Expert: John Locke - 4/27/2007

Question
Why do we feel 25°C and over as "hot" and why do we start sweating at around those temperatures, when our body temperature is 37°C ?

Answer
Jeronimo, thank you for using AllExperts and I'd be happy to answer your question.

This question requires an intersection of biology and physics (though, thankfully, neither is particularly complex). All warm-blooded organisms maintain a constant body temperature by the breakdown of food into simpler molecules, which can be used directly by the body. The body temperature maintained by each species varies (in fact, the body temperature of any single organism varies over the course of a day), but it always relates to metabolism: the higher the metabolism, the higher the temperature. The ambient temperature at which each organism will feel most comfortable depends upon their body temperature--perhaps an obvious statement, but a necessary one for the reasoning to follow.

An organism feels most comfortable when is it in a state known as thermal equilibrium: that is, the amount of heat lost through the skin equals the amount of heat that enters through the skin. The basic equations of thermodynamics--none of which I will write here!--tell us that thermal equilibrium occurs when the external temperature equals in the internal temperature of the body. Thus, for humans, we would expect to reach a state of thermal equilibrium at 37C ambient temperature. If more heat enters the body than escapes, it responds with heat-reduction mechanisms (sweating, flushing); in the opposite case, the body responds with heat conservation mechanisms (shivering, blood vessel constriction).

Why, then, do humans feel uncomfortable at 37C? The answer is actually pretty simple--the free exchange of heat to the air is impeded by clothing. A naked human reaches a state of thermal equilibrium at 37C (and thus feels comfortable there), while a clothed person reaches it at a lower temperature, because less heat is able to escape through the skin. That clothed person will feel hot at 37C; more heat enters the body than is able to escape.

In addition, the more clothing worn, the more it impedes with the free exchange of heat--you can even reach thermal equilibrium at 0C in the winter by wearing a heavy jacket, gloves, hat, and pants. Other animals have adaptations to prevent heat loss (fur, feathers, large stores of body fat) but humans have lost many of these mechanisms and have responded with the development of clothing. In general, any organism that wants to survive in an environment where the ambient temperature is lower than its body temperature (like humans in most temperate climates, deer in the tundra, seals in the Arctic, etc.) must have some kind of heat-loss prevention mechanism. We have the ability to shed ours when the outside temperature gets too warm.