Summer heat can be deadly unless people take precautions to protect themselves, health authorities warned yesterday as Washington temperatures soared into the 90 degree-plus range for the fourth consecutive day.

The most vulnerable are the very young and the elderly, those with diabetes or heart trouble, and anyone taking drugs or tranquilizers that interfere with perspiring. Others at risk are people who run long distances in hot weather.

"Runners should drink a pint of water 15 minutes before running any time the temperature is over 80 degrees," said Dr. Sidney M. Wolfe, director of the Public Citizen Health Research Group, a nonprofit consumer advocacy group. "And if the person runs more than one or two miles, he should stop and drink water along the way."

Wolfe, who once did heat research at the National Institutes of Health, also recommended that anyone who doesn't have access to air conditioning should drink a gallon of liquid a day when the temperature tops 95. The very large, the overweight and those exercising in the heat need even more than a gallon, he said.

Before increasing fluid intake, however, anyone with epilepsy; heart, kidney or liver disease; or fluid retention problems should consult their physician, according to U.S. Surgeon General C. Everett Koop.

Other precautions suggested by health authorities include:

Avoid alcoholic beverages, direct sunlight and unnecessary physical exertion.

Wear lightweight, loose-fitting clothing and, if you must go into the sun, wear a hat.

Stay as cool as possible. If you don't have air-conditioning at home, go to a theater, a library or a shopping mall. Another way to cool off is to take frequent showers or place icebags or wet towels on the body.

The Surgeon General advises against using salt tablets without consulting a doctor, "because salt can compound existing medical problems such as high blood pressure."

Early symptoms of heat exposure--feeling hot, uncomfortable and listless--are mild and usually pose no threat unless they persist for an unreasonable period, according to a special report published by the federal Office of Consumer Affairs. But medical attention is important, the report said, if a person experiences dizziness, rapid heartbeat, diarrhea, nausea, cramps, throbbing headache, dry skin (no sweating), chest pain, great weakness, mental changes, breathing problems or vomiting.

"These symptoms can also signal other major problems, such as heart failure," the report said. So anyone experiencing them should "call a doctor immediately."

The most serious condition triggered by hot weather is heat stroke, which the Surgeon General describes as a "failure of all body cooling mechanisms" and a medical emergency that demands immediate attention and treatment by a doctor. Symptoms include faintness, dizziness, staggering, headache, nausea, loss of consciousness, body temperatures of 104 degrees or higher, strong rapid pulse and flushed skin. In severe cases, blood pressure drops as circulation fails.

Studies by the Centers for Disease Control show that heat stroke is primarily an urban disease that mostly affects the old, the poor and minorities. "Heat stroke rates in persons 65 or older were 12 to 13 times the rates in the remainder of the population," the CDC said.

Poor people and blacks also have high rates of heat stroke, in part because they do not have air conditioners or fans and often resist or delay treatment of disease, the report said. Others found to have a high rate of heat stroke were alcoholics, people with mental illness and people who could not take care of themselves.

The District of Columbia's Medical Examiner's office has recorded one case of a heat-related death so far this summer. The victim, a 42-year-old black male who died July 15, had been working outside all day when he became ill, officials said. They said he was dead on arrival at a local hospital.

Dr. Wolfe, the health advocate, warned that Washington now is in the grips of "the same kind of heat wave we were having in 1980, when temperatures were in the 90s with high humidity over a prolonged period. During a normal year, the nation has about 200 heat-related deaths but during 1980 the number zoomed to 1,265.