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baloo_the_bear t1_izb0vj6 wrote

Metabolic activity continuously produces heat as a byproduct. Without a method of dissipating that heat, a person would quickly overheat and die.

Normal responses to hyperthermia include vasodilation and sweating as methods to facilitate heat transfer out of the body, but if the surrounding environment is already ‘too hot’ to allow dissipation the heat is trapped in the body and core temperature rises.

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perrochon t1_izb3wsy wrote

Just to add some napkin physics....

A human is producing ~100 watts of power at rest. Water's specific heat capacity is 4.186J/g. Humans consist of much water. Heating a 75kg bag of water 1 degree C using 100W takes 52 minutes.

https://www.google.com/search?q=4.184+joules%2Fg+*+75kg+%2F+100+watt

Normal body temperature is ~37C, life threatening is ~40C. ~41C starts irreversible protein denaturation. So give or take 4 hours to serious damage.

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NakoL1 t1_izb94r0 wrote

also of note if anyone's interested, is that 100W over 24 hours is 2.4 kWh or 8,640 kJ or 2,065 kcal

obviously the actual metabolic rate varies greatly, from around 60-70W or so during deep sleep to >1,000W during intense exercise. As well as between different people

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cowox93112 t1_izb8wyd wrote

The reason people feel most comfortable around 25C (77F) is that this is roughly the equilibrium point, where the heat passively dissipated (no sweating, no wind blowing etc.) into the environment balances with the waste heat generated by the metabolism.

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Halogen12 t1_izbqj46 wrote

That's interesting. I guess I'm more of a furnace because I'm most comfortable at 20C (68F). I like feeling a bit of cool on my skin. I work in a place where some days we have a lot of open flame and I get cranky when it gets over 22C (72F). I live in western Canada, though, so maybe I'm just acclimatized to long winters.

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urzu_seven t1_izccq9n wrote

You don't even need a theoretical blanket to do this, just a hot/humid enough environment. When the air is hot and humid enough that evaporative cooling no longer works human bodies can no longer dissipate heat and bad things start happening. This is called the wet-bulb temperature. Basically when the wet bulb temperature (thermometer whose measuring point is covered by a pocket of water) is equal to the dry bulb temperature (thermometer in open air) it means the air is fully saturated. A human in the shade with unlimited water could not survived more than a few hours when wet bulb reaches 35 °C (95 °F).

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HuntingTheWumpus t1_izbso78 wrote

People die from falling asleep in saunas because they are unable to lose heat due to high humidity. This is also a problem in caves with high humidity, and people have died in relatively cool temperatures because they are unable to cool themselves with perspiration.

There are growing numbers of "death zones" across the planet where human life is impossible without air conditioning because wet bulb temperatures are so high that it is physically impossible for the body to radiate enough heat to keep from cooking itself to death; as planet temperatures rise, inevitably there will be a power failure in one of these death zones and there will be catastrophic loss of life on a monstrous scale.

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NonEuclideanGal OP t1_izcuz2m wrote

I’ll take catastrophes resulting in massive loss of life that I wish I didn’t know existed for 400, Alex.

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eaumechant t1_j07ehm8 wrote

The theoretical blanket you've described is effectively what a space suit is. The solution is an undergarment called the liquid cooling garment (LCG) which is made of a thin material with narrow pipes sewn into it. The pipes carry cooled water which circulate back to a heat exchanger.

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