Hopes for relief from the heat and humidity of the past week were dashed yesterday after a cool, dry air mass from Canada stopped north of the metropolitan area. The result was another day of over 90-degree heat and an extension of the air pollution alert at least until near today.

The National Weather Service reported that the Canadian air mass had moved as far south as northern Delaware and Maryland before easterly winds had pushed it into Pennsylvania and New York State.

As a result, temperatures are expected to remain in the low 90s with continued high humidity thrugh the week.

The day only possibility for a slight relief comes from a weak cold front now over the western Dakotas that may reach here by Wednesday night.

"Even if it gets this far," said Charles Chilton, forecaster at the weather service, "it will be here a very short time."

"It looks like we're just stuck with summer weather for a while," he added.

The weather service reported a high of 91 degrees at 4 p.m. at National Airport yesterday, making it the seventh consecutive day that the thermometer topped the 90-degree mark.

The Metropolitan Council of Governments (COG) air-quality index showed a high reading for the region of 130 at 4 p.m., making the air "very unhealthy" for the sixth day running.

The longest period of "very unhealthy" air quality was for 13 days between Aug. 25 and Sept. 6, 1973, according to Dennis Bates, director of the health and environmental protection at COG.

Bates, referring to the successful water conservation efforts last week by residents of Montgomery and Prince George's counties, stressed the need to drive less.

"People can make a decision to form a car pool now," he said. They can make a decision to use public transit. They can make a decision to make the trips they take mor efficient."

Yet despite weather that made Washington feel like a giant steam room, the National Zoo was as crowded with visitors as on any fine spring day.

Couples and children with their parents - all wearing cool clothing - strolled through the trails or rode the zoo train.

Many of them stopped to see and photograph the two giant panda's Ling Ling and Hsing Hsing, who appeared more comfortable than their visitors as they ate bamboo in their individually air-conditioned cages.

Dr. Theodore Reed, director of the zoo, explained to a reporter later that the pandas require the air-conditioning because their natural habitat is in the wet, high mountains of Szechwan, China. He said the animals would go into heat prostration in temperatures above 73 degrees.

Reed said most of the animals were coping with the heat by doing what humans do - staying inside and limiting their activities. The polar bears have been moved to open to the public in September, where they will be able to keep cool by swiming in a large pool.

Yesterday marked the 14th day of the last 15 that the temperature here has passed 90 degrees. The official high was 91, recorded at National Airport at 4 p.m. For at least one family visiting the zoo, however, the steamy weather was just like back home.

Tue Ngo, an automobile parts salesman who fled Vietnam in 1975 and now lives in Norfolk, explained while eating hamburgers with his wife and two children that Washington weather resembled that of Saigon during the hot, dry season there from March to July.

And what about air pollution? Tue Ngo was asked. "Actullay, over there we didn't care much about the pollution, but over here people pay serious attention to it." he said.