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
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.
"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
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.