How Hot Is Too Hot to Exercise Outdoors?

Sure, another hot summer ahead means the potential for extra beach days, pool parties, and fun in the sun. But high temperatures are a public health threat that only intensifies as the world warms, and a serious problem for the Olympic athletes who have to break world records outdoor (without A/C in the Olympic Village, no less). Olympian or not, precautions everyone should take include hydrating properly, reapplying SPF regularly, investing in ways to keep your home cool on hot summer nights with air conditioners or sweat-wicking sheets, and retooling your fitness routine to account for exercising in the heat.

We get the impulse to keep sweatin' it out in the sun: The majority of American adults would rather be physically active outdoors, according to a survey OnePoll conducted for the fitness app Verv last year. Of the 2,000 participants, it found that 75 percent of men and 51 percent of women prefer to work out al fresco.

That's not surprising, considering exercising in nature provides bonus health benefits: Studies find that exercising outdoors increases your physical activity level¹, while making workouts feel easier. It’ll also reduce your stress and cortisol levels while boosting your mood and self-esteem.

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But unlike with indoor workouts, extreme temps of any sort and other weather factors, like humidity, need to be taken into consideration when it comes to outdoor workouts.

How exercising in the heat affects your body

"During exercise in hotter temperatures, our body does this great thing called thermoregulation, which is the body's ability to maintain our internal temperature within a safe range,” Heather Milton, CSCS, a board-certified clinical exercise physiologist at NYU Langone’s Sports Performance Center tells Well+Good.

Every time you sweat, that’s an indicator that your body is thermoregulating. Increased blood flow is another sign, says Milton. "The two combine to cause higher heart rates to perform the same amount of work as compared to a temperate environment,” she explains.

You’ve likely experienced this for yourself if you’ve ever tried hot yoga and it felt more challenging than when you’ve done either practice in a non-heated room—more heat equals more work.

How exercising in the heat affects your recovery

The way your body keeps you cool during a hot workout can have implications after the workout is over. The water you lose through all that extra sweating, and the blood flow that's diverted from the muscles to the skin, can make for a more difficult recovery.

"Higher temperatures cause a longer recovery time for your heart rate to return to normal," Bethany Stillwaggon, fitness trainer and master coach for Row House, previously told Well+Good about hot exercise recovery. "With enough water and electrolyte loss, someone could start feeling the effects of complications associated with water loss like headaches or dizziness, muscle cramping, easy fatigue, and muscle weakness."

Your muscles need water and blood flow to both regain strength² after a workout and repair the damage sustained, which is how you build muscle. Because blood flow can be restricted, and dehydration can be even more extreme after working out in the heat, muscle recovery may be impacted³.

How your body can adapt to exercising in the heat

It’s possible to train your body to better thermoregulate itself by increasing the intensity of your workouts and conditioning your body to perform at higher heart rates, says Ally McKinney, a personal trainer at Gold’s Gym.

“The better we are at regulating heat, the more effective we can be with our workouts,” McKinney says. “Like any other type of stress, adapting to and overcoming the intensity of these workouts is something to acclimate to. We can use this same technique when working to train for hotter temperatures.”

Conditioning can only take you so far. There is a point when temps can get too high for your body to be able to thermoregulate itself.

Conditioning can only take you so far, though. There is a point when temps can get too high for your body to be able to thermoregulate itself.

How hot is too hot to exercise in the heat?

Every body responds differently to heat depending on how used to high temps it happens to be. But all of us should be mindful when the thermostat rises past 90 degrees, according to Milton.

“Exercising in temperatures higher than 91.4°F can increase the risk of heat exhaustion, which occurs when the body is unable to maintain proper blood flow to all organs and the skin for thermoregulation at the same time," she says. Signs of heat exhaustion include fainting, fatigue, and no longer being able to exercise, she says.

Signs of heat exhaustion include fainting, fatigue, and no longer being able to exercise.

At about 92 degrees, your internal temp will be about 98.6 to 105 degrees, Milton says, and that’s about the end range for exercising in heat without risking heatstroke. "[Heatstroke] is even more serious and is paired with collapse and central nervous system dysfunction—confusion, dizziness, irrational behavior, etc. This situation requires immediate cooling," Milton cautions.

The best way to avoid such symptoms is by not working out in extreme heat. This could mean choosing to exercise earlier or later in the day, rather than when the temps are highest, or staying inside an air-conditioned room. But you also want to make sure you’re priming your body before, during, and after your outdoor workouts to ensure that’s it’s able to regulate your internal temp to the best of its ability.

How to prepare your body for exercising in the heat

In a word: hydrate. "Before exercise, always drink two glasses of water, then during the activity try to drink four to six ounces of fluids every 20 minutes, and always drink again after you are finished,” says Jennifer Haythe, MD, a leading cardiologist and director of cardio-obstetrics at New York Presbyterian Hospital Columbia.

Because one of the primary ways your body cools itself down is by sweating, you’re losing water through your skin. You’re also losing are electrolytes—sodium in particular, Haythe says. “Sodium is one of the most basic minerals our body needs to complete the fundamental processes of the cell.”

Downing water or sports drinks with electrolytes is one way to replenish your reserves. "Try to look for drinks that provide 14 grams of carbohydrates, 28 mg of potassium, and 100 mg of sodium per 8 ounces," says Haythe. Refueling with foods that are high in sodium, magnesium, and potassium—like cottage cheese, olives, bananas, and beets—helps, too.

In general, "for every pound of weight you lose due to sweat, replace it with at least a half of a liter of water,” Milton says. “You may need to take in up to 20 percent more fluid than usual."