Health / Expert Advice

What Exactly Is Heat Exhaustion and How Do I Prevent It?

How to stay safe while pushing yourself under the sun this summer.

Evolutionary evidence tells us that human beings are excellent at thermoregulation, or maintaining our internal core temperature. After all, our ancestors had to run vast distances between food sources without overheating. However, heat exhaustion, or the body’s inability to effectively exercise in the heat, has been our Achilles heel for millennia. Run too hard or too long in the heat and, like a smartphone left in a hot car, everything just shuts down. We enlisted the help of Rachel Katch, MS, LAT, ATC, of the Korey Stringer Institute at the University of Connecticut, to review the symptoms of heat exhaustion and to find out how you can prevent it.

Before you hit “play” on that Aaptiv outdoor run, check the temperature outside. Or, opt for an indoor workout, you have hundreds to choose from. 

What is heat exhaustion?

The distinguishing factor of heat exhaustion is dubbed cardiovascular inefficiency, which is essentially low blood pressure, or hypotension. “Your heart is having difficulty providing enough oxygenated blood to your working organs,” Katch says. “When you’re exercising, most of your blood is taken away from your gut and your organs and is moved to your biggest organ, which is your skin. [Your skin] helps with the evaporation process of heat dissipation from the body.” When a combination of heat and high-intensity exercise puts oxygenated blood in short supply, the skin begins to struggle with heat dissipation, and the cardiovascular system becomes deprived of the necessary oxygen to maintain exercise.

Heat Exhaustion v. Heat Stroke

The term “heat exhaustion” is often used interchangeably with “heat stroke.” Although both are heat illnesses, they don’t exist on a continuum. Crucially, exertional heat stroke entails a degradation of central nervous system (CNS) function, which doesn’t happen in heat exhaustion. “You will see some potential CNS symptoms, such as a headache and dizziness and nausea and chills with heat exhaustion. But the body will try to stop itself from overworking and going into that exertional heat stroke stage,” Katch says. Therefore, someone who has heat exhaustion has already shut down and likely won’t enter exertional heat stroke.

Getting “heat cramps,” or exercise-associated muscle cramps, doesn’t mean you’re entering heat exhaustion, either. “It’s not directly related to elevated body temperature,” Katch says. “Typically it occurs after you’re exercising and you’re not replacing fluids.” A combination of dehydration and electrolyte imbalance cause exercise-associated muscle cramps. Although heat can factor into one’s dehydration, cramping alone isn’t a sign of heat exhaustion.

How to Tell If You’re Entering Heat Exhaustion

The sensation of heat exhaustion is markedly different from regular dehydration, Katch says. However, the symptoms can arrive quickly, especially if you’re working out at a high level. “You start feeling like you can’t go on, you might potentially faint, you feel extremely weak. You might start sweating more heavily than you normally would,” she says. Headaches are another sign of heat exhaustion, as are the chills on a hot summer’s day. “Heat exhaustion is going to be one of the worst fatigues you’ve ever felt,” Katch says.

Pace yourself in the heat.

Heat exhaustion is marked by an elevated core body temperature that nears 105 degrees. Although, the only way to accurately take your core body temperature while exercising is via a rectal thermometer. So, unless you carry one of those on the run, you’ll have to closely monitor your exertion and symptoms as you become fatigued, especially if you’re by yourself. “Some people can go right into heat stroke without having heat exhaustion,” Katch says. “They could be working at such a high level of intensity that they completely disregard all of the different symptoms that might be coming on.”

Katch and her colleagues conduct heat stroke research at the Falmouth Road Race, a 7-mile event run in the mid-August Massachusetts heat. “Typically people in that heat stroke state don’t remember being treated, or parts of the race,” Katch says. “I know a person who ran two miles and didn’t remember they did it. They woke up in the medical tent.” In 2015, a particularly hot year, Katch’s team saw 42 heat strokes. All were treated with favorable outcomes. Runners exercising by themselves, though, might skip heat exhaustion altogether and enter heat stroke, which is extremely dangerous. “People who have exertional heat stroke and they’re by themselves most likely will not have a very good outcome because they’re not aware of the CNS dysfunction,” Katch says.

The bottom line: Never push yourself to the limit in the heat unless you’re being monitored by medical professionals.

Surviving Heat Exhaustion

Once you’ve observed the symptoms of heat exhaustion, the first thing to do is to stop exercising, which would seem obvious. “You do get people that are the warrior mentality folks,” Katch says of her experience treating heat illness. “Those are the ones you have to watch out for because they’ll ignore the symptoms. Those are the people that end up getting heat stroke.”

Get to a cool, shaded place and shed unnecessary layers to help the skin dissipate heat. You can apply ice bags or cold towels to expedite the process. Begin replacing fluids immediately, but don’t drink a gallon of water all at once. Do your best to replace the amount of fluid you lost during exercise (the KSI sweat rate calculator is handy for this), and temper the water with electrolytes to avoid hyponatremia, or low blood sodium.

After you’ve recovered, be sure to stay in a cool place and continue this hydration regime for the rest of the day. It’s important to monitor your hydration levels and core body temperature after the event. “There have been a lot of people who had no case the day before and had heat stroke the next day because their bodies hadn’t recovered,” Katch says. If you’ve done all the right things and you feel fine the next day, you should be set to resume exercising.

How to Prevent Heat Exhaustion

Staying out of the heat is naturally the best preventative measure against heat exhaustion. At 90 degrees, Katch says, it’s time to take the workout indoors. If you can’t avoid being outside, know the following risk factors and adjust accordingly.

Acclimate to the heat.

Katch works with high school football programs to acclimatize athletes to heat over a period of 7 to 14 days. For runners, that means taking it easy on day one and allowing the body time to adapt to the conditions. “The first things you’ll see are cardiovascular adaptations. You’re going to sweat more to evaporate heat, and you’re going to be able to handle more cardiovascular loads, which will help with heat dissipation as well,” she says.

Set a forgiving work-to-rest ratio.

“Working out for an hour and doing 55 minutes of intensity and a 5-minute break isn’t going to be enough,” Katch says. Aim for a work-rest ratio closer to 1:1 or 2:1. Use rest periods to get out of the sun and replace fluids lost during the work periods.

Replace fluids according to your sweat rate.

It’s key to start the workout well-hydrated. But adequately replacing fluids as you exercise is just as important. To calculate your sweat rate, take your body weight in kilograms pre- and post-exercise on a hot day. Write down the change in body weight in grams, and note the volume of fluid you consumed pre-exercise in milliliters. Adding the weight lost and volume of fluid consumed (1g = 1 mL) yields your sweat loss, which you divide by your exercise duration to find your sweat rate. Use that figure to gauge your rehydration for future hot-weather efforts.

Know your risk factors.

A variety of personal attributes determine your risk of heat exhaustion. People with high BMIs and bigger body surface areas have more metabolic heat production, Katch says, which means a higher risk of heat illness. People starting at lower fitness levels are also at greater risk of overheating due to earlier onset of fatigue. Certain medications, too, can increase your risk. Getting evaluated by a doctor before entering an exercise regime or race in the heat is your best bet to mitigate your own potential risk factors.

If it’s too hot to workout outside, try doing a strength class with Aaptiv in your living room, or make it your rest day.

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