Washington's summer has been cooler than normal this year, but the hottest--and most dangerous--days are on their way.

Most people are at least uncomfortable when it's hot, but 90-degrees-plus weather--combined with the area's high humidity--poses special dangers for the elderly. Because of medication the elderly may be taking, physical problems they may have or because their bodies are less able to cope with extreme temperatures, elderly persons need to take extra precautions in hot weather.

Until this week, that has not been so necessary, since there had been only four days above 90 degrees before yesterday compared with 10 in 1981. But that may be changing.

Yesterday's high was 91 degrees at 5 p.m. Today's high is expected to be between 90 and 95 degrees before some afternoon thundershowers. The weekend is expected to be somewhat cooler, but weather forecasters say that the temperature will be near 90 or above early next week again. The area's hottest weather generally occurs in mid to late July.

Temperatures that reach the mid 90s are the beginning of the danger level for older persons, according to Dr. Milo D. Leavitt, special assistant to the director of the National Institute on Aging. Persons with hypertension or diabetes will feel the effects of the heat at even lower temperatures, Leavitt said.

Older persons, Leavitt said, are more susceptible to hyperthermia, defined as temperature of 106 degrees or higher resulting from a breakdown of the body's thermal regulatory capacity. Leavitt said it is a particular problem for elderly persons. The incidence of hyperthermia increases as the weather gets hotter. The vulnerability of older persons to hyperthermia and the severity of the problem also increases as the weather stays hot for longer periods of time, taxing the body's ability to cool itself.

Often, says the National Institute on Aging, the best precaution in hot weather is to remain indoors in a cool place. Persons with air conditioning should use it. Those who don't have it should consider going to an air-conditioned library, movie theater or store during the hottest hours of the day. A cool bath or shower also can help offset the effects of high temperature.

Leavitt and the aging institute offer these suggestions for hot weather:

* Stay out of direct sunlight and avoid strenuous activity.

* Wear light, comfortable clothing, cotton rather than polyester or other synthetics since cotton absorbs perspiration and synthetics do not, making it more difficult for the body to cool itself. Wear a hat outdoors.

* Drink lots of liquids--water, fruit and vegetable juices. Avoid alcoholic beverages, including beer. According to Leavitt, beer and other alcoholic drinks "stimulate the production of urine and, ultimately, the loss of body fluids."

* Don't increase salt intake without consulting a physician.

* Persons living alone should have a relative or neighbor check in on them. This point is especially important, Leavitt said, since persons who are affected by the heat "don't have the presence of mind to call anybody."

* Take the heat seriously and don't ignore signs such as nausea, dizziness, fatigue and fever. If these symptoms occur, seek medical attention.

Weakness, heavy sweating, lightheadedness and nausea may indicate heat exhaustion, which results from the loss of body fluids and salt and is treated by resting away from heat, drinking cool fluids to restore body liquids and lowering body temperature by applying wet towels or taking a sponge bath.

Heat stroke, which is a complete failure of the body's cooling mechanisms, has the same symptoms as heat exhaustion except that the sweating stops and is replaced by extremely high body temperature--104 degrees or higher--rapid pulse and flushed skin. Heat stroke, which can result if heat exhaustion is not checked, is a medical emergency that requires immediate attention and treatment by a physician.

Once a person has suffered from heat exhaustion or heat stroke, Leavitt said, he or she should take that as a warning of predisposition to hyperthermia. "They should be very, very careful," he said.