Pittsburgh has spent years trying to live down its once-deserved "Smoky City" image, but a new American Lung Association report saying it has overtaken Los Angeles for having the sootiest air in the nation won't help.

According to the association's annual national report card on air pollution that's being released today, the five-county Pittsburgh metropolitan area has the worst 24-hour soot levels and the second-worst annual soot level, behind Los Angeles.

It's the first time in the nine-year history of the association's report that a metropolitan area not in California was rated No. 1 for levels of the tiny airborne particles of ash, metals, diesel exhaust, chemicals and aerosols, collectively referred to as soot. Last year, Pittsburgh ranked second for soot on both the 24-hour and annual scales.

The "State of the Air: 2008" report, which used U.S. Environmental Protection Agency air pollution data for 2004, 2005 and 2006, says aggressive emissions controls in the Los Angeles area have reduced year-round particle levels by about one-third over the last seven years, while Pittsburgh earned the top spot by making only marginal improvement.

Guillermo Cole, an Allegheny County Health Department spokesman, said Pittsburgh doesn't deserve the ranking, which hangs on high soot readings in the Monongahela River Valley caused by emissions from U.S. Steel's Clairton Coke Works.

"Where the soot levels are high in the Clairton area, we're concerned. But it's a localized issue and the problem area is very small," said Mr. Cole, noting that 25,000 people live in the Liberty-Clairton area, which includes Glassport, Port Vue and Lincoln, while 1.2 million reside in Allegheny County and 2.5 million live in the Pittsburgh metropolitan area.

"The fact of the matter is that the ranking only applies accurately to the Liberty-Clairton area, and Pittsburgh, the rest of the county and the surrounding counties have much better air," Mr. Cole said. "Liberty-Clairton is a unique situation. We have a large source, the coke works, sitting in a river valley, so it's a real challenge. There's no other area of the U.S. like that."

The Liberty-Clairton area does not meet federal air quality standards for airborne particles, but neither does the surrounding metropolitan area, even though soot levels there are lower.

Mr. Cole said significant improvements to the air quality in Liberty-Clairton are expected to result from U.S. Steel's planned $1 billion upgrade of the coke works. County permits for the first new battery of coke ovens could be issued by the end of June and they could be operational by the end of 2011, with a second battery planned for 2014.

"Our projection is that the Liberty-Clairton area will come into attainment for airborne particles by 2015," Mr. Cole said, "while the rest of the region, where particulate levels are much lower, will come into attainment by 2010. We have a plan in place, and the new coke batteries and pollution controls will improve the air dramatically."

Kevin Stewart, director of environmental health for the American Lung Association of Pennsylvania, said he understands Pittsburgh's frustration with the No. 1 ranking, but even if the Liberty-Clairton area is removed from the calculations, the metropolitan area would still rank 16th worst out of 222 metropolitan areas covered in the report.

"I respect the Allegheny County Health Department, and the lung association supports what the health department and U.S. Steel are doing to clean up the place," Mr. Stewart said, "but the importance of the report is to get the word out to the general public about these serious pollution issues."

The Group Against Smog and Pollution and Clean Water Action, two environmental groups, issued a joint statement saying the report should be a call to action for improving air quality now in the Liberty-Clairton area and throughout the region.

The report says more than 10 million Pennsylvanians -- five out of every six statewide -- live in metropolitan areas that received failing grades for dirty air.

Airborne particles pose a serious health risk because they are breathed deeply into the lungs. Exposure to airborne particles causes approximately 20,000 Americans to die prematurely each year -- primarily from heart disease -- and almost 70 million live in areas that violate the federal limits. Each year, soot also causes nearly 300,000 asthma attacks and 2 million lost workdays due to respiratory ailments.

The fine airborne particles also play a major role in climate change by forming droplets in clouds that affect how much sun is able to pass through and the amount of moisture that is returned to the surface. That strong influence on cloud formation can alter the global climate.

Although soot levels are a growing concern, the report says ground-level ozone, the primary component of unhealthy smog, continued to decline from peak levels in 2002. Pittsburgh's ozone ranking dropped from 17th two years ago to 34th in this year's report, and 20 other counties in the state also reported improvement.

Tom Rathbun, a spokesman for the Pennsylvania Department of Environmental Protection, said ozone levels across the state have dropped by 10 percent in the last five years, because of cleaner-burning fuel and vehicle emissions and a more-comprehensive vehicle-inspection program. He said new controls on power-plant emissions will help the Pittsburgh region do better on soot soon.

"We're on the verge of some significant improvements," he said. "We still have air quality problems whether we can see them or not, and reports like this one by the lung association create more public awareness and throw more public support behind efforts to make changes and improvements statewide."

Following Pittsburgh on the list of metropolitan areas most polluted by short-term airborne particles are Los Angeles/Long Beach/Riverside, Fresno/Madera and Bakersfield, all in California, and Birmingham, Ala.

For annual soot levels, Pittsburgh switches places with Los Angeles, followed by Fresno/Madera, Bakersfield and Birmingham.

Los Angeles, despite its ranking at or near the top of the pollution lists, has experienced continued improvements in air quality, dropping its annual particle pollution level by about one-third over the last decade, as well as significantly reducing its levels of ground-level ozone, the primary component of smog.

"The air quality in several cities has improved, but in others, declines in pollution have stalled," said Bernadette Toomey, president and chief executive officer of the American Lung Association. "The trends tell us loud and clear that we need to do more to protect Americans from breathing air that's simply hazardous to their health."