How to Exercise in the Summer Heat

The summer of 2021 came in sizzling, with June temperatures in many parts of the United States shattering records, baking landscapes and prompting those of us who usually exercise outside to question when, how — and if — we should continue to work out in nature’s furnace.

Helpfully, a group of exercise scientists wrote a comprehensive scientific review about training and competing in scorching heat, in preparation for the upcoming Summer Olympics in torrid Tokyo. Published in the aptly titled journal Temperature, the review focuses on elite athletes — but, the authors agree, the advice can be adapted for those of us training for a summer fun run or charity bike ride or aiming simply to stay active and safe outside until fall. What follows is a compilation of their expert recommendations, including when to down a slushie, why you might want to take a hot shower and whether to freeze your underwear.

When we exercise, we generate internal heat, which our bodies shed by sweating and shunting warmed blood away from our cores and toward the skin. If ambient temperatures rise, though, this process falters. Body heat builds up. Our hearts labor to send additional blood toward the skin. We glisten with sweat, and the same run, stroll or ride that felt tolerable during cooler weather now drains us.

To sidestep these conditions, we can move our workouts indoors, into air-conditioned comfort, or schedule them strategically. “I would always recommend the morning,” especially for city dwellers, says Oliver Gibson, a senior lecturer in exercise science at Brunel University London and lead author of the review. “In an urban area, it is likely that the concrete will have retained a high amount of residual heat that will radiate back” at exercisers later in the day, he says. Unshaded sidewalks similarly will be hotter than parks and leafy pathways.

We also should accustom ourselves, slowly, to unfamiliar swelter, Dr. Gibson says, a process known to exercise scientists as acclimatizing, which involves working out sometimes, by choice, when the day is warmest. This approach helps to condition our bodies to better cope with the heat. Once acclimatized, we will sweat earlier and more abundantly than before, dissipating internal heat better and leaving us feeling bouncier and less fatigued.

Acclimatizing should be gradual, however. To start, slather on sunscreen, fill a water bottle, head outside after about 10 a.m., when temperatures intensify, and try to complete a gentler version of your standard workout, says Carl James, a senior physiologist at the National Sports Institute in Kuala Lumpur, Malaysia, and co-author of the review. If you usually run for 30 minutes, for instance, maybe jog for 20, and monitor how you feel. If your heart seems to be racing, he says, or you feel lousy, “slow down.”

After a few acclimatization sessions, you should notice your clothes and skin are drenched, Dr. Gibson says. Congratulations. “Earlier and more profuse sweating is a great sign that heat adaptation is taking place,” he says. Most of us acclimatize after about five to 10 hot workouts, he adds, although women, who tend to sweat less freely than men, may require an extra easy session or two to be fully prepared for harder workouts in the heat.

After each acclimatization session, head for the showers, but dial up the heat. Standing under a warm shower spray or soaking in a hot bathtub for 10 minutes or so after a sweltering workout prompts our bodies to continue acclimatizing, Dr. Gibson says. “It extends the stimuli for heat adaptation,” he points out, “and is therefore welcome and beneficial.”

An icy beverage before a hot workout “will help with hydration and provide a combination of perceptual and actual cooling,” Dr. Gibson says. Aim to drink about 16 ounces of cold fluid 20 minutes or so before you head out. Drinking closer to the session’s start could cause stomach upset during your workout.

But these techniques can be risky, too, he cautions, because the cooling effects are limited and short-term, and potentially deceptive. “We sometimes see people cool before exercise, feel great, then head out too fast or hard,” he says, winding up prematurely winded and possibly on the cusp of heat problems.

If you feel nausea, headache, dizziness or cramping during a hot workout, slow down or stop and hunt for shade, Dr. Gibson says. These could be signs of incipient heat illness. (You can learn more about the symptoms of heat illness and heat stroke at the website of the Centers for Disease Control and Prevention.)

Unfortunately, heat illness also clouds thinking, says Neil Maxwell, a lecturer in environmental physiology at the University of Brighton in England and the review’s senior author. “Your judgment becomes impaired,” he says, and you may not realize you are overheating.

He and his co-authors strongly recommend exercising with a partner in the heat. If either of you starts to feel “seriously hot or shows signs of cognitive dysfunction,” he says, such as sudden confusion, get off the path, under a shady tree or awning, and call for help. “Rapid cooling is essential within the first 30 minutes” of such an episode, Dr. Maxwell says. Immediately applying a cool cloth could help to start lowering body temperature.

You might also protect yourself and your training partners by the simple expedient of rejiggering your routes, Dr. Gibson says. “On hot days, do shorter loops” than normal and include “a dedicated water station,” he suggests, such as a public drinking fountain. Refill your water bottle there or stick your head under the flow each time around. Plus, “if you are feeling the heat,” he concludes, running in short loops “makes ending the session early more realistic.”