A bank of steamy hot air settled over the city and its suburbs this week, cooking up a smoggy haze that led officials yesterday to issue the area's first air quality health warning since 1988.
The warning came on the hottest day of the year so far, when 97 degrees was recorded at National Airport.
The Metropolitan Washington Council of Governments, which issued the advisory at midday, canceled it late in the afternoon because smog readings at monitoring stations did not exceed federal health standards, as had been expected. The advisory had been issued because of unhealthy readings Thursday.
"It was a real close call," said David Foerter, the council's air quality specialist. "We're still going to be in the watch stage" Saturday.
The health advisory, which warns smog-sensitive people and those exercising to stay indoors, kicked off the summer smog season. Smog is produced when hydrocarbons and nitrogen oxides -- which in this area come mainly from cars, trucks and buses -- react with hot sunlight to form ozone.
In the upper level of the atmosphere, ozone protects the planet from the sun. But when ozone is produced at ground level, it worsens health problems of the young, old and asthmatic, and can produce respiratory ailments in otherwise healthy people. Some research indicates it can cause lung damage. It also harms crops and trees.
The Washington area is in the grip of a low-pressure system that is expected to continue over the weekend, breeding a familiar summer pattern of high temperatures, humid air and possible late-day thunderstorms that do little to cool things off.
Yesterday's high temperature at National Airport was 97, according to the National Weather Service, and came nowhere near cracking the 1934 record of 101. The high at Baltimore-Washington International was 98 and at Dulles was 94, neither a record.
Weather experts struggled yesterday for new ways to describe typical summer misery.
"It's fascinating because it's so normal," said Jerry Stenger, research coordinator for the Virginia state climatologist.
Throughout the region, people shed ties, plunged into swimming pools and turned up the air conditioning. Utilities reported high power use but no problems.
At the District's McKinley Pool in Northeast, attendance was slightly above normal, but more people than usual were in the water rather than lounging outside. "I see only a handful on the deck," said a pool attendant who declined to give his name.
The Bay Bridge was clogged with a four-mile backup late yesterday afternoon, and volume was slightly higher than last year, said Kathie Raynor of Reach-the-Beach, Maryland's new traffic advisory service.
Amid the heat, Foerter said the Council of Governments imposed the health advisory because monitoring stations in Mount Vernon, Takoma Park-D.C. and Greenbelt recorded ozone levels late Thursday afternoon above the federal health standard.
The ozone levels translated to a reading of 110 on a smog scale at which 100 is considered unhealthy. In the hot, dry summer of 1988, some readings reached 190, Foerter said.
High ozone levels were reported on June 14 and June 16, but health advisories were not issued because the next day's weather was not expected to produce an ozone buildup.
The region's air problems are dwarfed by those of Los Angeles or Denver, but they are serious enough that the Environmental Protection Agency has ordered the area to take new steps to scrub its smog.
The Clean Air Act under consideration in Congress is expected to require states to mandate the same vapor-trapping gasoline pump nozzles that have been installed at District service stations.
Environmental officials say the nozzles, which many consumers do not like, are the single biggest step the region can take to clean its air.