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
EDIT: I finally found the answer. The article is talking about 31°C wet bulb temperatures, which is equivalent to 100% rH.
At 100% rH, 31°C is a heat index temperature of 49°C (120°F or 322k).
So if it hits 49°C at ~33%rH you might want to turn up the aircon a notch.
~~So 31c at checks notes 92% rH? Wow I sure am worried now!~~
~~Even in the eternally damp UK rn the rH is 31% and at that level it'd need to be 41c before it causes any problems at all~~
~~This tracks with my personal experience where 30-35c feels pretty warm but not too warm for a nice summer day. Past 35c I'd avoid public transport and maybe wear white and would probably keep a fan going indoors occasionally~~
~~Am I wrong? Please dunk on me if I just don't get it, because otherwise I feel like this article is kind of misleading~~
I haven't run the numbers, but your assessment seems about right. The lower the humidity, the greater the potential for evaporative cooling, and the higher the max tolerable temperature. No reason to avoid moving to the tropics anytime soon.