Alexandria's historic Old Town section, the lowest and warmest place around the nation's capital, also suffers from some of the area's worst air pollution.

Alexandria's downtown air-monitoring station, located five blocks from the river's edge, recorded the Washington areas highest air pollution readings on three of the six days of last week's smog alert. The Alexandria station also recorded the second highest readings on the other three days.

Its air pollution levels frequently were two and three times higher than those recorded elsewhere around Washington, although Suitland and Bethesda in suburban Maryland also had almost daily air quality index readings above 50, or in the "very unhealthy" range for children, the elderly and thos with respiratory and heart ailments.

This time last summer, during 12 smoggy days in which two alerts were declared, Alexandria recorded the area's peak pollution readings on each day, all in what the Metropolitan Washington Council of Governments (COG) considers to be the "very unhealthy" range.

Alexandria has the distinction of being the hottest inhabited area around Washington, which aggravates air pollution for its 20,000 downtown residents. The city's downtown section is in the heart of what meteorologists call an "urban heat island" along the river, the lowest area, where warm air settles.

In both summer and winter, temperatures in Alexandria and at nearby National Airport - where the National Weather Service records Washington's official daily temperatures - frequently are several degrees above surrounding Washington neighborhoods and as much as 20 degrees hotter than Dulles International Airport, said Weather Service meteorologist Joseph Moyer.

It has its advantages in winter, when warmer weather means lower fuel bills. And frequently it rains there when other areas around Washington are getting snow. Spring comes earlier in Alexandria, where flowers regularly bloom as much as a week or two sooner than around Dulles Airport, Fall, too, lingers longer in the seaport town.

But beside being a heat collector, the low-lying city appears to be the recipient of much of the slow-forming summer smog generated almost every morning rush hour by Washington's heavy motor vehicle traffic.

COG's chief smog expert, Dennis Bates, says Alexandria "tends to be on the high side most of the time and was high throughout (last week's) alert." But he says the production and movement of smog is a complicated process, and it's not clear why the city's pollution readings should be so high.

Washington smog is caused primarily by auto and other motor vehicle exhaust fumes being chemically changed into ozone in sunlight. The process can take as much as three to five hours, which is why high air pollution readings usually occur in early afternoon, long after the morning rush hour has ended.

Areas most affected usually are downwind of congested traffic centers. Nantuckett Island gets New York City smog, late in the day, and a recent study in St. Louis found the highest pollution readings in the area were in a wheat field 150 miles east of the city, wafted there on the prevailing westerly winds.

The Bethesda monitoring site at the National Institutes of Health and the Alexandria monitoring site have had the lion's share of high pollution readings since regular monitoring was begun here in the early 1970s. But they are not downwind of any particular smog-producing areas, although downtown Alexandria has the Beltway, I-95, Rte. 1 and heavy commuter traffic to the west of it.

"It may be that over Alexandria, there is better mixing of lower air and upper air, where smog collects," says Environmental Protection Agency smog specialist Joseph Paisie. "There may be heat-induced air turbulence over the city pulling down the upper air." He said the proximity of the river might affect this. But he doubted that the 600 commercial jets that daily blast into and out of National Airport, most of them along the edge of the city, have any significant effect on Alexandria's pollution because their emissions generally would have been carried away by the wind.

"It also may be a question of location. The Alexandria and Bethesda sites may be better located than some of the other stations," says Paisie. "It doesn't mean there aren't places with higher pollution readings around Washington.

Alexandria and Bethesda frequently have pollution two and three times greater than the levels recorded at COG's four other full-time monitoring sites at Rte. 50 in Cheverly, the Census Bureau in Suitland, Sligo Creek Parkway in Silver Spring and in downtown Washington at Jersey Avenue and E Street NW.

The Washington area's highest single air-quality-index reading of 180, however, was recorded at Suitland on Aug. 1, 1975.

While this is considered very unhealthy for some people, it is below the 250 level officially considered "dangerous," the level at which normally healthy people begin to be affected. Although Washington is considered by EPA to be among the country's 10 cities with the worst air pollution, really high levels have been recorded only in Los Angeles and Houston, both of which have had readings above 250 in recent weeks.

Under federal air pollution laws, originally scheduled to have gone into effect by 1975, then 1977 and now by no later than 1987, Washington and other cities are to permit a maximum air quality index above 50 on only one day per year. Already this year in Washington there have been 36 days when the air quality index was above 50, 15 of them above 100. In 1977 there were 77 days above 50, 47 of them exceeding 100.