In the summer of 2023, a dozen people willingly walked into a steel chamber at the University of Ottawa designed to test the limits of human survival. Outfitted with heart rate monitors and temperature probes, they waited in temperatures of 42 degrees Celsius, or 107 degrees Fahrenheit, while the humidity steadily climbed, coating their bodies in sweat and condensation. After several hours, their internal body temperatures began ratcheting upward, as the heat cooked them from the outside in.
“Few people on the planet have actually experienced temperatures like this,” said Robert Meade, a postdoctoral researcher in epidemiology at the Harvard School of Public Health who led the study. “Imagine moisture condensing on the skin like a glass of water on a hot day. That’s how hot it was, compared to skin temperature.”
Their experiment tested the body’s ability to cope with extreme heat by exposing participants to temperatures at which they could no longer cool themselves. Their study confirmed that this dangerous threshold is much lower than scientists had previously thought: a so-called wet-bulb temperature, which accounts for heat and humidity, of 26 to 31 degrees C.
https://archive.ph/Lj16Y
As far as I can tell, this same research methodology would say that humans can't survive temperatures below 18 degrees Celsius. Put someone in a small room and tell them not to do anything useful to solve their problem, and they're going to do very poorly.
We are defined by our tool use, and not being able to survive the outdoors without wearing anything to suit the local environment is pretty common across the world.
So what do you wear to survive lethal wet bulb temperatures for a couple of days? As a rural third worlder with no access to power or air conditioned spaces?
Lethal according to this study? A sarong.
Properly lethal? Thick insulation, and undergarments stuffed with heat sinks. For example cold mud from a well, basement, or cenote, made relatively cold by nighttime temperatures and traditional air flow engineering. It probably wouldn't last more than an hour, but that's enough to travel. Then if that's not enough, shelter in place. Spend daytime in basements and hope supplies last.
That said, the most practical and historically supported option is migration. Humans have migrated away from unsuitable climates for as long as we've existed. Disregarding politics, from an economic perspective "rural third worlder with no access to power or air conditioned spaces" is not losing a lot of production assets. Maybe if the first world stops being colonialist shits for one second, these migrants can benefit the common good more than before, when they're given access to the means of production rather than being treated as detritus around the edges of colonial exploitation.
Imagine a billion extra farmers and engineers, ready to do the massive amount of labor necessary to turn habitable/temperate zones into sustainable climate-hardened permaculture. The issue isn't technology or economic feasibility, it's white supremacy.
You don't have to live in the third world to lack access to A/C either. Just being poor is enough in some places, and other places, even if they could afford it, may not have ever thought of it that now will. The American Northwest had that stint of temperatures a few years back that were causing all sorts of trouble, and part of the issue was people who had only heating because it wasn't normal to get that hot.
I would go in a basement, or a hole in the ground, or swim in a river or in the ocean, or find some wind while pouring more water on myself to complement the sweat.
The latter doesn't work for lethal wet bulb temperatures.
Actually it does work because the 'lethal" wet bulb temperatures are below human body temperature, so wind and extra water do indeed bring down the skin temperature, just less effectively. Now if the wet bulb temperature rises above body temperature it's a different story and it's significantly above lethal wet bulb temperature.
Please reread https://www.pnas.org/doi/10.1073/pnas.2421281122
Perspiration does not cool down at lethal wetbulb so wetting the body with ambient temperature water does not help since there is no evaporation enthalpy. Forced convection air buys you almost nothing. Immersion in ambient temperature water, especially agitated, buys you a few degrees higher tolerance due to its higher heat capacity.
Retreating underground into previously prepared subterraneous shelters would help. Starting digging too late will just give you a heat stroke from physical exertion.
Realistically, lethal wet bulb spell of a few days in an unprotected population is a mass casualty event. Given that grid failure is likely under the circumstances, the size of the unprotected population is larger than many expect.
I'm looking at
And I'm thinking that showering or swimming in water at 31°C is considered "quite hot" and is representative of ocean water temperatures in some tropics like in Oceania.
I researched a bit further and swimmers have died racing swimming in hot water, but possibly idling in hot water might give about 1-2°C margin of survivability compared to sweating in a wetbulb heatwave on land.
However swimming pools are very often built underground in the style of an open top basement, and water often comes from wells, so I still think there's some chances of surviving with the help of water.
Now when ground temperatures rise above that lethal temperature, everyone without a heat pump is going to die.
"Just give you a heat stroke from physical exertion" I wonder how many lives can be saved by a someone with a powered excavator (likely diesel) digging a trench so that people can access fresh cool earth at the bottom of the trench
Yeah, I hate when the temperature falls below 18 degrees Celsius! There have been times when I've gotten so uncomfortable that I've had to put some pants on. It's possible to survive it, for sure, but it sucks.