How to Workout Safely (and Avoid Heat Exhaustion) When It's Boiling Hot Outside

When it's hot as can be, is it still safe to go forth with your normal outdoor workout? Sure, you could cave and join a gym that blasts AC all day. The other option is to give up on exercise entirely until the temperature subsides, but that could be months (or never, if you're in a perennially warm location), and you can't stay cooped up and sedentary forever! We've all got to move.

If you're an avid outdoor exerciser, there's no need to let the summer heat or scorching sun sabotage your fitness habits altogether—but you do need to make a few tweaks to stay safe and healthy (read: avoid heat exhaustion, dehydration, muscle cramps, and the like). And this is even more important for groups who may be more sensitive to high heat, like adults over 65 or people on certain medications such as diuretics, beta-blockers, antihistamines, tranquilizers, or antipsychotics, says Alexis C. Colvin, M.D., an orthopedic sports medicine surgeon at Mount Sinai. We asked Dr. Colvin for her top tips on working out safely when it's really hot, smart ways to tailor your workouts to accommodate a heat wave, and the biggest signs that it's time to bring your sweat session indoors.

Portrait of sporty black woman in yellow top drinking water
Portrait of sporty black woman in yellow top drinking water

Prostock Studio/Getty Images

RELATED: 4 Reasons Why Sweating Is Actually Great for You (Besides Cooling You Down)

Why You Need to Be More Careful When Exercising in Extreme Heat

"Heat affects the body in a number of ways, including increasing body temperature, increasing heart rate, and increasing blood flow to the skin ([your] blood vessels dilate to increase sweating)," Dr. Colvin says. When it's super-hot—like, 80 to 90-plus degrees Fahrenheit—your body is more susceptible to heat-related issues, especially when exerting lots of energy from physical activity. Dr. Colvin notes that dehydration, heat cramps, and heat exhaustion are some of the most common conditions related to exercising in hot and/or humid temps.

What is heat exhaustion?

"Heat exhaustion occurs when the body isn't able to cool itself down, typically due to a combination of physical activity and high temperature (especially with high humidity)," she explains. "When the humidity is high, sweat doesn't evaporate as readily from your skin, which is the body's normal cooling mechanism." Matters are made worse if you're not properly replenishing the water and electrolytes you're sweating out (in other words, you're dehydrated).

Some signs and symptoms of heat exhaustion to watch out for, according Dr. Colvin: headache, dizziness, fatigue, nausea, vomiting, cramping, heavy sweating, fast but weak pulse, and/or cold, pale, clammy skin.

How hot is too hot to exercise?

Dr. Colvin says the best way to determine how safe it is to exercise or work vigorously outdoors is the WBGT, or wet bulb globe temperature, which "is a measure of the heat stress in direct sunlight," according to the National Weather Service, and takes into account "temperature, humidity, wind speed, sun angle, and cloud cover." This is different from the heat index, but if the WBGT isn't available for your location, the heat index works, too.

"In general, [temperatures] below 70 degrees F have a low risk of heat illness," Dr. Colvin says. "Caution is recommended when exercising between 80 and 90 degrees F—and extreme caution is advised when the heat index is over 90 degrees F." If you're not acclimated to these high temps (and few are), you'll want to head into an air-conditioned, shady space when it gets into the 90s For those more sensitive groups, you may want to do so even if the heat index is below 80 degree F.

RELATED: How Cold Is Too Cold to Exercise Outside? Here's How to Stay Safe and Warm During Winter Workouts

Smart Tips for Safe Workouts in the Heat

Workout before 10 a.m. or after 4 p.m.

To avoid the strongest sun and highest temperatures, Dr. Colvin says to limit physical activity between the hours of 10 a.m. and 4 p.m. "If you must be outside during those times, take precautions to prevent sun exposure: Use and reapply sunscreen, use a wide-brimmed hat, sunglasses, and light-colored, loose, long-sleeved clothing," she says. "Take frequent breaks, have cooling methods available (such as ice towels), and stay properly hydrated."

Know your limits based on normal temperatures.

According to Dr. Colvin, it's important to establish a baseline level of fitness in cooler temperatures before grinding away in the sun and heat. Gradually increase the hours and days of activity in the hotter climate over several weeks, she says. Start slow and listen to your body—don't push it. Keep things lower impact and modify to make it easier on your system.

Always take the time to warm up and cool down.

"Warming up before a workout helps to acclimate your body to the heat and humidity," Dr. Colvin says. "Cooling down afterward helps to bring your body temperature, heart rate, and blood pressure back to normal levels."

Her favorite dynamic warm-up moves to try: jumping jacks, inchworms, butt kicks, jogging with arm circles, lunge with a twist, skaters, and high knees.

And some static stretches for cooling down: cat cows, child's pose, pigeon pose, runner's lunge, cross-arm and overhead triceps stretch, and doorway pectoral muscle stretch. (Here's a quick, full-body stretching routine you can do every day.)

RELATED: 3 Gentle Stretches for a Stiff Upper Back and Neck Pain


And not just after your workout or if you happen to feel thirsty during a break. "Mak[e] sure you're hydrated when you begin the activity, stay hydrated during the activity, and replace fluids after the activity," Dr. Colvin says. "Each individual's hydration needs are different, and it can be determined by measuring your sweat rate."

Hydration guidelines to follow, according to Dr. Colvin: When doing an activity for less than one hour, water is going to be the ideal drink. But when sweating and exerting energy vigorously in the high heat for anything longer than one hour, drink fluids that contain both carbohydrates and sodium (like sports drinks) to replenish lost glucose and electrolytes.

RELATED: 10 Super-Hydrating Drinks That Are Way More Interesting Than a Glass of Water