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Comment by andsoitis | Hacker News Reader

At what point the heat becomes unbearable is personal, but a study published in 2010 estimated that a "wet-bulb" temperature of 95F (35C) at 100% humidity, or 115F at 50% humidity is probably as hot as most humans can maintain a healthy core body temperature by sweating. Above that "critical environmental limit" our body temperature rises continuously and the risk of heat-related illnesses such as heat stroke increases.

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A 2022 study experimentally found the limit to be 30.5C ± 1C at 100% humidity, and lower in dry environments.

https://journals.physiology.org/doi/full/10.1152/japplphysio...

That 35C temperature is the upper limit, assuming that you're in the shade and have unlimited water and air fans to assist you. In reality, wet-bulb temperatures have caused fatalities as low as 28C

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7209987/

It’s always interesting reading stats like these as an Australian. Our most recent heatwave was something like a maximum of 100F during the day and a minimum of 85F during the night at 60-80% humidity. So there was this period of probably >24 hours that was at or above the “critical environmental limit”. It’s too hard to find good stats on how many deaths this caused (you could simply count the number of the deaths on the day to get an upper bound, but how many of those deaths will be “people who would have died this month because of their fragile health and the heat wave merely “gathered” those deaths on that day? Ideally you want something like QALY impact instead), but I believe I recall studies indicating heatwaves like these increase mortality by 5% (so from 40 per 100,000 to 42 per 100,000).

> ... 95F (35C) at 100% humidity

Last summer in Europe there's been 38C for weeks in several places/countries. I was on the Atlantic side (for once), not even much south, and had 38C. We were cycling daily (without electric assistance) to the beach, under the sun, and had zero AC at home. It was hot but nobody there died. I used my car once while the rest of the group was cycling because I wanted my car's AC and music but that's it. It was very hard to enter a water at 17C (Atlantic side, huge tides, very cold water there) while the temperature was 38C outside but besides that meh.

Is "100% humidity" a value that even make sense? Spain regularly gets 40C+. I lived in Spain about five months a year for five years straight. 35C is really common there.

So I don't get it: "You can die at 28C with 100% humidity". OK, I don't dispute that.

But where do you get 100% humidity?

For example Morocco / Marrakesh is 41C today. It's pretty normal there.

https://en.wikipedia.org/wiki/Wet-bulb_temperature

> Is "100% humidity" a value that even make sense?

According to the National Weather Service, "If the relative humidity is 100 percent (i.e., dewpoint temperature and actual air temperature are the same), this does NOT necessarily mean that precipitation will occur. It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at."

https://www.weather.gov/lmk/humidity#:~:text=If%20the%20rela....

You get 100% humidity basically any time you're near a large body of water that has been hot and then the air cooled. So usually on hot days like that along the coast, as the sun goes down you'll notice lots of things just getting wet, that's due to the 100% humidity.

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