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Heat Cramps, Heat Exhaustion, and Heat Stroke

Warm weather is wonderful. However, sometimes you can overdo the warmth -- especially if you are active or exercising.

Here are three problems children -- and adults --can have in hot weather. These conditions are largely brought on by heat and dehydration -- and with proper care you can prevent them.

Heat Cramps

Heat cramps are muscle contractions, usually in the gastrocnemius or hamstring muscles (the muscles at the back of the calves). These contractions are forceful and painful.

These cramps seem to be connected to heat, dehydration, and poor conditioning, rather than to lack of salt or other mineral imbalances. They usually improve with rest, drinking water, and a cool environment.

Heat Exhaustion

Although partly due to exhaustion -- and feeling like exhaustion, as the name implies -- heat exhaustion is also a result of excessive heat and dehydration. The signs of heat exhaustion include paleness, dizziness, nausea, vomiting, fainting, and a moderately increased temperature (101-102 degrees F) which, in this case, is not truly a fever, but caused by the heat. Rest and water may help in mild heat exhaustion, and ice packs and a cool environment (with a fan blowing at the child) may also help. More severely exhausted patients may need IV fluids, especially if vomiting keeps them from drinking enough.

It's also important to measure temperature properly. In particular, if you suspect problems with heat cramps, heat exhaustion, or heat stroke you should measure temperature using an oral or rectal thermometer, not an ear or forehead thermometer (which may give a falsely low temperature, especially with forehead thermometers and lots of sweat).

Heat Stroke

If your child has these symptoms, stop right here and call your doctor or EMS. Heat stroke is a medical emergency!

Heat stroke is the most severe form of heat illness. It can occur even in people who are not exercising, if the weather is hot enough. These people have warm, flushed skin, and do not sweat. Athletes who have heat stroke after vigorous exercise in hot weather, though, may still be sweating considerably. Whether exercise-related or not, though, a person with heat stroke usually has a very high temeperature (106 degrees F or higher), and may be delirious, unconscious, or having seizures. These patients need to have their temperature reduced quickly, often with ice packs, and must also be given IV fluids for rehydration; they must be taken to the hospital as quickly as possible (EMS is appropriate here), and may have to stay in the hospital for observation since many different body organs can fail in heat stroke.

Preventing Heat-Related Illnesses

You can prevent heat-related illnesses. The important thing is to stay well-hydrated, to make sure that your body can get rid of extra heat, and to be sensible about exertion in hot, humid weather.

Your sweat is your body's main system for getting rid of extra heat. When you sweat, and the water evaporates from your skin, the heat that evaporates the sweat comes mainly from your skin. As long as blood is flowing properly to your skin, extra heat from the core of your body is "pumped" to the skin and removed by sweat evaporation. If you do not sweat enough, you cannot get rid of extra heat well, and you also can't get rid of heat as well if blood is not flowing to the skin. Dehydration will make it harder for you to cool off in two ways: if you are dehydrated you won't sweat as much, and your body will try to keep blood away from the skin to keep your blood pressure at the right level in the core of your body. But, since you lose water when you sweat, you must make up that water to keep from becoming dehydrated. If the air is humid, it's harder for your sweat to evaporate -- this means that your body cannot get rid of extra heat as well when it's muggy as it can when it's relatively dry. One way to determine the effect of humidity with high temeperature is the heat index.

The best fluid to drink when you are sweating is water. Although there is a little salt in your sweat, you don't really lose that much salt with your sweat, except in special circumstances; taking salt tablets may raise your body's sodium level to hazardous levels. (Your doctor can tell you whether or not you need extra salt.) "Sport drinks" such as Gatorade® will also work, but water is usually easier to obtain.

It's also important to be sensible about how much you exert yourself in hot weather. The hotter and more humid it is, the harder it will be for you to get rid of excess heat. The clothing you wear makes a difference, too: the less clothing you have on, and the lighter that clothing is, the easier you can cool off. Football players are notoriously prone to heat illness, since football uniforms cover nearly the whole body, and since football practice usually begins in late summer when the temperature outside is highest. Therefore, football players should pay extra attention to the fluids they drink and lose: teams and coaches should limit practice and wear light clothing for practice on very hot days, and athletes must be able to drink all the water they want during practice.

Humidity, Temperature, and the Heat Index

Since high humidity reduces your body's ability to get rid of excess heat by sweating, for a given air temperature the higher the humidity the higher the apparent temperature, or heat index. For example, if the air temperature is 86 degrees Fahrenheit (or 30 degrees Celsius), but the relative humidity is 50 percent, the apparent temperature will be about 88 degrees Fahrenheit (31 degrees Celsius). That may not sound like a huge difference... but if the humidity is 90 percent, the heat index will be 105 degrees Fahrenheit (40.7 degrees Celsius). In other words, your body will have to sweat as much to get rid of extra heat at 86 degrees Fahrenheit in 90 percent humidity as it would in a dry desert at 105 degrees Fahrenheit.

The calculator below will compute the heat index for temperatures of 80 degrees Fahrenheit (26.7 degrees Celsius) and above, and for relative humidity of 40 percent or above. The formula, like the formula for the wind chill index, is based on experimental measurement in volunteer subjects. It is taken from an article published on the Web by the US National Weather Service. The calculator is written in JavaScript, so you must have JavaScript enabled in your browser to use it.

Heat Index Calculator

Relative Humidity: percent

Air Temperature: in degrees:

For more information on heat-related illnesses, a good source is Extreme Heat: A Prevention Guide to Promote Your Personal Health and Safety, at the United States Centers for Disease Control and Prevention Web site.

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PLEASE NOTE: As with all of this Web site, I try to give general answers to common questions my patients and their parents ask me in my (real) office. If you have specific questions about your child you must ask your child's regular doctor. No doctor can give completely accurate advice about a particular child without knowing and examining that child. I will be happy to try and answer general questions about children's health, but unless your child is a regular patient of mine I cannot give you specific advice.

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Copyright © 1998, 1999, 2001, 2005, 2007, 2008, 2011 Vinay N. Reddy, M.D. All rights reserved.
Written 07/11/98; major revision 01/12/07; last revised 07/19/11 counter