DEAR DOCTOR K:

My brother and I are planning a long bike trip this summer. I'm worried about the heat. What can I do to prevent heat stroke?

DEAR READER:

Normally, the human body does a good job of regulating its temperature. As you'd expect, your body tends to heat up more easily when it is surrounded by hot air. It also heats up during exercise, since hard-working muscles generate heat. When you exercise in hot weather, your body is particularly challenged to get rid of excess heat so that it can maintain a normal body temperature.

A part of our brain is constantly monitoring how hot we are. When our body temperature starts to rise above normal, the brain sends signals that cause the blood vessels near our skin to open wide, which in turn causes us to sweat. Heat leaves the blood vessels and enters the air around us. Evaporation of the sweat pulls even more heat from the body.

Spending too much time in the heat without drinking enough fluids can hamper the body's cooling system. As a result, body temperature can rise high enough to make a person sick.

The first symptoms of heat illness are called heat exhaustion. They include headache, nausea, vomiting, muscle cramps and fatigue. If you experience these symptoms, get out of the heat immediately. Seek out air-conditioning. Drink water. If possible, take a cool shower or use a hose to soak yourself. If you don't take steps to cool yourself, heat exhaustion can worsen into heat stroke.

Heat stroke is serious and potentially life-threatening. Body temperature rises to 105 degrees Fahrenheit or higher. This can cause confusion, staggering, faintness, hallucinations, agitation or unconsciousness. It can affect internal organs and lead to coma or death. If you or your brother experience symptoms of heat stroke, seek emergency help immediately.

While you wait for help, cool the person's body from the outside. Remove tight or unnecessary clothing, spray the person with water, blow cool air on the person or wrap the person loosely in wet sheets. If you have ice packs, place them at the person's neck, groin and armpits to accelerate cooling.

During your bike trip, do the following to help you stay cool and avoid heat illness:

-- Drink lots of water throughout the day.
-- Stay indoors in an air-conditioned area whenever you feel too warm.
-- Wear lightweight, light-colored clothing.
-- Avoid strenuous activity in the hottest part of the day (between 10 a.m. and 4 p.m.). If you must participate, take frequent breaks.
-- Limit the time you wear a helmet by taking it off between activities.
-- Drink less caffeine and alcohol, which can contribute to dehydration.

If you take these precautions and stay alert for early symptoms of heat illness, you should have a fun -- and safe -- bike trip.

(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)

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