Francisco Karmelic
“Humans maintain a core body temperature near 37 °C that varies slightly among individuals but does not adapt to local climate. Human skin temperature is strongly regulated at 35 °C or below under normal conditions, because the skin must be cooler than body core in order for metabolic heat to be conducted to the skin (17). Sustained skin temperatures above 35 °C imply elevated core body temperatures (hyperthermia), which reach lethal values (42–43 °C) for skin temperatures of 37–38 °C even for acclimated and fit individuals (18, 19, 20, 21). We would thus expect sufficiently long periods of TW > 35 °C to be intolerable.”
S.C. Sherwood and M. Huber. An adaptability limit to climate change due to heat stress.
PNAS, 2010. http://www.pnas.org/content/107/21/9552.full