Inside a small, sealed room, Olivia Leach swallowed a pill that would soon monitor her internal temperature. She then hopped on a stationary bike and prepared to sweat. Slowly, as she began to pedal, the room started to heat up at a rate of one degree every five minutes.
It felt “like a muggy, hot summer day where you’re just drenched in sweat,” said Ms. Leach, a doctoral student at Pennsylvania State University.
Ms. Leach works in the lab of W. Larry Kenney, a physiology and kinesiology professor at Penn State. The lab’s work is part of a growing body of research exploring how the body deals with heat, and how to help people acclimate to hotter weather.
Ms. Leach and other experts said that this research had become more urgent as more parts of the world faced an increasing number of dangerously hot days. In the United States alone, extreme heat caused at least 2,302 deaths in 2023, and some research suggests that number may be far higher.
Without the right preparation, pushing the body too hard in extreme heat can be deadly. Almost half of all heat-related deaths among workers occur on the first day on the job, and more than 70 percent occur within the first week, according to the Occupational Safety and Health Administration. Recognizing those risks, the Biden administration recently proposed new rules to protect people who work in the heat.
The proposal would entitle workers to so-called acclimatization plans, which would let new employees who aren’t used to high temperatures safely adjust to heat by gradually increasing their hours.
“Heat is a stress, and our bodies adapt to stress when we’re exposed repeatedly,” said William Adams, a kinesiologist at the University of North Carolina, Greensboro.
Heat acclimatization is often used to help people who regularly perform intense activity outside, like athletes, outdoor workers and members of the military. But even for people who don’t regularly work outdoors, high temperatures can cause heat exhaustion, heat stroke and even death. As temperatures rise, experts said, many people can benefit from trying to build their heat tolerance.
“Start making an effort to get outdoors, get exposed to it and work your way up,” said Michael Sawka, an adjunct professor of biological sciences at the Georgia Institute of Technology who has studied heat acclimatization. “You’ll find that your tolerance increases, and it’ll help you.”
The benefits of adapting to heat
When it’s hot, your body looks for ways to keep cool. You start to sweat, and the heart begins to pump harder.
But extreme heat can cause the heart to beat too hard. Your blood pressure can start to drop. You can sweat so much that you become dehydrated or dizzy.
People can feel these effects at “drastically different” temperatures, depending on their health and what level of heat they’re used to, said Alexandra Heaney, a climate and health researcher at the University of California, San Diego. “Extreme heat is a very relative term,” she said. Certain medications and health conditions, such as kidney disease and heart failure, can also make people more vulnerable to heat stress.
The idea behind acclimatization is to train your body to better handle the stress of heat, starting with small doses. You might begin by doing half an hour of light or moderate exercise in the heat for a few days, followed by an hour for the next few days. You gradually build up to more intense activity in greater heat and over longer periods of time. Generally, two weeks of daily exposure to heat while doing a physical activity for 60 to 90 minutes are enough to help the body adapt.
This period of adaptation reduces the strain on your body in the heat, and it bolsters its ability to protect itself from heat-related illness. Your body gets better at regulating its core temperature. Your total body water increases, and your blood plasma expand. This means that your heart fills faster and doesn’t have to beat as often to circulate oxygen. And it allows more blood to be sent toward the skin, where it can be cooled.
Over time, you sweat sooner, and more. This helps the body keep cool for longer in greater heat. You retain more salts, too, which means that fewer electrolytes are lost through sweat. Your body also produces more heat shock proteins, which help protect against heat stress and repair from it.
Some of these effects can be seen in just a few days.
These changes can make people feel more comfortable and capable of exerting themselves for longer periods in high temperatures, said J. Luke Pryor, a clinical associate professor of exercise and nutrition sciences at the University at Buffalo.
How to acclimatize yourself
What the process looks like will depend on your health, your fitness level and what you need to do in the heat.
For someone who is new to an outdoor job, for example, the Centers for Disease Control and Prevention recommends working 20 percent of a normal day during the first shift in heat, followed by 40 percent the next day. Only on the fifth day should someone work as much as a full shift.
Your body will adjust to the level of work you put it through; light exercise in dry heat, for example, only acclimatizes you to light exercise in dry heat.
“You can adapt by just having a resting exposure to the heat, but you’re not going to have the same adaptation as if you’re physically active and in the heat,” Dr. Sawka said. You shouldn’t start in the middle of a heat wave, he warned. It’s best to begin in more moderate weather, when you know higher temperatures are on the horizon.
The process can be tiring, particularly in the first few days, Dr. Pryor said: “It kind of hits you like a sledgehammer.” It’s important to take breaks in cooler temperatures when you feel uncomfortable, and to stay hydrated. The C.D.C. recommends drinking a cup of water every 15 to 20 minutes when working in the heat.
Pay attention to how you’re feeling, experts said, and keep an eye out for any signs of heat-related illness, including dizziness, nausea, headache, a rapid heart rate or muscle cramps. Excessive heat can also cause you to vomit, become confused or lose consciousness. If you feel sick, stop what you’re doing in the heat and seek medical attention if needed, experts said.
You “don’t want to overdo it,” Dr. Sawka said. “But you want to adapt so that if you are exposed to it, it’s not harmful to your body.”
Some people can’t safely acclimatize, Dr. Pryor noted, including people who have had heat strokes, burn victims with substantial scarring and those with multiple sclerosis.
And while acclimatization can make it easier to manage hot days, every person has a limit, Ms. Leach said. “There is a ceiling to the adaptations possible,” she added.
If you are able to increase your heat tolerance, working in hot temperatures one or two days each week should help your body maintain its adaptations, Dr. Adams said.
Without periodic heat exposure, though, the effects will wane within about a month. However, the body remembers some of the adaptations, which should make it easier to acclimatize the next time around.
Ms. Leach has been in the hot room enough times to notice how her own body has adapted. Over time, it has gotten easier to pedal the bike — and she no longer counts down the time she has left in the room.
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