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
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.