°C/95% RH to 50°C/5% RH. Corresponding dry heat losses ranged from 14 W·m^-2 and -72 W·m^-2 (negative = heat gain) and evaporative heat losses ranged from 39 W·m^-2 to 104 W·m^-2. The wet bulb temperature (Twb) at the CELs ranged from 33.3 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.