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When a person is hot, their skin cools them down by dissipating heat generated internally. When the external environment is hotter than body-temperature (98.6F, 37C), this is still possible using evaporative cooling from sweat. If the humidity is roughly 100%, however, sweat no longer evaporates.

Is it possible for humans to cool down in an environment above body temperature and at roughly 100% humidity?

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2 Answers 2

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In 100% humidity in a temperature above body temperature you will die from over heating... quickly.

Sweating is one of the most biologically efficient ways of reducing body heat. however it is influenced by humidity. Sweating operates through evaporative cooling, which requires a relatively low-humidity environment. As humidity goes up, less water can evaporate, leading to less loss of heat from the warm, moist object.

If you are in an environment above body temperature, like a sauna, you can be quite comfortable as long as you can sweat. As the humidity rises you will feel less comfortable, until the air reaches saturation. At this point your body will no longer be able to lose heat and you will die.

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I'm in Thailand at the moment and it is horrific. I should be able to pin my heartrate up at 164 (about 300 watts according to the Wattbike), but I can't get above 140, and I'm saturated with un-evaporated sweat, because the temperature and humidity are so high. One unit of power is two of heat, you can't evaporate the sweat so no E = ML loss of heat - it doesn't matter who you are the laws of physics apply. It has nothing to do with hydration or acclimatisation. At 35C and 80% humidity you move like a ninety year old.

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More of a comment than an answer –  FredrikD Jul 19 at 15:18

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