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In theory, heat goes from uncomfortable to lethal at a wetbulb temperature of 35C, this being the point at which the body no longer has a way to shed heat to keep an internal temperature of 37C even with unlimited water supply.

I occasionally hear casual claims of wetbulb temperature exceeding this, in times and places where people and animals seem to be still alive. Saw such a claim in a documentary just now, decided to follow up on it.

https://www.youtube.com/watch?v=C8Q2ptLaFj0&ab_channel=FreeDocumentary

The narrator claims conditions in the Bangladesh mangrove swamp were 40C at 95% humidity. https://www.omnicalculator.com/physics/wet-bulb says this corresponds to a wet bulb temperature of over 39C. In theory, this should be flat-out lethal to pretty much all complex animal life, yet the film crew, tigers and other local animals were all still walking around uncomfortable but alive.

How?

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  • 1
    Please edit the question to provide some kind of reliable source of information, and one that isn't a video. SE questions and answers are supposed to be self-contained, and it's not reasonable to expect people to click through and watch a video just in order to find out what is being claimed and what the evidence is. This question could also have benefited from some research. Googling quickly brought me to the WP article "Wet-bulb temperature," which says the highest wet bulb temperature ever recorded was 36.3 C.
    – user2169
    Jul 25 at 19:55
  • @BenCrowell That is why I reported what the narrator said as well as providing the link to the video; now the question is self-contained. Are you saying you think the video narrators are an unreliable source of information, and that this one was wrong about the temperature, humidity or both? If so, that would be a perfectly valid answer, albeit one that could've been phrased more constructively.
    – rwallace
    Jul 25 at 22:22
  • @JonathanReez If one plans to be outdoors in such tropical regions, surely it is useful to know whether the answer is 'yes, the heat and humidity will kill you unless you take the following precaution no one has ever mentioned', 'no, the narrator was wrong, it's not actually anywhere near that hot' or some third option I haven't thought of?
    – rwallace
    Jul 26 at 3:50
  • 1
    Also an interesting read: doi.org/10.1073/pnas.0913352107 Jul 26 at 21:52
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Its hard to confirm what the video crew was experiencing but generally speaking wet-bulb temperatures over 35C (approximate lethal limit for humans) are still very rare on this planet. An interesting study from 2020 dives deep into this subject:

The study identifies thousands of previously rare or unprecedented bouts of extreme heat and humidity in Asia, Africa, Australia, South America and North America, including in the U.S. Gulf Coast region. Along the Persian Gulf, researchers spotted more than a dozen recent brief outbreaks surpassing the theoretical human survivability limit. The outbreaks have so far been confined to localized areas and lasted just hours, but they are increasing in frequency and intensity, say the authors.

They give an example of a 35C heat wave in Iran:

While air conditioning may blunt the effects in the United States and some other wealthy countries, there are limits. Before the new study, one of the previously highest heat/humidity events ever reported was in the Iranian city of Bandar Mahshahr, which almost reached a 35C wet-bulb reading on July 31, 2015. There were no known deaths; residents reported staying inside air-conditioned vehicles and buildings, and showering after brief sojourns outside.

So it seems like wet-bulb temperatures around 35C could be survived if they don't last too long and you stay indoors. The authors also provide a nice map of the highest recorded wet-bulb temperatures per city and it seems like the area around the mangrove swamps isn't particularly record-breaking. Wikipedia likewise has a list of temperature records showing that anything above 35C is very rare (at least for now).

Can humans survive a wet-bulb temperature of 39C over a long period of time? Probably not. Did the film crew actually encounter such a temperature? Almost definitely not.

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