Heat exhaustion is a condition whose symptoms may include heavy sweating and a rapid pulse, a result of your body overheating. It's one of three heat-related syndromes, with heat cramps being the mildest and heatstroke being the most severe.

Causes of heat exhaustion include exposure to high temperatures, particularly when combined with high humidity, and strenuous physical activity. Without prompt treatment, heat exhaustion can progress to heatstroke, a life-threatening condition. Fortunately, heat exhaustion is preventable.

Signs and symptoms of heat exhaustion may come on suddenly and resemble those of being in shock. Your skin may feel hot and moist and appear flushed. Other possible heat exhaustion symptoms include:

• Heavy sweating
• Faintness
• Weak, rapid pulse
• Low blood pressure
• Nausea
• Low-grade fever
• Headache
• Dark urine

Your body's heat combined with environmental heat results in what's called your core temperature — your body's internal temperature. Your body needs to regulate the heat gain (and in cold weather, heat loss) from the environment to maintain a core temperature that's normal, approximately 98.6 F (37 C).

Impaired cooling mechanism
In hot weather, your body cools itself mainly by sweating. The evaporation of your sweat regulates your body temperature. However, when you exercise strenuously or otherwise overexert in hot, humid weather, your body is less able to cool itself efficiently.

As a result, your body may develop heat cramps, the mildest form of heat-related illness. Signs and symptoms of heat cramps usually include heavy sweating, fatigue, thirst and muscle cramps. Prompt treatment usually prevents heat cramps from progressing to heat exhaustion.

You usually can treat heat cramps by drinking fluids containing electrolytes (such as Gatorade or other sports drinks), getting into cooler temperatures, such as an air-conditioned or shaded place, and resting.

Other causes
Besides hot weather and strenuous activity, other causes of heat exhaustion include:

• Dehydration, which impedes your body's ability to sweat and maintain a normal temperature
• Alcohol use, which can affect your body's ability to regulate your temperature
• Overdressing, particularly in clothes that don't allow sweat to evaporate easily

Anyone can develop heat exhaustion, but certain factors increase your sensitivity to heat. They include:

• Young or old age. Infants and children younger than 4 and adults older than 65 are at higher risk of heat exhaustion. The body's ability to regulate its temperature isn't fully developed in the young and may be inhibited by illness, medications or other factors in older adults. Both age groups tend to have difficulty remaining hydrated, which also increases risk.
• Certain medications. Drugs that affect your body's ability to stay hydrated and respond appropriately to heat include those that narrow your blood vessels (vasoconstrictors, such as ergotamine), regulate your blood pressure by blocking adrenaline (beta blockers, such as atenolol), rid your body of sodium and water (diuretics, such as hydrochlorothiazide), alleviate allergy symptoms (antihistamines), calm you (tranquilizers, such as phenothiazines, butyrophenones and thioxanthenes), or reduce psychiatric symptoms such as delusions (neuroleptics, such as olanzapine).
• Obesity. Carrying excess weight can affect your body's ability to regulate its temperature and cause your body to retain more heat.

If you think you're experiencing heat exhaustion, then first try to cool off by:

• Moving to a cooler place
• Drinking cool water or sports drinks
• Resting with your legs elevated above heart level

Contact your doctor if your signs or symptoms worsen or if they don't improve within 30 minutes. Seek immediate medical attention if your body temperature reaches 104 F (40 C) or higher.

Untreated, heat exhaustion can progress to heatstroke, a life-threatening condition that occurs when your body temperature reaches 104 F (40 C) or higher. Heatstroke requires immediate medical attention to prevent permanent damage to your brain and other vital organs or death.

In most cases, you can treat heat exhaustion yourself by doing the following:

• Rest in a cool place. Getting into an air-conditioned building is best, but at the least, find a shady spot. Rest on your back with your legs elevated higher than your heart level.
• Drink cool fluids. Stick to water or sports drinks. Don't drink any beverages that have alcohol or caffeine, either of which can contribute to fluid loss.
• Apply cool water to your skin. If possible, take a cool shower or soak in a cool bath. Don't use alcohol on your skin.
• Loosen clothing. Remove any unnecessary clothing and make sure your clothes aren't binding.

If you don't begin to feel better within a half-hour using these treatment measures, seek prompt medical attention. You may be given intravenous (IV) fluids to help you rehydrate. Immersion in cold water, misting your skin, and placing you in front of fans or using cold or ice packs and cooling blankets are some of the techniques that may be used to bring down your body temperature.

You can take a number of precautions to prevent heat exhaustion and other heat-related illnesses. When temperatures climb, remember to:

• Wear loosefitting, lightweight, light-colored clothing. Excess, dark or tight clothing holds in heat and doesn't let your body cool properly because it inhibits sweat evaporation.
• Avoid sunburn. If you're going to be outdoors, wear a lightweight, wide-brimmed hat or use an umbrella to protect yourself from the sun, and apply sunscreen to any exposed skin. Having a sunburn reduces your body's ability to rid itself of heat.
• Seek a cooler place. Being in an air-conditioned building, even for just a few hours, is one of the best ways to prevent heat exhaustion. If your home doesn't have an air conditioner, consider spending time at a library or shopping mall. At the least, find a well-shaded spot. Fans alone aren't adequate to counter high heat and humidity.
• Drink plenty of fluids. Staying hydrated will help your body sweat and maintain a normal body temperature.
• Take extra precautions with certain medications. Ask your doctor or pharmacist whether the medications you take make you more susceptible to heat exhaustion and, if so, what you can do to keep your body from overheating.
• Avoid hot spots. When parked in the sun, your car can reach a temperature of more than 160 F (71 C). Let the car cool off before you drive it. Never leave children or anyone else in a parked car in hot weather for any period of time.

It's best not to exercise or engage in any strenuous activity in hot weather, but if you must, follow the same precautions and rest frequently in a cool spot. Taking breaks and replenishing your fluids during that time will help your body regulate your temperature.

If you're otherwise healthy, your body can adapt to heat after several weeks, and its ability to regulate its temperature becomes more efficient. You'll still need to take precautions, but working or exercising in heat should become more tolerable.