Climate change is making extreme heat more common and more severe, as we’ve seen in the heat waves that have swept the western US for the past two weeks. Some climate models predict that swaths of the globe will become inhospitable to humans in the next century.

But what makes a place unlivable isn’t as straightforward as a specific temperature, and even accounting for humidity doesn’t fully explain the limits of the human body in extreme heat. Tolerance can vary from person to person, and someone’s ability to withstand heat can change. Understanding our limits and what determines them will be more important as global temperatures creep upward and extreme weather events become harder to predict.

“You would think that, at this moment, we will have choices between the good and the bad,” says Camilo Mora, a climate researcher at the University of Hawaii. But now, when it comes to extreme heat, “the choices are more of this or a lot more of this.”

For a study published in Nature Climate Change in 2017, Mora and his team analyzed hundreds of extreme heat events around the world to determine what combinations of heat and humidity were most likely to be deadly, and where those conditions were likely to occur in the future.

They found that while today around 30% of the world’s population is exposed to a deadly combination of heat and humidity for at least 20 days each year, that percentage will increase to nearly half by 2100, even with the most drastic reductions in greenhouse-gas emissions.

Other researchers have found that climate change is making extreme heat waves up to hundreds of times more likely and causing over a third of heat-related deaths. We’re changing our planet—what are the limits of what we can endure?

Cooling off

As warm-blooded mammals, humans have a constant body temperature, around 98 °F (37 °C). And our bodies are designed to work pretty much right at that temperature, so there’s a constant balance between heat loss and heat gain.

Problems start when our bodies can’t lose heat fast enough (or lose it too fast in the cold, but let’s focus on heat for now). When your core temperature gets too hot, everything from organs to enzymes can shut down. Extreme heat can lead to major kidney and heart problems, and even brain damage, says Liz Hanna, a former public health researcher at the Australian National University, who studies extreme heat.

Your body works to maintain its core temperature in hot environments mostly by using one powerful tool: sweat. The sweat you produce evaporates into the air, sucking heat from your skin and cooling you down.

Humidity cripples this cooling method—if it’s so humid that there’s already a lot of water vapor in the air, then sweat can’t evaporate as quickly, and sweating won’t cool you down as much.

Extreme heat can lead to major kidney and heart problems, and even brain damage.

Researchers like Mora and his team often use measures like heat index or wet-bulb temperature to consider how excessive heat and humidity interact. This way, they can focus on a single number to identify unlivable conditions.

Heat index is an estimate that you’ve probably seen in weather reports; it factors in both heat and humidity to represent how the weather feels. Wet-bulb temperature is literally what a thermometer measures if a wet cloth is wrapped around it. (The temperature in the forecast is technically a dry-bulb temperature, since it’s measured with a dry thermometer.) Wet-bulb temperature can estimate what your skin temperature would be if you were constantly sweating, so it’s often used to approximate how people would fare in extreme heat.

A wet-bulb temperature of 35 °C, or around 95 °F, is pretty much the absolute limit of human tolerance, says Zach Schlader, a physiologist at Indiana University Bloomington. Above that, your body won’t be able to lose heat to the environment efficiently enough to maintain its core temperature. That doesn’t mean the heat will kill you right away, but if you can’t cool down quickly, brain and organ damage will start.

The conditions that can lead to a wet-bulb temperature of 95 °F vary greatly. With no wind and sunny skies, an area with 50% humidity will hit an unlivable wet-bulb temperature at around 109 °F, while in mostly dry air, temperatures would have to top 130 °F to reach that limit.

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.

“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.”

By ASNF