How to Build Up Your Heat Tolerance for a Hotter World

s 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.

To see a published study, click here Nature Climate ChangeMora and his coworkers analyzed thousands of extreme heat events worldwide in 2017. They found that while 30% of people were exposed for more than 20 days each year to heat and humidity, this percentage could rise to almost half by 2100.

Both heat and humidity can cause severe health problems. Another 2017 study was published in The Cardiovascular Quality of CirculationMora gave 27 reasons why a heatwave can cause death, including inadequate blood flow to the brain and liver. “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.

Is heat tolerance defined?

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.”

Bernard says that age is also a contributing factor. Seniors and very young children are especially at high risk for heat illness. Other factors that can affect heat tolerance include drug and alcohol abuse, chronic or acute illness, obesity, and diabetes. However, improving your cardiovascular fitness may increase it.

Continue reading: Extreme Heat: Why Extreme Heat Can Hurt Your Human Body

The ability to cope with heat is also affected by how healthy you are. 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.”

Are you able to improve your heat tolerance?

You can improve your ability to adapt or tolerate changes in the environment by doing many different things. 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. Kenney states that it takes between 9 and 14 exposures for an average person to get acclimatized. “The fitter you are, the shorter that time is.”

Continue reading: 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. Additionally, our sweat will have a lower concentration of salt so it is less concentrated and more likely to be found 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 suggests that those who want to improve their heat tolerance slowly expose themselves to high temperatures over the course of one or two weeks. For those who have never worked outside before, it is a good idea to start slowly by spending 20 percent of your first day in heat. Then gradually increase that amount for each subsequent 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?”

Are humans able to adapt to heat extremes?

Since the early days of climate science, 95 degrees Fahrenheit was considered to be the highest temperature humans could regulate. (“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.

Continue reading: We can expect more disease outbreaks in the future if we live in a hotter world.

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. Matthew Huber of Purdue University is a climate scientist. Huber says that the temperature thresholds could be broken if there’s another 2.5 to 3.0 degrees Celsius (or 4.5–5.4 degrees Fahrenheit) of global warming.

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. This could be the new norm elsewhere.

A better access to cooling will be needed around the world, along with cheaper and more efficient forms 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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