When is it too hot to exercise? How to safely work out in the heat

As unprecedented heat waves become more common, exercisers increasingly have to weigh the joys versus the risks of an outdoor workout.

There’s no simple 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 such as age, usual exercise routine, workout environment and intensity and whether that person is used to being active in the heat, says Stavros Kavouras, director of the Hydration Science Lab at Arizona State University.

Even when you’re at rest, your body produces heat. The harder you work out, the hotter your body gets.Credit: Getty Images

Exercising in humid heat poses unique challenges, he says, but being active in dry heat can be just as risky.

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

If the temperature outside is greater than 32 degrees or if the sun is shining, your body will also be heated by the environment, Kavouras says.

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

The main way the body sheds heat is through the evaporation of sweat, which cools the surface of the skin, Kavouras explains. In dry heat, sweat can evaporate so quickly that you may not notice it.

During high-intensity exercise, most people lose 1.5 to 2 litres of water per hour, although some people can lose even more. As a person becomes dehydrated, sweat production slows and it becomes harder to cool off. Those who exercise less regularly, who aren’t used to the heat, are sleep-deprived, are sick or are older have more trouble cooling off.

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

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

If you exercise and start to feel unwell, stop, rest in the shade and remove excess clothing, Tirabassi says. Symptoms of heat-related illness can include cognitive or mood changes, rapid pulse, headache, tunnel vision, dizziness, fainting or nausea.

Feeling cold or developing goose bumps are clear signs of a medical emergency, Kavouras says. Cool down by drinking cold fluids, spraying yourself with water, covering yourself with a cold towel or taking a cold shower. Work out with a partner in case one of you starts to feel sick.

Even if you don’t feel yourself sweating as you exercise, drink lots of water, Kenny says. The most you want to drink is about 1.5 litres of water an hour, which is the body’s absorption limit, Kavouras says. If you plan to do high-intensity exercise in the heat for over an hour, consider a hydration drink with added electrolytes – minerals such as sodium, potassium and magnesium – to replace what you lose, Kavouras says. Otherwise, you can experience cramping, dizziness and become prone to fainting, he says.

Sodium is especially important. “If you’re an athlete, especially if you exercise in a hot environment and you sweat profusely, you do need a lot,” he says.

If you exercise in the high heat and your body isn’t used to it, be sure to give yourself ample time to rest between workouts, too. Consider not exercising every day.

“If you push your body day after day, there is a progressive deterioration in your body’s ability to dissipate heat,” Kenny says. “Your body needs to recover.”

The New York Times

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