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How hot is too hot for the human body? [technologyreview.com]:

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 [sciencemag.org] 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.

“Everyone is susceptible—some more than others,” says Hanna, the Australian public health researcher. Children and elderly people usually can’t regulate their temperature as well as young adults, and people on certain medications have a decreased ability to sweat.

People’s heat tolerance can also change over time—your body can become more acclimatized to heat with exposure, sort of like the way it can acclimatize to lower oxygen levels at high elevations.

Heat tolerance

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.

There are limits to acclimatization, Hanna points out. We won’t be able to evolve past the conditions that climate change is likely to bring in the coming decades. She also says that while physiological limits are important, we must also consider other factors, like behavior and infrastructure.

If you’re moving around or working outside, the temperature doesn’t have to get nearly as hot to be deadly, Hanna says. Of the total energy you use to do a task, whether that’s running a race or washing dishes, 20% goes to actually moving your muscles, and the other 80% turns into heat. So more movement means more heat for your body to get rid of, which means that if you’re exerting yourself, you won’t be able to handle temperatures you could endure if you were just lying around.

Living in Australia, Hanna is especially attuned to how extreme heat affects people and communities. Australia is one of the hottest countries on Earth, with some places already pushing the limits of human tolerance. Helping people understand the dangers of heat is more urgent to her than ever as the extremes become the norm.

“The world is warming,” Hanna says, “and it’s going to go beyond what normal physiology can cope with.”

Original Submission