How to Build Up Your Heat Tolerance for a Hotter World


As the world continues to feel the effects of climate change, research suggests that the severity and frequency of extreme weather events—like unrelenting stretches of heat—will only worsen with time.

“We shouldn’t be worried—we should be terrified,” says Camilo Mora, an associate professor in the department of geography and environment at the University of Hawai‘i at Mānoa. “What we are dealing with here is between something bad and something terrible,” depending on what actions are taken to curb climate change.

For a study published in Nature Climate Change in 2017, Mora and his colleagues analyzed hundreds of extreme heat events around the world and determined that, while about 30% of the population was exposed to a deadly combination of heat and humidity for at least 20 days annually, that percentage would increase to nearly half by the year 2100.

Heat and humidity can be pernicious. In another 2017 study, published in Circulation: Cardiovascular Quality and Outcomes, Mora described 27 ways a heat wave can kill, such as inadequate blood flow to the brain, heart, kidneys, liver, or pancreas. “It’s like a horror movie with 27 endings to choose from,” he says.

However, it’s possible to prepare for a hotter world by building up your heat tolerance, though experts say doing so isn’t necessary for everyone. Here’s what to know about how humans can adapt to rising temperatures, and the ways in which they can’t.

What is heat tolerance?

The Occupational Safety and Health Administration (OSHA) defines heat tolerance as “the physiological ability to endure heat and regulate body temperature at an average or better rate than others.”

Heat tolerance likely has a genetic component, though that connection isn’t yet well-understood. “Our nervous systems don’t all function exactly the same,” says Thomas E. Bernard, a professor in the College of Public Health at the University of South Florida who studies occupational safety and health in the heat. “Just like you have high performers in terms of intelligence, you have high performers in a neurophysiological sense. There’s nothing you can do to change that.”

Age is another contributing factor: very young children and seniors are at particularly high risk of heat illness, Bernard says. Beyond that, drug and alcohol use, the presence of acute or chronic illness, and obesity can negatively affect heat tolerance, while improving cardiovascular fitness will increase it.

Read More: Why Extreme Heat Is So Bad for the Human Body

Hydration status also plays a role in how well someone fares in warm weather. Drinking enough fluids “doesn’t make you superhuman, but it allows you to continue to tolerate the heat,” Bernard says. (Once you’re well-hydrated, however, “more doesn’t help.”)

Other factors that affect a person’s heat tolerance are more situational, like how long a heat wave has lasted. Heat tolerance tends to decline when it’s extremely hot for many days. If you’re working outside on the fourth consecutive day of high temps, for example, you likely won’t do as well as you did on day one.

While no one is immune to the heat, most people have “an inherent ability to tolerate quite a bit,” says Michael F. Bergeron, who advises the Women’s Tennis Association on performance health and has extensively researched heat. “Human beings who are healthy and used to the hot conditions, and who don’t overexpose themselves to undo levels of work or exercise in the sun, can tolerate a lot.”

Can you improve your heat tolerance?

People can do plenty of things to enhance their ability to tolerate or adapt to changes in the environment. The best method is heat acclimatization, which is “the process of the body gearing up all these physiological systems to better handle heat stress,” says W. Larry Kenney, a professor of physiology and kinesiology at Penn State. To get acclimatized, he says, you could go outside on a hot day and engage in mild activity—like taking a walk—for a very short period of time—about 15 minutes—and then repeat the process the following day. It takes the average person between nine and 14 exposures to become acclimatized, Kenney says. “The fitter you are, the shorter that time is.”

Read More: How to Cool Down When It’s Really Hot Outside

Several things happen during the acclimatization process that improve people’s ability to tolerate heat. Most notably, blood volume expands. “That allows the heart to not work as hard, and it provides more fluid for sweating,” Kenney says.

After the first few days of acclimatization—which are all about cardiovascular adjustments—“the sweating mechanism starts to gear up, and we produce more sweat,” Kenney says. Plus, the sweat we produce will be more diluted, meaning we lose less salt, and will occur more frequently on the limbs. “When people are unacclimatized, most of their sweating is on the trunk, the face, the back, and the chest,” Kenney says. “But the best way to evaporate sweat is to get it all over the body. So being able to sweat more on the limbs, which are moving through space quite a bit, allows that sweat to evaporate better.”

Heat acclimatization is often a focus for athletes, people who work outside, and those in the military, says Brenda Jacklitsch, a health scientist with the U.S. Centers for Disease Control and Prevention’s National Institute for Occupational Safety and Health. For example, some farm workers who spend their days spraying pesticides have to wear protective equipment such as long sleeves, pants, and respirators, all of which increase the odds of a heat-related illness—hence the need to become acclimatized.

Jacklitsch advises people who are trying to build up their heat tolerance to slowly introduce themselves to hot environments over one to two weeks. Those who are new to working outside benefit from easing in, perhaps spending 20% of their first day in the heat and then gradually increasing that time for the rest of the week.

Even when someone is fully acclimatized, she notes, they’re still susceptible to heat stress and could become sick. That’s why it’s important to always be around other people, take breaks in the shade, and stay well-hydrated. Also, heat acclimatization isn’t static: “Once you get better tolerance, you have to maintain it, because if you aren’t in the heat any longer, that resilience can decay,” Bergeron says.

While acclimatization is helpful for those who can’t escape the heat, experts agree it’s not necessary for the average person, and pushing yourself could lead to heat illness. There’s nothing wrong with hunkering down in the air conditioning during heat waves. “You’re not doing yourself a disservice,” Bernard says. “Your tolerance to heat might not be maximized—but it’s uncomfortable [to become acclimatized]. Why would you want to do that unless you have to?”

Will humans adapt to extreme heat?

For years, climatologists described a wet-bulb temperature of 95 degrees Fahrenheit as the upper limit for humans to be able to safely regulate their body temperature. (“Wet-bulb” temperature is a measurement used by researchers that accounts for both heat and humidity. It’s the temperature that would be read by a thermometer that was covered in a water-soaked cloth; at 100% humidity, it’s equal to the air temperature.)

Thinking about the maximum sustainable wet-bulb temperature has evolved, in part thanks to Kenney’s research. He and his colleagues send volunteers of all ages into environmental chambers and adjust the humidity and temperature, while monitoring participants’ core temperature. Participants swallow a pill that allows researchers to monitor their deep body temperature, and while they’re inside the chamber, they move around, perhaps walking on a treadmill as the temperature and humidity fluctuate.

Read More: A Hotter World Means More Disease Outbreaks in Our Future

Kenney’s findings indicate that the “critical upper limits,” even for healthy people, are closer to a wet-bulb temperature of 88 degrees Fahrenheit—which would mean, for example, 88 degrees at 100% humidity or 100 degrees at 60% humidity. At that point, “the sweat you produce doesn’t evaporate,” Kenney says, so the body can no longer cool itself. But that’s not synonymous with instant death. “People would stop the activity, go inside, find shade, and drink more fluids. Nobody would keep going above those limits for long periods of time.”

These conditions aren’t yet widespread on Earth, though some areas approached them during recent heat waves. If the world experiences another 2.5 to 3 degrees Celsius of warming (or 4.5 to 5.4 degrees Fahrenheit), significant parts of the population could start routinely breaking these thresholds, says Matthew Huber, a climate scientist at Purdue University.

Experts agree that, physiologically, humans will not be able to adapt to such extreme heat—even within the next few centuries. As Huber puts it, humans’ internal body temperature is “a shared trait, from 100 million years ago. It’s not something that changes quickly.”

That doesn’t mean, however, that the human species will cease to exist. Rather, we’ll need to depend on behavioral adjustments and other interventions. In some areas of the tropics and subtropics, Huber says, it’s already common for workers to do time-shifting, like working from 4 a.m. to 11 a.m., and then spending the hottest part of the day inside. That could become the norm in other places.

The world will also need improved access to air conditioning, as well as cheaper, more energy-efficient forms of cooling, like electric fans. We might see “swamp coolers” more often—devices that use moisture to cool air, Kenney says. “I think what will have to happen is better engineering controls that provide those sorts of cooling devices to more and more people who can’t afford them.”

Plus, Huber stresses, we’ll need to shift our mindset on the heat—and not push ourselves to go for a run when it’s really hot outside, or try to tough it out because “grandma used to live in this house without an air conditioner.”

“There’s going to have to be a change in mindset that people have, where they have to start thinking about hot, humid conditions actually as a threat and not something to be overcome,” he says.

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