Experts on how the body handles heat say that scorching days in Washington are no joke. They can kill you.

"Washington at 100 degrees is actually more dangerous than Texas at 110," said Dr. Michael Rolnick, director of the emergency department at Georgetown University Hospital. "The real key is our humidity's so high."

If an archvillain wanted to design an environment that would foil human beings' ability to keep cool, he would come up with Washington, D.C., in the summer. A heat wave like this one foils every mechanism we have for keeping the body's temperature normal.

So Washington-area residents -- especially the elderly, those with illnesses such as heart disease and diabetes and exercise enthusiasts -- are vulnerable in the medical hazard of hot weather: heat cramps, heat exhaustion and heat stroke. All of these ills are treatable, but doctors say prevention is vital.

Stay in a cool place. If your home is not air conditioned, go to a store, a library or a movie. Or take frequent tepid baths or showers.

Drink plenty of water -- but not water alone. Since water contains little salt or minerals, it is best to also have juice, soda or beer. Athletes should drink plenty of fluid before exercising.

Wear light-colored clothing made of cotton or another material that absorbs fluid and reflects light, Rolnick said that one of last summer's heat exhaustion victims was a runner in a nylon jogging suit. Wear a hat.

Reduce your exercise. Run or cycle more slowly, with frequent breaks, and try to do it in the morning or evening when it's cooler. The safest exercise for hot weather is swimming.

Be especially careful if you are overweight, diabetic, or have a problem such as heat disease, lung ailment or an overactive thyroid. Don't medicate yourself with salt pills -- which doctors no longer recommend -- and call your physician if you have questions about what level of activity is safe for you.

Take the heat seriously. Watch for danger signs like nausea, dizziness, mental confusion, loss of appetite or loss of the ability to sweat.Your temperature should remain normal, so if you develop a fever, seek medical treatment at once. If you are elderly and living alone, have a relative or neighbor check on you regularly.

Hot weather sickness is really a spectrum of symptoms, all caused by overtaxing the body's mechanisms of keeping cool, according to Dr. Ronald G. Crystal chief of the pulmonary branch of the National Heart, Lung and Blood Institute. He said the most important reason for the breakdown is always that the body has lost too much water.

Rolnick said the reason for water loss is that in temperatures higher than body temperature -- around 96 degrees Fahrenheit -- sweating is the only way we have to rid of excess heat. The other forms of heat loss -- radiation, conduction and convection -- become useless. Since the environment is warmer than we are, it transfers heat to us instead of vice versa.

But on a humid day, perspiring is ineffective too. The air is already so saturated with water that little more can evaporate from the skin. It's like trying to wipe up a spill with a waterlogged sponge.

So the sweat stays on the skin, we feel hot and sticky, and we perspire all the more -- losing more fluid -- as the heat control center in the brain's hypothalamus responds to the body's increasing temperature.

The first to feel the effects of water loss may be joggers, cyclists and tennis players who do not moderate their exercise when the temperature rises. They can get heat cramps: sharp pains in the limbs or abdomen caused by the muscles' reaction to inadequate fluid and minerals.

"When one of the major muscles goes into spasm, that type of cramp can be excruciatingly uncomfortable," said Dr. Samuel Fox, director of Georgetown University's preventive cardiology program.

The treatment is to stop exercising cool off and drink fluids.

Heat exhaustion, which can affect even those who stay indoors and sit still, is probably more common than heat cramps. It's symptoms range from light-headedness and a slight headache to fainting and vomiting.

Again, the cause is fluid and mineral loss. But in most cases, according to Rolnick, the patient's temperature remains normal and his skin is moist, showing he is still protected by the ability to sweat.

Mild symptoms of heat exhaustion can be relieved by taking the victim to a cool place or bathing him, and having him drink fluids. But it is a dangerous disorder, because it can quickly progress to full-blown heat stroke, in which fluid loss is so severe that the victim can no longer sweat. His body temperature can rise rapidly to more than 105 degrees.

Crystal said heat stroke is a medical emergency, very often fatal unless a patient is treated at a hospital with ice baths, intravenous fluid and medicine. It often strikes the elderly -- because their hearts and circulation are unable to deal with the physical demands of heat, and because once overcome by heat exhaustion, they may be alone and too weak to seek help.

"In heat stroke . . . the temperatures are higher and various organs are gettind fried," he said.

Crystal said one way to prevent heat stroke is to watch for warning symptoms and take your temperature. "What a fever means is your body is not getting rid of heat it's producing, and that abnormal."