Exercise safely in the heat

As unprecedented heat waves become more common, athletes must increasingly weigh the joys and risks of an outdoor workout.

There is no easy answer to the question of how hot is too hot. A person’s ability to stay safe while exercising in the heat depends on many factors, including age, usual exercise routine, exercise environment and intensity, and whether that person is used to being active in the heat, said Stavros Kavouras, director of the Hydration Science Lab at Arizona State University.

Exercising in humid heat presents unique challenges, he said, but being active in dry heat can be just as risky. (If you’re exercising in humid heat, here’s a guide.)

Why heat can be dangerous

Even when you’re at rest, your body produces heat — and the amount increases as your muscles burn fat and carbohydrates during exercise. The harder you work, the hotter your body gets.

When the outside temperature is above 90 degrees or when the sun is out, your body is also being heated by the environment, said Dr. Kavouras. (Although the body’s average internal temperature is 98.6, the skin often hovers around 90, so higher temperatures increase the risk of overheating.)

“As you add this huge external source of heat, the body has to deal with it,” says Glen Kenny, a physiologist who studies the body’s stress response at the University of Ottawa.

The main way the body dissipates heat is through the evaporation of sweat, which cools the surface of the skin, explains Dr. Kavouras out. This happens more easily in dry heat than in humidity, but in dry heat, sweat can evaporate so quickly that you may not notice it.

“You don’t even see it and you don’t even realize you’re getting so dehydrated,” said Dr. Kavouras.

During intense exercise, most people lose one and a half to two liters of water per hour, although some people can lose even more. If a person becomes dehydrated, sweat production slows down and it becomes more difficult to cool down. Some people are better at dissipating heat than others. Those who move less regularly, are not used to the heat, lack sleep, are ill or are older have more difficulty cooling down, putting them at increased risk for heat-related illness, said Dr. Kenny.

People can get used to exercising in the heat to a limited extent, he added. In a small study from 2019, healthy men between the ages of 50 and 70 improved their ability to dissipate heat by 5 percent after a week of daily exercise in temperatures of 104 degrees. But it’s unclear how many people following a more relaxed regimen will acclimate, he said.

Stay safe

Try to exercise during the coolest time of day, which is often early morning in dry heat regions, said Dr. Jill Tirabassi, a physician with expertise in sports medicine at the University at Buffalo. Seek shade and wear porous, light-colored clothing made of moisture-wicking material. The more bare skin the better.

Avoid cotton, which holds water instead of allowing it to evaporate, and backpacks, because you produce a lot of sweat around your spine that can get trapped, said Dr. Kavouras.

If you exercise and start to feel unwell, stop, rest in the shade and remove excess clothing, said Dr. Tirabassi. “Warning signs can sometimes be subtle,” she said, but symptoms of heat-related illness can be Involving cognitive or mood changes, rapid pulse, headache, tunnel vision, dizziness, fainting or nausea.

Feeling cold or getting goosebumps are clear signs of a medical emergency, Dr Kavouras said. Cool down by drinking cold liquids, spraying yourself with water, covering yourself with a cold towel, or taking a cold shower. Exercise with a partner in case one of you starts to feel sick.

Even if you don’t feel yourself sweating during exercise, drink plenty of water, Dr. Kenny said. “Water is always the most important thing,” he said. The most you want to drink is about 1.5 liters of water per hour, which is the body’s limit of absorption, said Dr. Kavouras. If you’re planning on exercising for more than an hour in the heat, consider a hydration drink with added electrolytes — minerals like sodium, potassium and magnesium that are released during sweat — to replace what you lose, said Dr. Kavouras. Otherwise, you could experience cramping, dizziness and become prone to fainting, he said.

Sodium is especially important. “If you’re an athlete, especially if you train in a hot environment and you sweat profusely, you need a lot,” he said. If nothing else, on days when you’re exercising intensely for more than an hour, “it’s a good idea to salt your food a little bit more.”

If you’re training in the high heat and your body isn’t used to it, make sure you also give yourself enough time to rest between workouts. Consider not exercising every day.

“When you put stress on your body day in and day out, your body’s ability to dissipate heat gradually deteriorates,” said Dr. Kenny. “Your body needs to recover.”