A tropical air mass smothering a third of the nation turned the Washington area into a polluted steam bath yesterday, sending the temperature up to a record 100 degrees and prompting a new high in consumption of electrical power, much of it for air conditioning.

Cooler air hovers only a short distance to the north, where temperatures in Wilmington, Del., usually only two degrees below those in Washington, were in the low 80s yesterday. But U.S. Weather Service forecasters said the cool front which extends from eastern Canada to northern Maryland is not strong enough to dislodge the hot air mass enveloping the Washington area.

With the temperature at 100, the humidity over 50 per cent and the air quality index at an officially "very unhealthy" 125 at 5 p.m., there was plenty of reason to feel rotten when venturing out-of-doors.

The air pollution alert of the past two days remains in effect today, with the blanket of stationary hot air getting dirtier and dirtier.

According to Weather Service forecasters there is no relief in sight from either the heat or the pollution. There is a chance it may less humid over the weekend, they said, and the temperatures could drop into the upper 80s. But they stressed the "could" and said "it'll still be hot."

And it was hot yesterday: The hottest July in Washington since 1911: The hottest day here since the summer of 1969. It was a record-breaking 105 in Richmond: 103 in Winterset, Iowa, and 103 degrees in Grand Island, Neb.

Gunther George, a 49-year-old fruit vendor working the corner of 15th and L Streets NW from 8 a.m. until 5 p.m. yesterday, said it for virtually everyone: "I'm about to burn up. This heat makes me feel lazy and evil."

The reason we feel lazy and evil, explained Dr. Estelle Ramsey, professor of physiology an biophysics at Georgetown University Medical School, is that "we are what is known as homeotherms, we maintain a fairly constant body temperature. That's why it's significant when the body temperature rises or drops.

"The price that you pay to try to maintain that fairly constant temperature is that you have to get rid of the heat you generate within the body . . . If you have a machine in a small room and there was no way to dissipate the heat the same way," she explained.

"When you place a warm body producing heat in a hot environment," she continued, "the heat exchange isn't going to be as efficient.

"An important way to dissipate heat is sweating," said Ramey. "If water reaches the surface of the body it has to evaporate. What happens when the humidity goes up is it's more difficult to evaporate the mositure if you have a layer of mositure round the body."

In other words, because the air is already laden with moisture it is impossible for it to absorb all the moisture from the body it would normally absorb, making it harder for the body to maintain equilibrium.

Cooling the body becomes even harder on such days, she said, when "you're doing something difficult, like breathing, and it compounds the problem if you have to walk across the street. The air pollution, a toxin [poison], acts on the lungs to make breathing even more difficult."

Hence persons with respiratory and circulatory problems are warned to stay inside in such weather.

Asked if the usual advice to forgo all outdoor activities, such as jogging and tennis, applies. Ramey broke into song: "Mad dogs and Englishmen go out in the midday sun.

"The answer to this is to reverse everything, not to generate any more heat than you have to and not to expose yourself to the burning sun. Get into the shade and drink a long, cool, drink . . .

"We are very finely tuned," she said. "We have some leeway but it's not nearly as much as some people think. We have a pretty narrow range of limits."

What people were clearly doing yesterday when the got home was turning on air conditioners. At one moment between 3 and 4 p.m. Pepco consumers used 3,730,000 kilowatts of electrical power, a record for power consumption at the peak of a 60-minute period.

The peak in power usage is reached around 4 p.m., explained Pepco spokesman John Grasser, because "some people have left the office and gone home. They've turned on their air conditioners, turned on the TV and started cooking dinner. At the same time the offices are still open.

"There are enough generating units to meet the demand," said both Grasser and a spokesman for Vepco, which serves much of Northern Virginia. However, added Grasser, "that's not to say there won't be an unforeseen problem at one of the generating plants."

Pepco, he said, has the potential of providing 5.013 megawatts, or 5.013,000 kilowatts of electrical power. "We're expecting a peak (consumption) this summer of 3.710 megawatts."

If consumer demand for power exceeds Pepco's capabilities, either because one of its six plants breaks down, or because of a prolonged, intense heat spell, the utility can draw power from the PJM Interconnection, a group of utilities in Pennsylvania, he said.

The peak in power usage is reached around 4 p.m., explained Pepco spokesman John Grasser, because "some people have left the office and gone home. They've turned on their air conditioners, turned on the TV and started cooking dinner. At the same time the offices are still open.

"There are enough generating units to meet the demand," said both Grasser and a spokesman for Vepco, which serves much of Northern Virginia. However, added Grasser, "that's not to say there won't be an unforeseen problem at one of the generating plants."

Pepco, he said, has the potential of providing 5.013 megawatts, or 5.013,000 watts of electrical power. "We're expecting a peak (consumption) this summer of 3.710 megawatts."