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 lead to heatstroke, a life-threatening condition. Fortunately, heat exhaustion is preventable.

Signs and symptoms of heat exhaustion may develop suddenly or over time, especially with prolonged periods of exercise. Possible heat exhaustion signs and symptoms include:

• Cool, moist skin with goose bumps when in the heat
• Heavy sweating
• Faintness
• Dizziness
• Fatigue
• Weak, rapid pulse
• Low blood pressure upon standing
• Muscle cramps
• Nausea
• Headache

When to see a doctor

If you think you're experiencing heat exhaustion:

• Stop all activity and rest
• Move to a cooler place
• Drink cool water or sports drinks

Contact your doctor if your signs or symptoms worsen or if they don't improve within one hour. If you are with someone showing signs of heat exhaustion, seek immediate medical attention if he or she becomes confused or agitated, loses consciousness, or is unable to drink. You will need immediate cooling and urgent medical attention if your core body temperature (measured by a rectal thermometer) reaches 104 F (40 C) or higher.

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).

Your body's failure to cool itself

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 or sports drinks containing electrolytes (Gatorade, Powerade, others), 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 reduces 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 age 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 reduced by illness, medications or other factors in older adults.
• Certain drugs. Medications that affect your body's ability to stay hydrated and respond appropriately to heat include some used to treat high blood pressure and heart problems (beta blockers, diuretics), reduce allergy symptoms (antihistamines), calm you (tranquilizers), or reduce psychiatric symptoms such as delusions (antipsychotics). Additionally, some illegal drugs, such as cocaine and amphetamines, can increase your core temperature.
• Obesity. Carrying excess weight can affect your body's ability to regulate its temperature and cause your body to retain more heat.
• Sudden temperature changes. If you're not used to the heat, you're more susceptible to heat-related illnesses, such as heat exhaustion. Traveling to a warm climate from a cold one or living in an area that has experienced an early heat wave can put you at risk of a heat-related illness because your body hasn't had a chance to get used to the higher temperatures.
• A high heat index. The heat index is a single temperature value that considers how both the outdoor temperature and humidity make you feel. When the humidity is high, your sweat can't evaporate as easily and your body has more difficulty cooling itself, making you prone to heat exhaustion and heatstroke. When the heat index is 91 F (33 C) or higher, you should take precautions to keep cool.

Untreated, heat exhaustion can lead to heatstroke, a life-threatening condition that occurs when your core 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 that can result in death.

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

• Wear loosefitting, lightweight clothing. Wearing excess clothing or clothing that fits tightly won't allow your body to cool properly.
• Protect against sunburn. Sunburn affects your body's ability to cool itself, so protect yourself outdoors with a wide-brimmed hat and sunglasses and use a broad-spectrum sunscreen with an SPF of at least 15. Apply sunscreen generously, and reapply every two hours — or more often if you're swimming or sweating.
• Drink plenty of fluids. Staying hydrated will help your body sweat and maintain a normal body temperature.
• Take extra precautions with certain medications. Be on the lookout for heat-related problems if you take medications that can affect your body's ability to stay hydrated and dissipate heat.

• Never leave anyone in a parked car. This is a common cause of heat-related deaths in children. When parked in the sun, the temperature in your car can rise 20 degrees Fahrenheit (more than 11 C) in 10 minutes.

  It's not safe to leave a person in a parked car in warm or hot weather, even if the windows are cracked or the car is in shade. When your car is parked, keep it locked to prevent a child from getting inside.

• Take it easy during the hottest parts of the day. If you can't avoid strenuous activity in hot weather, drink fluids and rest frequently in a cool spot. Try to schedule exercise or physical labor for cooler parts of the day, such as early morning or evening.
• Get acclimated. Limit time spent working or exercising in heat until you're conditioned to it. People who are not used to hot weather are especially susceptible to heat-related illness. It can take several weeks for your body to adjust to hot weather.
• Be cautious if you're at increased risk. If you take medications or have a condition that increases your risk of heat-related problems, such as a history of previous heat illness, avoid the heat and act quickly if you notice symptoms of overheating. If you participate in a strenuous sporting event or activity in hot weather, make sure there are medical services available in case of a heat emergency.

If you need medical attention due to heat exhaustion, it may be apparent to medical personnel that you have heat exhaustion, or they may take your rectal temperature to confirm the diagnosis and rule out heatstroke. If your doctors suspect your heat exhaustion may have progressed to heatstroke, you may need additional tests, including:

• A blood test to check for low blood sodium or potassium and the content of gases in your blood
• A urine test to check the concentration and composition of your urine and to check your kidney function, which can be affected by heatstroke
• Muscle function tests to check for rhabdomyolysis — serious damage to your muscle tissue
• X-rays and other imaging to check for damage to your internal organs

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 very least, find a shady spot or sit in front of a fan. 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 alcoholic beverages, which can contribute to dehydration.
• Try cooling measures. If possible, take a cool shower, soak in a cool bath, or put towels soaked in cool water on your skin. If you're outdoors and not near shelter, soaking in a cool pond or stream can help bring your temperature down.
• Loosen clothing. Remove any unnecessary clothing and make sure your clothes are lightweight and nonbinding.

If you don't begin to feel better within one hour of using these treatment measures, seek prompt medical attention.

To cool your body to a normal temperature, your doctor may use these heatstroke treatment techniques:

• Immerse you in cold water. A bath of cold or ice water has been proved to be the most effective way of quickly lowering your core body temperature. The quicker you can receive cold water immersion, the less risk of death and organ damage.
• Use evaporation cooling techniques. If your core body temperature is not in the heatstroke range and if cold water immersion is not available, health care workers may try to lower your body temperature using an evaporation method. Cool water is misted on your body while warm air is fanned over you, causing the water to evaporate and cool your skin.
• Pack you with ice and cooling blankets. Another method is to wrap you in a special cooling blanket and apply ice packs to your groin, neck, back and armpits to lower your temperature.
• Give you medications to stop your shivering. If treatments to lower your body temperature make you shiver, your doctor may give you a muscle relaxant, such as a benzodiazepine. Shivering increases your body temperature, making treatment less effective.