Blistered by another day of 90 degree heat, summer-weary Washington also was choked yesterday by filthy air, prompting extension of the year's first air pollution health advisory.

The advisory was issued Thursday when authorities formally found that breathing the air here was "unhealthful." It was extended yesterday when the Council of Governments concluded that breathing this air would remain unhealthful at least through today and possibly through the entire long Labor Day holiday weekend.

The advisory cautions persons with heart or breathing problems to restrict their outdoor activities during the peak afternoon pollution period from noon to five p.m. Healthy persons were also advised against overexertion.

The advisory was triggered by a combination of "unhealthful" readings on COG's pollution meters, and the lingering and unwelcome presence in this area of a mass of stagnant air that has brought six successive days of 90-degree weather, and threatens to bring more.

Yesterday's principal pollutant -- ozone -- is produced by the action of bright sunlight on automobile and other exhausts. On a cool, breezy day, the ozone is blown away. On a warm, windless day, such as yesterday was and today is expected to be, the ozone accumulates.

The thermometer at National Airport has registered 90 or above on 46 of the 70 days since this searing summer began June 21.

When the temperature reached 90 yesterday, on its way to the day's high of 96, it marked the 20th day this month that the mercury reached that level or above.This set a record for an August.

Last month, the hottest in Washington weather history, area residents sweltered through 21 90-degree days, matching a record for that or any month.

With two days remaining in August, and no relief in sight from the fiery late summer sun, the record tied last month may fall this weekend.

"We're stuck in this warm humid air mass," National Weather Service forecaster Harold Hess said gloomily. "There's no prospect for change."

Hess was little more sanguine about the prospects for those who join the Labor Day weekend migration to nearby shore resorts.

If the temperatures will be in the 90s here, he said, they will be near 90 there. "It will certainly be considered a warm weekend by everyone going to the shore," the forecaster added.

One reason the miseries of Washington summer have not been exacerbated this year by more officially recognized air pollution episodes has been a change in the standards.

Under new, federally imposed standards, 50 percent more ozone is required in the air before officials begin issuing their advisories. Last summer, after the new standards were instituted, no advisories were issued. Previously, they would occur several times a summer.

A reading of 100 indicates air that is "unhealthful." Yesterday's high reading was 165, in Fairfax City. A reading of 200 is "very unhealthful," and would trigger a request for voluntary curbs on driving.

Meanwhile the city government said all 44 of its outdoor pools will close for the summer on Saturday or Sunday. The nine remaining open until Sunday are Langdon Park, Takoma, Banneker, Georgetown, East Potomac, Fort Dupont, Francis, Oxon Run and Randall.