Reader PartiallyDeflected wrote in to ask, “Since our body temperature is around 98 degrees, why do we feel hot when it’s 90?”
Pretty much everything your body does, whether physical (like muscle contractions) or chemical (like some stages of digestion), produces heat as a byproduct. You’re constantly generating it, and constantly losing it to the environment. The hypothalamus, an almond-sized chunk of the brain that rests deep within its squishy confines, acts as the body’s thermostat and tries to keep the amount of heat created and the amount lost close to each other and maintain normal body temperature.
Normally, this is easy enough. Heat seeks equilibrium, a state where everything is the same temperature as everything around it. It’s why a bowl of hot soup and a glass of ice water will both reach room temperature if you leave them out on the counter long enough. Usually, the environment around you is cooler than your body, so your little thermostat can just dump the excess heat into it with thermoregulatory processes like sweating (where the heat is lost by evaporation) and increasing bloodflow through capillaries close to the surface of the skin (where the heat is lost through radiation, convection and conduction).
When there’s a big temperature difference between your body and your environment, heat flows out of you and into the air pretty easily, and you cool down quickly. When the environment is warmer and closer to our body temperature, though, the heat doesn’t transfer as readily or quickly via radiation, convection, and conduction. You’re stuck hanging on to some of your excess heat for longer, and you feel hot and uncomfortable (and if the ambient temperature goes higher than your body temp, heat’s quest for equilibrium means that you’ll take on excess heat from the environment). If conditions are hot and dry, the body can deal with these situations by ramping up sweat to get rid of more heat through evaporation. When it’s hot and humid, though, you really feel hot and gross because the high moisture content of the air makes it more difficult for the sweat to evaporate.
If you spend enough time in a situation where the heat you generate or absorb from the environment exceeds the heat you’re getting rid of, your core temperature will rise and you can suffer from heat illnesses.