In a small, enclosed room, Olivia Leach swallowed a pill that would soon monitor her internal temperature. She then hopped on a stationary bike and prepared to work up a sweat. Slowly, as she began pedaling, the room began to warm up at a rate of one degree every five minutes.
“It was like a hot, humid summer day where you’re drenched in sweat,” said Leach, a doctoral student at Pennsylvania State University.
Leach works in the lab of W. Larry Kenney, a professor of physiology and kinesiology at Penn State. The lab’s work is part of a growing body of research exploring how the body responds to heat and how to help people acclimate to higher temperatures.
Leach and other experts said the research has become all the more urgent as more parts of the world grapple with a growing number of dangerously hot days. In the United States alone, extreme heat caused at least 2,302 deaths in 2023, and some research suggests the number could be much higher.
Without proper preparation, overexerting the body in extreme heat can be deadly. Nearly half of heat-related deaths among workers occur on the first day of work, and more than 70% occur within the first week, according to the Occupational Safety and Health Administration. Recognizing these risks, the Biden administration recently proposed new rules to protect people working in the heat.
The proposal would entitle workers to so-called acclimatization plans, which would allow new employees who are not used to high temperatures to safely adjust to the heat by gradually increasing their hours.
“Heat is a stressor, and our bodies adapt to stress when we’re exposed to it repeatedly,” said William Adams, a kinesiologist at the University of North Carolina at Greensboro.
Heat acclimatization is often used to help people who regularly engage in strenuous activities outdoors, such as 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, many people can benefit from building their heat tolerance, experts say.
“Start making the effort to get out, expose yourself to heat, and work your way up,” said Michael Sawka, an assistant professor of biological sciences at the Georgia Institute of Technology who has studied heat acclimation. “You’ll find that your heat tolerance increases, and that will help.”
The benefits of adapting to heat
When it’s hot, your body looks for ways to cool down. You start sweating and your heart starts beating faster.
But extreme heat can cause your heart to race. Blood pressure can start to drop. You may sweat so much that you become dehydrated or dizzy.
People can experience these effects at “dramatically different” temperatures, depending on their health and the 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 heat acclimatization is to train your body to better handle heat stress, starting with small doses. You might start by doing half an hour of light or moderate exercise in the heat for a few days, then an hour on subsequent days. You gradually increase the intensity of the activity in more intense heat conditions and for longer periods of time. Typically, two weeks of daily exposure to heat while exercising for 60 to 90 minutes is enough to help the body adapt.
This adaptation period reduces the stress on your body from the heat and strengthens your body’s ability to protect itself from heat-related illness. Your body becomes better at regulating its internal temperature. The total amount of water in your body increases and your blood plasma expands. This means your heart fills up faster and doesn’t have to beat as often to circulate oxygen. It also allows more blood to be pumped to your skin, where it can be cooled.
Over time, you sweat faster and more profusely. This helps your body stay cooler for longer in hot weather. You also retain more salts, which means fewer electrolytes are lost through sweat. Your body also produces more heat shock proteins, which help protect against and recover from heat stress.
Some of these effects can be seen in just a few days.
These changes may make people feel more comfortable and able to exercise for long periods in high temperatures, said J. Luke Pryor, associate clinical professor of exercise and nutrition sciences at the University at Buffalo.
How to acclimatize
The process will depend on your health, fitness level and what you need to do in hot weather.
For someone new to outdoor work, for example, the Centers for Disease Control and Prevention recommends working 20 percent of a normal day on the first shift in hot weather, then 40 percent the next day. It’s not until the fifth day that you should work a full shift.
Your body will adapt to the level of work you put it through; light exercise in dry heat, for example, only acclimates you to light exercise in dry heat.
“You can adapt by just resting in the sun, but you won’t have the same adaptive capacity as if you’re physically active and in the heat,” Dr. Sawka said. You don’t want to start in the middle of a heat wave, he cautioned. It’s best to start in more moderate weather, when you know higher temperatures are on the horizon.
The process can be tiring, especially the first few days, says Dr. Pryor: “It’s like a sledgehammer.” It’s important to take breaks when temperatures are cooler and to stay hydrated. The CDC recommends drinking a glass of water every 15 to 20 minutes when working in the heat.
Pay attention to how you feel, experts advise, and watch for any signs of heat illness, including dizziness, nausea, headache, rapid heartbeat or muscle cramps. Excessive heat can also cause vomiting, confusion or loss of consciousness. If you feel sick, stop what you are doing in the heat and seek medical attention if necessary, experts advise.
“You don’t want to overdo it,” Dr. Sawka said. “But you have to adapt so that if you are exposed to it, it’s not harmful to your body.”
Some people cannot acclimatize safely, Dr. Pryor noted, including people who have suffered heat stroke, burn victims with significant scarring and those with multiple sclerosis.
And while acclimatization can make hot days easier to cope with, everyone has their own limits, Leach said. “There’s a ceiling on how much you can adapt,” she added.
If you’re able to increase your heat tolerance, working in high temperatures one or two days a week should help your body maintain its adaptations, Dr. Adams said.
Without periodic exposure to heat, the effects wear off after about a month. However, the body remembers some adaptations, which should make acclimatization easier the next time.
Leach has been in the heated room enough times to notice how her own body has adapted. Over time, it has become easier to pedal the bike and she no longer counts the time she spends in the room.
Megan Thielking contribution to the report.