Heat Exhaustion

Medically Reviewed by Poonam Sachdev on June 11, 2024
Written by Kim Painter, Rick Ansorge
8 min read

Heat exhaustion is an illness that can happen when your body gets too hot and can't cool itself. It's a response to losing too much water and salt, usually because of too much sweating. It can happen when you work or exercise outside on a hot, humid day or spend too much time in a very hot environment, such as a home or factory without air conditioning during a heat wave.

Heat exhaustion is one of three major heat-related illnesses. The other two are heat cramps, which are less serious, and heat stroke, which is the most serious.

Because these conditions can progress from one to another, they are sometimes described as the three stages of heat illness.

Heat exhaustion and heat stroke are different conditions, but they are related. If you have symptoms of heat exhaustion and don't take quick steps to cool your body down, you can develop heat stroke.

Heat stroke is a very serious, potentially deadly condition that, unlike heat exhaustion, can permanently damage your brain and other vital organs. When you see symptoms of heat stroke, it's an emergency and you should call 911.

Some symptoms that could point to heat stroke, rather than heat exhaustion, include:

  • Body temperature at or above 104 F.
  • Confusion, agitation, or aggression
  • Hallucinations
  • Slurred speech
  • Fainting
  • Not being able to sweat

Is sunstroke the same thing as a heat stroke?

Yes, sunstroke is a name some people use for heat stroke.

Heat exhaustion symptoms may start rapidly or come on slowly, with signs of milder heat illness showing up first. You might get a red heat rash or painful heat cramps, often in your arms and legs.

Once heat exhaustion sets in, symptoms can include:

  • A body temperature over 100 F (though it's possible to have heat exhaustion with a normal temperature)
  • Dizziness or faintness
  • Cool, moist skin with goosebumps
  • Blurred vision
  • Fatigue
  • Headache
  • Nausea and vomiting
  • Heavy sweating
  • Rapid heartbeat
  • Rapid shallow breathing
  • Swollen ankles, feet, or hands (heat edema)
  • A drop in blood pressure when you stand

Symptoms of heat exhaustion in babies

Babies and young children are at especially high risk of heat illnesses, such as heat exhaustion and heat stroke. That's because they sweat less, making it harder for them to cool off.  But they might not be able to tell you how they are feeling. And they may show no clear early signs, other than seeming fussy and unwell.

But signs of heat exhaustion in a baby might include:

  • Paler skin than usual -- which might be most noticeable on the palms of hands, nails, eyes, gums, or tongue
  • Cooler than usual skin
  • Increased thirst and other signs of dehydration, such as peeing or crying less
  • A rise in body temperature

Usually, you can count on your sweat to act as your body's air conditioner: As your sweat dries, it carries heat away from your skin. But when the air is hot and moist, sweat is slower to evaporate and your body struggles to stay cool, especially when you're active. The more you sweat, the more you deplete your body's fluids and essential minerals (electrolytes), such as sodium and potassium. Those losses of water and minerals cause the main symptoms of heat exhaustion.

Heat exhaustion is strongly related to the heat index, which is a measurement of how hot you feel when the effects of relative humidity and air temperature are combined.

The risk of heat-related illness dramatically increases when the heat index climbs to 90 degrees or more. So it's important, especially during heat waves, to pay attention to the reported heat index and remember that the heat index is even higher when you are in full sunshine.

If you live in an urban area, you may be at especially high risk for heat exhaustion during prolonged heat waves. That's because of the "heat island effect," in which asphalt and concrete store heat during the day and gradually release it at night, resulting in higher nighttime temperatures and fewer opportunities to cool off. Stagnant atmospheric conditions and poor air quality can make things worse.

Other factors that can increase your risk of heat-related illness include:

Age. Infants and children up to age 4, and adults older than 65, are particularly vulnerable because they adjust to heat more slowly than other people.

Certain health conditions. These include heart, lung, or kidney disease, obesity or underweight, high blood pressure, diabetes, mental illness, sickle cell trait, alcoholism, sunburn, and any conditions that cause fever. People with diabetes are at increased risk of emergency room visits, hospitalization, and death from heat-related illness and may be more likely to underestimate their risk during heat waves.

Medications. These include some medicines in the following classes: diuretics, sedatives, tranquilizers, stimulants, heart and blood pressure medications, and medications for psychiatric conditions.

Alcohol use. Alcohol makes you pee more than other fluids and can negatively affect how your body balances fluids. That can contribute to heat exhaustion.

Sudden temperature changes. Your body can get used to higher temperatures to some extent, but if you go suddenly from a cold place to a hot one or your region gets an unseasonable heat wave, you might not adapt quickly enough.

If you have symptoms of heat exhaustion or see someone who does, get a temperature reading to rule out heat stroke. If you are at a sporting event or other location with a medical professional on hand, they might take your temperature rectally to confirm the diagnosis. They'll also ask about your activity -- such as whether you've been exercising outside, working in a hot building, or sitting in a hot car.

If your temperature is high enough to suggest you might be developing heat stroke -- 104 F or higher --  you might get other tests, such as:

  • A blood test for sodium, potassium, and blood gases
  • A urine test to look for kidney effects
  • Muscle tests to look for muscle damage
  • Imaging tests to look for organ damage

If you have symptoms of heat exhaustion, it's essential to immediately get out of the heat and rest, ideally in an air-conditioned room. If you can't get inside, try to find the nearest cool and shady place.

You should also:

  • Drink water or sports drinks (avoid caffeine and alcohol) -- taking small sips rather than big guzzles over about an hour
  • Remove any tight or unnecessary clothing
  • Take a cool shower, bath, or sponge bath
  • Apply other cooling measures such as fans or ice towels

If you don't feel better within an hour, seek medical help, because untreated heat exhaustion can progress to heat stroke. If your temperature rises or your symptoms get worse, don't wait — call 911 or go to an emergency room.

If you are showing signs of heat stroke, you may be put in an ice bath or cooling blanket or, if those aren't available, misted with cool water. You might also get medication to help you stop shivering.

How long does it take to recover from heat exhaustion?

Heat exhaustion usually goes away with rest and fluids. You should feel better within an hour.

Still, you'll probably be more sensitive to high temperatures during the following week. So, it's best to avoid hot weather and heavy exercise until your doctor tells you that it's safe to resume your normal activities.

When heat exhaustion symptoms don't get better with an hour of rest and fluids, seek medical care. Get help immediately if someone with signs of heat exhaustion:

  • Can't drink or keep fluids down
  • Has a body temperature of 104 F or more
  • Has trouble speaking, standing, or walking
  • Keeps sweating heavily
  • Seems confused
  • Passes out

When the heat index is high, it's best to stay inside in air conditioning. If you must go outdoors, you can avoid heat exhaustion by taking these steps:

Dress for the weather. Wear lightweight, light-colored, loose-fitting clothing, and a wide-brimmed hat.

Prevent sunburn. Getting burned can increase your risk for heat illness. So, in addition to wearing a hat, you should use a broad-spectrum sunscreen with an SPF of at least 15 and reapply it every 2 hours or after swimming or sweating a lot. Wear sunglasses as well.

Time your activities. Do your outdoor exercise, yard work, or other activities early in the morning or in the evening to avoid the hottest parts of the day.

Drink extra fluids. To prevent dehydration, drink plenty of water or other nonalcoholic, caffeine-free drinks. Because salt depletion can play a role in heat-related illness, consider an electrolyte-rich sports drink when it's very hot and humid. 

A general recommendation for those doing moderate- to high-intensity exercise is to drink 17 to 20 ounces of fluid 2-3 hours before exercise. Consider adding another 8 ounces of water or a sports drink right before exercise. During exercise, you should consume another 7-10 ounces of water every 20 minutes, even if you don't feel thirsty. Also, drink another 8 ounces within a half hour after exercise. Take additional precautions when exercising or working outdoors.

If you have epilepsy or severe heart, kidney, or liver disease, are on a fluid-restricted diet, or have a problem with fluid retention, check with your doctor before increasing liquid intake. Get advice on what kinds of fluid are best for you.

Get used to the heat. If you just arrived at a hot location or you're experiencing the first heat wave of the year in your neck of the woods, take some time to get used to the change. Limit your time working or exercising in the heat until your body adapts. If you work outside in the heat or a hot indoor environment -- such as a bakery or steel mill --  your employer should allow you to take these steps for your safety, according to U.S. government recommendations. Some states require such steps, along with water breaks and other heat illness prevention strategies.

Know your risks. If you have a medical condition or take medicine that increases your risks, you might have to take it easier than others around you. If you take part in organized sports that involve strenuous activity, make sure medical help is on hand for anyone who overheats.

Preventing hot car tragedies

Never stay or leave a child or pet in a hot car, even for a few minutes, even if the windows are cracked, or even if you parked in the shade. Children can quickly progress from overheating to heat exhaustion, then to heat stroke and even death.

Lowering risks inside

If your home doesn't have air conditioning, open windows and run fans to increase ventilation. If possible, get to cool indoor spaces, such as libraries or shopping malls, during the day. Some cities have cooling centers open during heat waves.

When you get too hot, symptoms of heat exhaustion, such as weakness, nausea, headache, and heavy sweating, are a warning — it's time to get into a cool place, rest, and drink fluids. If left untreated, heat exhaustion can quickly turn into a heat stroke, a life-threatening condition that can damage your brain and other organs. You can prevent heat illness by taking it easier on very hot and humid days, drinking plenty of fluids, and limiting your time in the heat.