[lazide]

I know of at least one instance where several well acclimatized individuals died in less than 15 minutes in open air in the Grand Canyon above the Colorado River due to solar heating and 95% wet bulb humidity.

I’d argue you just haven’t actually been in those types of situations either. In that case I think they estimated 110ish air temps.

But maybe I’m misremembering - I heard it from the investigating safety officer though.

I have yet to see anyone actually able to work or function in actual 100% humidity and 100+ degrees temps for more than a few minutes before having serious problems.

I’ve seen plenty of people have problems in 60% humidity which everyone agreed was terrible. Mostly heat stroke.

People’s core temp is already just a hair under 100F, and even at rest are producing 100ish watts of thermal energy. It really doesn’t take much for it to start spiking if cooling is literally impossible.

[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.

[CuriousCosmic]

With 100% humidity you definitely can't survive or function but with 95% at 100F exactly it's just barely feasible. That gives you just enough margin due to evaporative cooling that with sufficient air flow you can maintain a temperature of 97-98F via evaporative cooling. And importantly this only works in the shade. Outdoors it's unlikely to be feasible due to the rise in surface temperatures due to thermal radiation from the sun.

That of course requires a strong fan blowing and regular, heavy hydration to sustain but it's feasible. So it's workable in an industrial environment where you can adjust the environment enough to get by but outdoors in large wild spaces like the Grand Canyon (as per your example) it's unlikely to be survivable for long.

And notably in an indoor environment there is a big difference between an operator running a machine or vehicle and an individual under heavy exertion. The added thermal stress of heavy exertion makes it less survivable as well.

So in the end it only really works in factory settings because:

- There's no sun to add radiative heat.

- There's fans and ventilation to maintain evaporative cooling.

- Workers can take regular or semi-regular breaks in a cooler or lower humidity environment to recover some from the thermal stress and to recover water and electrolytes.

- Those workers can limit their activity to rates of exertion/heat production that don't exceed the limited evaporative cooling they have access to.

As soon as you remove one of those advantages or increase the temperature much above 100F or increase the RH above 95%, survivability becomes way less likely.