Heat Exhaustion Symptoms and Prevention

Your body wants your temperature to always remain between 97° to 99°F Fahrenheit (F), or 36.1° to 37.2°Celsius (C), according to the US National Library of Medicine. And it usually does a pretty good job at regulating its temperature. On a hot, humid day, though? Sometimes not so much.

When your body loses its ability to self-regulate, you run the risk of developing a heat illness, one of the biggest summer health hazards. According to the Centers for Disease Control and Prevention (CDC), heat illness is a risk during outdoor exercise or outdoor physical activity, especially for people over age 65 or who take medications that increase sensitivity to high temperatures. Given the right conditions, though, it can happen to anyone in high temps at any time of the year.

Here are the heat exhaustion and heatstroke signs and symptoms you should look out for.

Heat Exhaustion vs. Heatstroke

When your body has to work extra hard to cool you down, you can develop heat exhaustion. According to the CDC, during heat exhaustion, the body's core temperature is usually less than 104 degrees Fahrenheit, blood pressure is low, and the heart is not pumping blood as efficiently as it should.

At this stage, the body is still doing what it should. "You'll be very fatigued and sweating a lot and thirsty—so those natural defenses against heat and dehydration are still working," said Peter Shearer, MD, associate director of the Mount Sinai Hospital emergency department in New York City.

Heat exhaustion does not necessarily lead to heatstroke—but it could, said exercise physiologist Michael Bergeron, PhD, former president and CEO of Youth Sports of the Americas. Bergeron described exertional heatstroke as "a clear medical emergency affecting multiple body systems," which usually occurs when the body's core temperature rises above 104 degrees.

According to a 2022 review published in Experimental Physiology, heatstroke causes the central nervous system to malfunction. It can also damage the brain, heart, liver, kidneys, spleen, and muscular tissue. "Your body loses the ability to thermoregulate, so at that point, it's much more difficult to reverse itself," said Dr. Shearer.

Warning Signs: Cramps, Nausea, or Headache

The first signs of a heat illness are often stomach cramping or nausea, which can indicate dehydration or an electrolyte imbalance in the body, according to the CDC. These are closely linked with heat illness since the body needs water (and electrolytes like sodium) to regulate its temperature properly, but it also loses them through sweat. Headaches are also a common first sign of dehydration.

It can be hard to pinpoint the exact cause of gastrointestinal (GI) distress while exercising. Bergeron told Health, "but, if conditions warrant you thinking this, best to assume that it is heat-related." Bergeron's advice? "Take a break [and] hydrate." Only if you're feeling better should you try going back to what you were doing.

Warning Signs: Thirst and Heavy Sweating

"Heavy sweating is a sign that your body is producing a lot of heat," said Bergeron, which could lead to heat exhaustion if conditions don't improve or if they worsen. According to a research article published in 2020 in Medico Legal Journal, heat is more dangerous on humid days because sweat can't evaporate off the skin—which normally produces a cooling sensation—as easily as it can with dry or breezy weather.

Feeling thirsty is also a sign that your body needs more water. Even if you don't feel parched, it's important to sip water frequently on hot days. If you're out in the heat for more than an hour, a sports drink can also help replenish lost electrolytes.

Warning Signs: Dark Urine

Watch out for urine trouble. "If you're out playing tennis for hours but you're drinking between sets and you take regular bathroom breaks and your urine is fairly clear, that's a sign you're hydrating well," said Dr. Shearer. "But if you go for a long run and you're not urinating a lot, and when you do go, it's very dark and concentrated, those are signs you're underhydrated. That means you won't be able to release heat as efficiently through sweat and evaporation, which puts you at risk for overheating," explained Dr. Shearer.

Warning Signs: Dizziness or Collapse

Heat exhaustion often makes people dizzy, need to sit down, or even pass out momentarily. "Which is sort of a good thing," Bergeron told Health. "That is, it stops your body from continuing before you get into more serious trouble," Bergeron said.

The dizziness may be accompanied by tachycardia (a rapid heart rate), and a person can also experience syncope (passing out).

The hotter it gets outside, the more common these symptoms are. But Bergeron said it's important to remember that, for people doing strenuous workouts, exertional heat illness can also occur in relatively cool conditions.

Warning Signs: Lack of Sweating

When the body reaches a certain internal temperature, its natural defenses—like sweating to release heat—begin to shut down. "If a person's been outside exercising in the heat and their skin is totally dry, that's a red flag," said Dr. Shearer.

However, said Bergeron, it's not always the case that a person will stop sweating as they enter the danger zone—so don't assume that just because someone is sweating heavily, they're okay. (If they have other signs of heat illness, get them help immediately.)

Warning Signs: Confusion, Convulsions, or Coma

If heatstroke progresses far enough, it can harm your brain. "You start to experience confusion and delirium," said Dr. Shearer. "And that's really dangerous because when you're confused you won't do the normal things to cool yourself down, like seek out shade or get water," Dr. Shearer warned.

In the most severe cases of heatstroke, fainting and convulsions can occur.

You Might Not Feel Hot

The signs of heat illness aren't always as obvious as they might seem. "In fact, with developing overheating—possibly leading to exertional heatstroke—one can feel chilled, even in the heat," said Bergeron.

The reason? When the body experiences exercise- or heat-related stress (or both), it protects itself by producing inflammatory proteins, as described in a 2022 review in Nature Reviews Disease Primers. But these proteins can also interfere with the body's thermoregulation, bringing on symptoms like chills, goosebumps, or cold, clammy skin.

You May Not Notice Heat Illness

Even if you feel overheated, nauseous, or a little "out of it," it can be hard to stop what you're doing and take a break—especially if you're in the middle of a killer workout or running a race. That's why it's important to recognize the earliest symptoms of heat illness and address them while you still can.

"A challenge is that an overheated athlete is often the worst person to make an objective assessment," said Bergeron. "As the body and brain heat up, the brain's cognitive capacity and objective reasoning are often compromised," Bergeron said.

What to Do: Listen To Your Body

Most of the body's heat is generated internally from muscle exertion. And for some people, this can manifest with muscle cramps. So in hot, humid conditions, it's smart to lower the intensity of your workout and take precautions beforehand—like wearing sweat-wicking clothing and drinking plenty of fluids.

If you still feel like you're starting to overheat, stop what you're doing, said Bergeron. In fact, Bergeron added, any significant deterioration in performance or signs of struggling should be reason to take a break.

What To Do: Get Out of the Sun

One of the most important things you can do for yourself or someone else who may be experiencing heat illness is to get them out of the sun. If you can, get to an air-conditioned area, but anywhere with shade can help. According to the U.S. Occupational Safety and Health Administration, a location bathed in direct sunlight can have a heat index value up to 15 degrees higher than a spot nearby in the shade.

What To Do: Increase Air Circulation

One way the body cools down is through convection—the process of air or water flowing across the skin and transferring away heat. You can help this process by increasing air circulation and the air's exposure to skin, said Dr. Shearer.

"If someone is overheating, you can loosen their clothing—make sure nothing's too restrictive, or remove items they don't need," Dr. Shearer said. Putting them in front of a fan, manual or electric, can also help them lose excess body heat.

What To Do: Elevate Legs

Blood flow throughout the body can be compromised during a heat illness, so sitting or lying down with feet elevated about 12 inches can help prevent swelling in the legs and improve blood flow to the brain, according to the 2022 review article in Experimental Physiology. It can also protect you from falling and injuring yourself if you're feeling faint.

What To Do: Drink Cold Water, and Use It to Cool Your Body

If a person is conscious and able to swallow, have them drink water or a sports drink that contains electrolytes. (Cool fluids are recommended, according to the CDC.) You can also make your own electrolyte solution by mixing a teaspoon of salt per quart of water. This will help replenish the sodium the person has lost through sweat. Sip until you're feeling better.

You can also use cold water or ice to cool down, but don't place ice directly on the skin. If the person's not sweating, spritzing them with water can help mimic the process and help them evaporate some of the heat off of their body.

For someone whose body temperature has reached dangerous levels, full-body immersion into an ice bath or a cold body of water is the most effective method of bringing temps down quickly, said Bergeron. Applying ice packs or cold-water-soaked towels to the body can also help.

The groin and armpits are especially effective spots for cooling, said Dr. Shearer. So is the neck, although Dr. Shearer cautioned against putting ice packs directly on this area since the carotid artery supplies blood to the brain. The palms of the hands, feet pads, and the forehead can be effective for cooling the body.

Get Medical Help

If a person shows signs of heatstroke, get them medical care immediately. They should have their blood pressure, temperature, heart- and respiratory rate, and central nervous system status monitored closely, even once they start to feel better.

Heat exhaustion, on the other hand, may not need professional attention. "If it's a person who is young and healthy and they're really sweaty and thinking clearly, they will probably be fine after cooling down and resting," said Dr. Shearer. "It's really the people who are older or who have other medical problems—heart disease, diabetes—who you want to get to a hospital to be safe."

Take It Easy Afterward

Even if you're young and healthy, that doesn't mean you should jump right back into your marathon or finish your shift doing manual labor in the hot sun after a heat-illness scare. "You may still be measurably dehydrated and/or at risk for rapidly overheating if you go at it again," said Bergeron.

In fact, anyone experiencing heat illness should take the rest of the day off from physical exertion, Bergeron added, even if your symptoms go away and you're feeling 100% better. "Best to rest and more fully recover and live to play another day," Bergeron said.

Protect Yourself for Next Time

Heatstroke can be dangerous, even fatal, and difficult to treat once it sets in. That's why it's important to prevent it from happening in the first place. Avoid situations that put you at risk. Don't push yourself too hard in the heat when you're recovering from an illness involving vomiting, diarrhea, or fever, taking medications that may make you more sensitive to heat or to the sun, or—of course—it's really hot out. Gradually ramping up your workouts over several weeks to acclimate your body to higher temperatures can also protect you during the summer.