Brazil is melting: Record heat index of 62.3 degrees scorches Rio de Janeiro

[FalloutShelter]

5. The number to be watched in these increasingly hot temperatures is the humidity-

This from Popular Science in 2021:

Perspiration can feel gross, but it’s an incredibly effective cooling system because water takes a lot of energy to heat up. You produce water on your skin, and as those droplets evaporate they take some of your body heat with them. This is why “a dry heat” is more pleasant than humid heat: dry air can absorb a lot of moisture, making it easier for sweat to evaporate. Even at a heat index of well over 100 degrees Fahrenheit, your body can stay around 98.6 degrees thanks to perspiration.

But there’s a point at which sweat stops working: once the wet bulb temperature passes 95 degrees Fahrenheit. That’s because, in order to maintain a normal internal temperature, your skin has to stay at 95 degrees or below. Since sweating is your skin’s mechanism for shedding heat, and wet bulb temperature is the lowest temperature that a wet surface can achieve through evaporation (read: that sweaty skin can reach), wet bulb temperatures past 95 degrees are extremely dangerous. Sustained skin temperatures above 98 degrees are considered fatal.

https://www.popsci.com/environment/wet-bulb-globe-temperature/

Aside: If you have read Kim Stanley Robinson's novel The Ministry for the Future, - this will terrify you.