This summer's record-setting heat wave helped produce some of the worst smog the Baltimore area has seen in four years.
There have been 14 days this year when ozone, the key ingredient in smog, has reached unhealthful levels in the city or its suburbs.
On those days, the Maryland Department of the Environment issued "code red" air quality alerts, urging people with breathing problems to stay indoors and asking the public to limit driving.
Last year, the Baltimore area experienced 10 "bad ozone" days, when ground-level ozone concentrations exceeded the federal safety threshold of 120 parts per billion of air. Maryland had 11 such days, the most of any state on the East Coast.
This summer, despite the increase in ozone alerts in Baltimore, New Jersey has edged Maryland out for the dubious honor of smoggiest East Coast state, state officials say.
Barring some unusually hot, stagnant weather in September, officials say, the smog season should be over.
"If we can stumble through the next two days and avoid anything untoward, it's history," said William F. Ryan, a meteorologist at the University of Maryland at College Park, who has prepared daily air quality forecasts for the state.
Ozone, which is in the earth's upper atmosphere and shields us from the sun's harmful rays, is harmful close to the ground. For most people, high ozone levels in the air may cause a slight tightness in the chest, a scratchy throat and possibly wheezing, even in healthy people if they exercise or work outdoors.
But it can cause debilitating breathing problems for asthmatics and others with lung disease. The American Lung Association estimates that as many as 600,000 people in Maryland are vulnerable.
Ozone is formed when hydrocarbons -- mainly from gasoline vapors -- mix in the air on hot, windless days with auto exhaust and nitrogen oxides from such sources as coal-fired power plants. Ozone is extremely dependent on weather, and this summer's weather, by almost any measure, was extreme.
Baltimore's ozone problem this year was fueled in large part by the midsummer heat wave, when the thermometer hit 90 or higher 25 days in a row starting July 12. That is the longest string of 90-plus days since the National Weather Service started keeping track in 1950.
Eight of the 14 "code red" ozone days occurred during the hot spell. Since then, smog has reached unhealthful levels on Aug. 14 and 21.
As bad as the air quality was this year, it could have been worse -- a point made in an advertising campaign launched last month by a business group.
The Baltimore Alliance for Clean Air Progress is airing radio ads featuring Brooks Robinson, the Orioles' Hall of Fame former third baseman, talking about how much the region's air quality has improved in the past 10 years.
The alliance represents a variety of business interests.
"The U.S. EPA [Environmental Protection Agency] says that back in 1984, Baltimore had 41 days of bad air pollution," Mr. Robinson says in the 60-second radio message. "I guess I was lucky I mainly was fielding grounders. The outfielders had to try to find fly balls in all that smog."
The Baltimore area actually tallied 41 smoggy days in 1983, not 1984, according to state figures. And last year's "bad ozone" streak stopped at 10, rather than the 14 Mr. Robinson mentions in the ad.
Statistical quibbles aside, region air quality has improved, officials say, and they credit air pollution controls imposed on factories and autos by federal and state laws.
The last time there was a similar run of hot days, in the summer of 1988, there were 36 smoggy days. And this summer's tally of 14 "bad ozone" days was topped only four years ago, when there 15 such days in the Baltimore area.
"It is important to note we are making progress in clearing the air," said Merrylin Zaw-Mon, air management director for the Department of the Environment. "But we are not there yet."