What are Heat Exhaustion and Heat Stroke?
The risks of exercising in hot and humid conditions
When summer arrives, it is important to think about preventing dehydration and heat-related illnesses. According to the Centers for Disease Control (CDC), about 300 Americans die every year of heat-related illnesses. Most of these deaths could be avoided if people better understood the dangers.
What is the difference between Heat Exhaustion and Heat Stroke?
Heat exhaustion develops in people who are exposed to high temperatures and who do not drink enough fluids. People who are especially at risk for heat exhaustion are the elderly, children, people with high blood pressure, and those who work or exercise in hot environments. During heat exhaustion, the body is able to maintain a normal temperature for a while, but only at the expense of other body functions. If not treated, heat exhaustion can lead to heat stroke. In athletes, heat exhaustion results in sudden decreased performance and exhaustion.
Symptoms of heat exhaustion include
- Heavy sweating
- Muscle cramps
- Dry tongue and thirst
- Nausea or vomiting
People who experience these symptoms or those who have heart problems need to see a doctor immediately. If someone is developing heat exhaustion, you should move him or her out of the sun right away and into a cool place. Remove any extra layers of clothing and give the person water or a sports drink to replenish the body's lost fluids. If the person does not feel better after an hour, seek medical attention.
Heat stroke is a medical emergency. It is unlikely that conditioned athletes will develop heat stroke, but young children, elderly people, chronically ill patients, and even pets may easily get heat stroke. Heat stroke is a severe form of hyperthermia (very high body temperature) and requires medical treatment right away. Because the body's cooling system has been overwhelmed by heat and dehydration, the body temperature may rise to 103°F or higher. If a person does not get treatment right away, heat stroke can lead to permanent damage to the body's organs, including the brain. In some cases, people who do not get help right away can die.
Some signs and symptoms of heat stroke include
- Body temperature of 103˚F or higher
- Red, hot, dry skin (no sweating)
- Rapid heartbeat
- Severe headache
If you think that someone is experiencing heat stroke, call 911 right away. While waiting for an ambulance, try to cool the person by moving him or her out of the sun and into a cool place. Attempt to cool the person by any available method, such as covering them with a wet sheet, applying ice packs, or placing them in a cool tub of water. If possible, offer the person water to drink.
Who is at Risk?
Everyone is at risk for heat-related illnesses such as heat exhaustion or heat stroke if he or she does not stay hydrated, but there are certain people who are at greater risk:
- Those who are active, exercise, or spend a lot of time outside.
- Elderly people.
- People with chronic (long-term) illnesses who are taking certain medicines.
- People who are severely obese.
- Outdoor workers, such as construction workers and landscapers.
- Athletes who train outside in the heat.
- People with low cardiac reserve whose hearts are unable to quickly adjust to the changes the body goes through in extreme heat, such as an increased heart rate. Heart failure patients and children younger than 4 usually have low cardiac reserve. They can become dehydrated very easily, even just by sitting in a house that is too hot or walking outside in hot weather.
How does sweating work, and why is it important?
Up to 60% of the human body is water, and only about 10% of this water is in the bloodstream. A lot of the water in the bloodstream can be lost through sweating. In fact, during intense exercise in hot and humid conditions, we can sweat up to 3 liters, which is almost all of the water in the bloodstream. To replace the water that is lost from the bloodstream, the body takes water from its tissues or uses the fluids that you drink during and after exercise.
In dry weather, sweat evaporates quickly, so only a small amount of sweat is needed to lower body temperature. But in humid conditions, sweat takes longer to evaporate from the skin because the air already contains a lot of water. So, in these humid conditions, the body tries to cool itself by sweating even more. If you do not replenish the water that your body loses, you will become dehydrated.
Normally, the body cools itself by increasing blood flow to the skin. In hot weather, blood flow to the skin may increase by 3 times for healthy people. But for heart failure patients, the heart muscle may be too weak and the blood vessels in the skin may not be able to enlarge enough for heat to be released through the skin. That means that heart failure patients have greater difficulty controlling their body temperature.
How much water should I drink during exercise?
Exercising vigorously in hot and humid weather can be challenging and even dangerous. But you can safely exercise in hot weather if you take the proper precautions. One of the most important things to do is to stay hydrated and decrease your exercise intensity on very hot days. Keeping your body hydrated during exercise helps replace the water lost from sweating and prevents fatigue and poor physical performance.
Feeling thirsty is not the best indicator of your body's water needs, because thirst occurs after your body is already dehydrated. Also, your thirst is usually satisfied even before your body's water supply is fully replaced. This means that during workouts, you should drink water even if you do not feel thirsty.
The amount of water your body needs to stay hydrated depends on your body weight, body temperature, and the type of exercise you are doing. If you are dehydrated after an exercise session, it will take time to replenish the body's water. Drink several glasses of water spaced out throughout the day. You are usually well hydrated if you pass a good amount of very light yellow or clear urine a couple of times before going to bed.
For workouts of less than 1-1/2 hours, you should
- Drink about 16 ounces (500 mL) of cool or cold water 1 to 2 hours before you exercise.
- Drink about 16 ounces (500 mL) of cool water or a sports drink 15 minutes before you exercise.
- Drink about 5 ounces (150 mL) of cool water every 10 minutes during exercise.
- Have about 34 ounces (1 L) of cool water on hand per hour.
- Drink about 16 ounces (500 mL) of cool or cold water or a sports drink just after exercise.
Other Pointers and Recommendations
- Wear loose-fitting clothing that will allow air to circulate but protect you from the sun.
- Avoid direct exposure to the sun. Protect yourself from the sun by wearing a hat, sunglasses, and sunscreen.
- Do not take salt tablets. Salt tablets make dehydration worse because they drain the water from your body.
- Drink cool water rather than cold water, because the body absorbs cool water faster.
- Do not drink juices or sodas during exercise, because these drinks contain more than 10% carbohydrates (sugar) and are not absorbed well during exercise. It is okay to drink sports drinks because they usually contain less than 8% carbohydrates, but these can lead to too many calories if you drink too much of them. When it comes to workouts lasting less that 1-1/2 hours, there is no difference between drinking sports drinks and cool water to stay hydrated. Sports drinks do replenish the salt and minerals lost through sweating, although a healthy diet is usually adequate for this.
Heart Failure Patients
Patients with heart failure and certain other chronic medical conditions should not exercise outside in very hot and humid weather. This is because their hearts have less reserve capacity to transport heat from the body, and their hearts can become overworked. Many heart failure patients are also trying to balance the fluid levels in their bodies by taking diuretic medicines that rid patients' bodies of excess water and reduce their ability to sweat. Because heat and humidity also reduce the amount of water in the body, patients taking diuretics may need to have their medicines adjusted in the summer months. Heat exhaustion and heat stroke can develop in patients just by their sitting in a house or car that is not air-conditioned or by being outside in the sun for too long on very hot days.
Tips for heart failure patients, elderly people, or chronically ill patients
- Continue your usual exercise habits but move indoors to cool, air-conditioned spaces. For example, try walking laps or climbing stairs in an air-conditioned mall instead of the park.
- Do not stay in a building or house without air conditioning if the power goes out. Stay with a friend or neighbor or cool off in a public building.
- Do not ride or sit in a car that does not have air conditioning.
- Keep a good fan on hand at home.
- Dehydration lowers your body weight, so weigh yourself everyday and write it down. If your weight drops more than 2 or 3 pounds from you usual "dry" weight, call your doctor to see if your medicines need adjusting.
- Be cautious and take frequent cooling breaks if you must be outside.
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Centers for Disease Control and Prevention
Updated August 2016