Telltale signs of heat exhaustion and how to treat it before it develops into a heat stroke

• Heat exhaustion symptoms include rapid pulse, nausea, fatigue, and lightheadedness.
• Heat exhaustion usually happens in hot, humid environments when the body is dehydrated and can't cool itself properly.
• To prevent heat exhaustion stay hydrated and try to avoid exercising, or otherwise exerting yourself, during the hottest parts of the day.
• This article was medically reviewed by Jason R. McKnight, MD, MS, a family medicine physician and clinical assistant professor at Texas A&M College of Medicine.

Heat exhaustion is a heat-related illness that happens when the body overheats and cannot cool itself effectively. It's often caused by exposure to extreme heat and humidity. Common symptoms include heavy sweating and rapid pulse.

Heat exhaustion can happen to anyone, but those most at risk include children under four and people over age 65.

Most often, it occurs when people exercise or work outdoors in hot or humid conditions. However, heat exhaustion can also happen indoors, like if you're in a car with the windows rolled up during the summer.

This article discusses the symptoms of heat exhaustion, who's most at risk, and how to treat and prevent it from happening in the future.

Heat exhaustion symptoms

Heat exhaustion is not as serious as heatstroke. However, if you're experiencing the following symptoms, you should seek treatment immediately since heat exhaustion can develop into heatstroke, a more dangerous heat-related illness.

The main symptoms of heat exhaustion are:

• Heavy sweating
• Rapid pulse
• Cool, clammy, or pale skin
• Fatigue
• Nausea
• Headache
• Lightheadedness
• Fainting
• Dizziness
• Confusion

You may also experience heat cramps, another type of heat-related illness that causes muscle spasms and can be a precursor to heat exhaustion.

Heat exhaustion causes

Heat exhaustion occurs when the body overheats. Normal body temperature is usually around 98.6 °F but it can vary between 97 °F and 99 °F. The body can easily overheat when it's dehydrated and in hot, humid environments. Exerting yourself under these conditions, like exercising, can accelerate the process.

"Your body temperature can increase about a degree for every five minutes of exercise you're doing," says Joshua Scott, MD, primary care sports medicine physician at Cedars-Sinai Kerlin-Jobe Institute.

Sweat is the main way your body cools itself down. But in humid environments, sweat doesn't evaporate as quickly, which can cause you to overheat. Add high temperatures and exercise to the mix, and you can easily overheat before you know it.

Heat exhaustion treatment

If you're experiencing symptoms of heat exhaustion, it's important to cool yourself down immediately. Try the following treatments:

• Get out of the heat. Seek shade or get indoors — and preferably in air conditioning — if you can.
• Hydrate. Sweating cools you down, but it causes you to lose fluids and electrolytes, which are minerals that help keep you hydrated. To treat heat exhaustion, you need to replenish those fluids by drinking water or a sports drink like Gatorade for electrolytes.
• Use cool water. Help to bring your body temperature down by taking a cool bath or shower, spraying yourself with a garden hose, or by putting wet, cool cloths on your body.
• Loosen your clothing. Remove any tight or unnecessary clothing if you notice symptoms of heat exhaustion. Multiple layers can trap heat and make it harder for your body to cool itself — light fabrics and loose clothing are always best when it's hot.

When to see a doctor

If heat exhaustion symptoms don't improve within an hour of onset or they get worse, you need to seek immediate medical attention.

These more severe symptoms can be a sign that heat exhaustion has progressed to heatstroke, a more dangerous heat-related illness that can be fatal.

Learn more about the difference between heat exhaustion and heat stroke.

Who's most at risk for heat exhaustion?

Heat exhaustion can happen to anyone, but those most at risk include children under four and people over age 65.

That's because the central nervous system isn't fully developed or has lost its ability to regulate temperature as well as most adults, Scott says.

In addition, those who are newer to exercise — or have taken a long break from working out — are also at risk because their body won't be as accustomed to regulating temperature during physical activity.

"We know that we become more efficient at cooling ourselves the more we exercise," Scott says. "So going out and exercising for the first time in five or six months can actually predispose you to heat illness."

Exercising in a hot, humid environment can also increase your risk of heat exhaustion. If you live somewhere like Florida or South Carolina where relative humidity levels can reach 60% to 80%, the sweat on your skin won't evaporate as much, and your body won't be able to cool itself as effectively.

Some medications, like antihistamines used to treat allergies or diuretics used to treat high blood pressure, may also affect your body's ability to sweat effectively or stay hydrated, and can predispose you to heat exhaustion, Scott says.

Talk with your doctor if you're worried that your medication may be causing heavy sweating, rapid pulse, lightheadedness, or nausea during high temperatures.

Heat exhaustion prevention

If you're going to be exercising or working outside in the heat, here are three of the best ways to protect yourself:

1. Avoid the hottest part of the day

Some jobs, like working construction, may require you to be outdoors all day, but if you can, try to avoid being outside around 3 p.m. to 4:30 p.m., when temperatures are highest.

If you're planning to go for a run, check the weather for the day, and if the high is more than 101 degrees Fahrenheit, choose a cooler time of day to be outside, like the morning or evening.

2. Take frequent breaks

When you're in hot and humid conditions, Scott recommends taking a 10-minute break for every 40 minutes of exercise.

Exercising quickly raises your body temperature. Giving yourself time to rest can help your body adjust to these changes.

3. Stay hydrated

When you're dehydrated, your body can't produce enough sweat to cool down. Make sure to take a drink about every 15 to 20 minutes while you work out, Scott says.

Overall, you should be consuming about 100 fluid ounces of liquid a day, even if you don't feel thirsty. You usually don't feel thirsty until you're already dehydrated, Scott says, so it's important to drink water throughout the day, and especially when you're working out.

"Dehydration can start to reduce muscle strength which can then reduce your body's ability to thermoregulate itself," Scott says. "So the problems start long before you get thirsty."

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