Before air conditioning, residents and foreigners alike regarded Washington as a tropical city with swamps that bred fevers and damp, oppressive weather that would descend every summer. Consider:

The British declared the District a hardship post and paid their diplomats a bonus to endure the sweltering summers.

In the 19th century, Congress would recess in the summer because of heat and the lack of indoor plumbing in the Capitol.

On July 4, 1850, President Zachary Taylor stood bareheaded in the sun for several hours while participating in the ceremonies to celebrate the building of the Washington Monument. His death three days later was attributed to sunstroke.

"We question why our founding fathers chose the spot they did for the nation's capital," said Herbert R. Collins, a curator at the Smithsonian Institution.

Washington summers have high temperatures that can hinder a person's ability to dispose of excess heat produced naturally by the body.

The body loses heat through urination and perspiration. As the water from perspiration evaporates, the skin is cooled. Therefore, a breeze is good because it causes water to evaporate more quickly, said Dr. T. Franklin Williams, director of the National Institute on Aging.

High humidity makes it much harder for body perspiration to evaporate and achieve the necessary heat loss. This causes a rise in body temperature and dehydration if people are not drinking enough fluids to match what they have lost through perspiration.

Body reactions to heat range from mild ones, such as heat cramps or fainting, to more serious conditions, like heat exhaustion. Heat stroke is the most severe reaction and requires immediate medical attention or the person will die.

While exercising in the heat, people may experience cramps in their limbs or briefly become unconscious, a condition known as heat syncope, said Dr. Edwin M. Kilbourne, medical epidemiologist at the Centers for Disease Control (CDC) in Atlanta.

Heat exhaustion is characterized by a feeling of weakness that comes on gradually due to fluid and salt imbalances in the body. Although this condition is serious and requires medical attention, it is not life-threatening.

Heat stroke occurs when the body's temperature rises above 105 degrees, Williams said. Symptoms include headache, dizziness, nausea and loss of consciousness. Immediate treatment is necessary and usually involves an ice massage or an ice-water bath. The cooling process is carefully managed so the person does not go into shock.

"Severe heat can cook your brain and cause permanent neurological damage," Kilbourne said.

Older people are at greatest risk of suffering heat stroke. "They are less well off than average, are less likely to have air conditioning and less likely to have transportation available to get relief," said Williams.

Today, the majority of homes in the District have air conditioning to deal with the high humidity and temperatures. Out of 181,000 residential customers serviced by the Potomac Electric Power Co. (PEPCO), 79 percent have some form of electric air conditioning.

Of the 538,000 residential customers system-wide, including the District and Montgomery and Prince George's counties, PEPCO estimates that 90 percent use air conditioning.

Some researchers have pointed out that the risk of getting heat stroke is linked to economic status -- those who can afford air conditioning don't die of it, while those who can't sometimes do.

The results of a 1980 study of risk factors associated with heat stroke showed that if people who could not afford air conditioning spent some time each day in a cool environment -- going to a grocery store or a movie -- their chances of heat stroke significantly decreased, said Kilbourne, one of the team researchers.

The study examined residents of St. Louis and Kansas City who suffered heat stroke during a July 1980 heat wave. There were approximately 1,500 deaths attributed to heat that year, said Kilbourne.

At that time, the CDC researchers also looked at the effectiveness of fans in reducing body temperature. They found that although some air movement aids the evaporation of perspiration to provide a cooling effect to the body, it is counterproductive at higher temperatures.

"If the temperature is 102 degrees and your body is 98.6, you are blowing cool air away from your body and replacing it with hot air," Kilbourne said. The study concluded that above 100 degrees, fans may be harmful.

To avoid heat-related illness: During a heat wave, drink more fluids than you normally would on the basis of thirst. Juices are good because they have small amounts of salt in them. Avoid alcohol. Try to remain in a cool environment for at least part of the day. Avoid sudden exercise in the heat if you are not used to it. At the first sign of dizziness or weakness, get out of the heat, drink fluids and rest for a few hours. People taking major tranquilizers such as haldol or thorazine are more susceptible to heat. More Information

For a brochure on "Heat, Cold and Being Old," write NIA Information Center, Heat, 2209 Distribution Center, Silver Spring, Md. 20910.