Some climate models predict that we're going to start hitting wet-bulb temperatures over 95 °F by the middle of the 21st century. Other researchers say we're already there. In a study published in 2020, researchers showed that some places in the subtropics have already reported such conditions—and they're getting more common.
While most researchers agree that a wet-bulb temperature of 95 °F is unlivable for most humans, the reality is that less extreme conditions can be deadly too. We've only hit those wet-bulb temperatures on Earth a few times, but heat kills people around the world every year.
[...] Heat acclimatization builds up over time: It can start in as little as a few days, and the whole process can take six weeks or longer, Hanna says. People who are more acclimatized to heat sweat more, and their sweat is more diluted, meaning they lose fewer electrolytes through their sweat. This can protect the body from dehydration and heart and kidney problems, Hanna says.
Acclimatization is why heat waves in cooler places, or heat waves early in summer, are more likely to be deadly than the same conditions in hotter places or later in summer. It's not just that places like Canada and Seattle are less likely to have air conditioning, although infrastructure is another big factor in how deadly heat waves will be. Residents of cooler places are also just less acclimatized to the heat, so wet-bulb temperatures below 95 °F can be deadly.