[Taek]

It takes about 300 watt hours of energy to raise the body temperature of a human by 6 degrees. If your human is starting at 99 degrees, 6 degrees will put you at 105, which is where you'll start to have immediate problems. (At 104 you aren't going to be happy, but you are going to survive and you're not likely to sustain organ damage).

If we assume that you are putting out 100 watts, you've got 3 hours at a wet bulb of 100F before you start having risk of death (if you are well adjusted to the heat).

And yes, I spend time every month in Saunas and Hammams with extreme temperatures. My favorite room is 195 degrees and 45% humidity. That translates to something like 150 degree wet bulb temperature, and I can happily stay in that room for about 20 minutes. (though 60 minutes would probably kill me). I've also spent plenty of time in Hammams (30+ minute sessions) where the temperature was 110F and the humidity was 100% (which means the entire room is fog and it's constantly dripping everywhere, practically raining). These aren't elite extremes in the sauna world, you'll find saunas close to these conditions all over the world.

[lazide]

Those calculations are assuming the only heat input is from internal metabolic activity, not metabolic activity + heat transfer from the environment, yes?

If the environment is 95-105f we can assume no external heat transfer, but the environments you’re referring to seem to be well above that.

Based on some quick googling, it seems like surface area is on average 1.6 m^2 for women, and 1.9 m^2 for men. I get lost in the math, but there has to be significant heat transfer if it’s 15+F hotter than body temperature in the room yeah?

[Taek]

Well yeah, if you are in a 195 degree sauna with 45% humidity there's a ton of heat transfer, that's why people like them. And it would definitely be unsafe to hang out in them for too long. People who aren't well adjusted to that sort of environment might even start having trouble after 10 minutes.

I know people who can tolerate that environment for 30+ minutes without injury (And I regularly do 20-25 minute stretches myself), which is why I'm so confident that 105F and 100% humidity is tolerable for a period of at least multiple hours.

Yes, eventually you'll heat stroke and die, you can't just live in a sauna. But it's not going to "kill you in minutes" if you are used to it, which is what a lot of people in this thread seem to be implying.