Washington area residents suffered through stagnant heat, stale air and record-setting 100-degree temperatures yesterday, but relief may be on the way in the form of afternoon thundershowers today and a cooling trend beginning Tuesday.

The Metropolitan Area Council of Governments reported that the air pollution index was at an "unhealthy" 65 yesterday afternoon, down from the "very unhealthy" 100 readings through the last three days. The pollution alert for the area was extended by the COG until noon today - the sixth consecutive day.

Forecasters at the National Weather Service, however, expect a break in the heat and stagnating pollution as an advancing cool front preceded by a line of thundershowers reaches the Washington area today.

The thundershowers, expected to begin about mid-day, and the front behind them should bring cooler temperatures for the rest of the week, forecaster Chet Henricksen said. Today's high temperature is expected to be in the low 90s, while Tuesday's high should be in the upper 80s.

"I definitely think we've seen the worst of it," Henricksen said.

Yesterday's record 100-degree reading was reached at 2:45 p.m. The oppressive heat combined with day-long high humidity created barely sufferable conditions for most persons involved in outdoor activities, including the thousands of tennis fans who viewed the Washington Star International tennis finals between Jimmy Connors and Eddie Dibbs. Courtside temperatures at the Carter Barron Amphitheater were said to have reached 105 degrees.

The record temperature also affected others throughout the city who chose to be outdoors or who could not avail themselves of the comfort of air conditioning.

"I could tell it was going to be a record heat day," said Robert Crawford, who was walking his dog, Mickey, along 16th Street NW near Rhode Island Avenue.

"When it got so hot early this morning, I just knew it. When I got outside to my car, it was like coming out into an oven," Crawford said.

Others, like the uniformed Secret Service guard who stood watch outside the Soviet Embassy on 16th Street NW, had no choice and had to endure the heat.

"It just stinks, what else can you say?" said the guard, who asked not to be identified. "I'd rather be out on the beach getting a tan."

Many area residents took to the water at beaches and swimming pools. Beaches at Ocean City, Md., were reported packed, as were area swimming pools. About 350 persons turned up at the Capital East swimming pool in the District - many more than the pool could accommodate. Acting pool manager Edwin Jones said he was forced to "change shifts of people allowed in the indoor pool" occassionally so that everybody could get wet.