What is heat tolerance?
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.”
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.
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.”
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.
