During the Washington-area's latest air pollution alert, Bethesda and Suitland were among the localities to suffer from the worst smog.
The Bethesda air-monitoring station at the National Institutes of Health recorded the peak air pollution index reading of 110, in the "very unhealthy" range for children, the elderly and those with respiratory and heart ailments.
Alexandria had the area's higest air-pollution readings on three of the six days of the smog alert and had the second highest readings on the other three days, but both Bethesda and Suitland were close behind. All three had levels of pollution two and sometimes three or four times higher than the other monitoring stations of the Metropolitan Washington Council of Governments.
Maryland's close-in suburbs also were hotter than less-densely populated areas, which aggravated the air pollution. Washington's hot spot was the low-lying "urban heat island" along the Potomac River which includes Alexandria and National Airport - where Washington's official daily temperatures are recorded.
Because of its location, National Airport temperatures sometimes can be as much as 20 degrees warmer than temperatures at Dulles International Airport, says Weather Service meteorologist Joseph Moyer. The difference is so great that spring regularly comes a week or two earlier around downtown Washington than it does 25 miles away at Dulles, and in the winter when Dulles gets snow, Washington often gets rain.
But beside being heat collectors, the close-in suburbs also appear to be the recipients of much of the slow-forming summer smog generated almost every morning rush hour by heavy motor vehicle traffic.
COG's chief smog expert, Dennis Bates, says Bethesda and Alexandria "tend to be on the high side most of the time," but he adds that the production the movement of smog is a complicated process and it's not clear why their air pollution readings should be so high.
Smog is caused primarily by auto and other motor vehicle exhaust fumes chemically changed by sunlight into ozone. 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. Nantucket Island gets New York City smog, late in the day, and a recent study in St. Louis found the highest pollution readings were in a wheat field 150 miles east of the city, wafted there on the prevailing westerly winds.
But the Bethesda and Alexandria monitoring sites are not downwind of any particular smog producing areas, although downtown Alexandria has the Beltway, I-95, Route 1 and heavy commuter traffic to the west of it.
"It may be a question of location. The Bethesda and Alexandria sites may be better located than some of the other stations," says Environmental Protection Agency smog specialist Joseph Paisie. "It doesn't mean there aren't places with higher pollution readings around Washington," although in general pollution blankets an area fairly evenly, Paise says.
The Silver Spring air-monitoring station, just off Sligo Creek Parkway near the Beltway, and some stations near heavily-traveled roads, actually may produce artificially low readings because of what is called the "scavaging or supression effect," says Paisie. "Within 50-100 feet of a heavily-traveled road you don't find as much ozone" because photochemical oxidants like ozone are volatile, constantly decomposing and reacting with freshly-released exhaust fumes along roads.
The Washington area's highest single air-quality-index reading of 180, however, was recorded at Suitland on Aug. 1, 1975. Both the Suitland monitoring station, at the U.S. Census Bureau, and the Bethesda station regularly record higher air pollution readings than the COG stations in Silver Spring, in Cheverly at Route 50, and at the District's downtown station at New Jersey Avenue and E Street NW.
While readings from 100-250 are considered very unhealthy for some people it is only air pollution above the 250 level that is 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.