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When I tried googling this question, I found the answers for "What is the lowest survivable body temperature" and "humans can survive for a few minutes in -200 degree weather". Neither of which are my question.

I've read that the internal temperature of igloos can be in the wide range between 19 F and 61 F (-7 C and 16 C). 61 degrees I could see being reasonable, but 19 degrees seems nearing too cold for survival. I'm wondering where that cutoff point is. What is the lowest temperature that living in it continuously (say, a week or more) poses a serious health risk?

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    Health risk to whom? If you're a healthy, active, and mentally competent adult, you could survive any temperature indefinitely that normal home A/C systems can produce. But anyone too young, old, or sick to warm and protect themselves could be in danger at prolonged low temperatures. It would help if you narrowed this down.
    – Carey Gregory ♦
    Nov 1, 2017 at 4:52
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    Wearing normal clothes, eating enough calories? I think you'd have to go sub-zero for a large mammal like a human.
    – jiggunjer
    Nov 1, 2017 at 5:31
  • @CareyGregory My question is about the limits of the average, healthy human body. The AC was an example, pretend my AC is magical and can go to any temperature I want.
    – Daffy
    Nov 1, 2017 at 5:56
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    People don't sit around naked in igloos, or any other cool-air dwelling. They wear clothes and sleep under blankets. I have slept in a tent at around the freezing mark and I was in no danger, because I was in a warm sleeping bag.
    – Kate Gregory
    Nov 2, 2017 at 13:11
  • @KateGregory I‘ve slept in tents at freezing mark freezing because my sleeping bag wasn’t warm enough :) - I wasn’t in any danger though, otherways I‘d have aborted the hike.
    – Narusan
    Nov 2, 2017 at 13:14

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There are just too many factors to give an easy clean answer to this question. To limit the problem, lets assume the individual is wearing 1 CLO of clothing in still dry air. Further, lets assume that this person has a surface area of 2 m (a little bigger than the typical male, but it makes the math easier). Finally, lets assume they can indefinitely produce 300 Watts of heat (this is about 6000 calories a day). This means their heat output is 150 W/m^2. Then solving

T = (31 − 0.155·P·R)°C

with R=1 and P=150 gives a temperature of 8°C.

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  • Basically, 8 degree Celsius is the amount where body heat would be self-sustainable, if I understand correctly? Might be worth explaining where the formula comes from.
    – Narusan
    Nov 3, 2017 at 17:35
  • But if my home was 8 C, I would probably wear more clothing and wrap a blanket around me. Honestly, there are people who work outside for 8-10 hours a day at -20 and they wear heavy coats, hats, gloves, boots etc but they get by.
    – Kate Gregory
    Nov 3, 2017 at 18:00
  • I notice from that link that a person will be at thermal equilibrium (which I believe means comfortable, a much more difficult bar to meet than "not damaging yourself" at -28 C if they are wrapped in a polar duvet.
    – Kate Gregory
    Nov 3, 2017 at 18:02
  • @KateGregory you need to achieve equilibrium in order to not damage yourself. If you are willing to add enough insulation, then you can live in whatever temperature you are faced with. The key is to adjust the calories you burn (P) and the insulation you wear (R) to achieve equilibrium with the temperature (T) that you live in.
    – StrongBad
    Nov 3, 2017 at 18:13

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