There's a new layer of grimy soot covering that "most livable city" title Pittsburgh claimed just five days ago.

According to the American Lung Association's annual "State of the Air" report, Pittsburgh ranks as the second dirtiest metropolitan area on both short-term and annual airborne particle measurement scales. Only Los Angeles' air is dirtier.

Last year's Lung Association report ranked the Pittsburgh metropolitan area third for year-around particle pollution and fourth for short-term particle pollutants. Los Angeles was first in both categories last year, too.

The report to be released today and based on 2003-05 pollution measurements, notes that the Pittsburgh metropolitan area's increased levels of soot are part of a trend toward increased airborne particle pollution in the eastern United States and declining particle pollution west of the Mississippi River.

Higher soot levels in the East, the report said, are linked to an increase in electricity generation by heavily polluting, coal-fired power plants. In the West, soot levels are dropping, especially in California, where new engine technologies and cleaner burning gasoline and diesel fuels are reducing sooty air emissions.

"The increased particle pollution in the east is a particularly troubling trend, because exposure to particle pollution can not only take years off your life, it can threaten your life immediately," said Terri Weaver, Lung Association chair.

"Protecting Americans from potentially deadly air pollution means we need more protective federal standards so that every community in the United States can have truly clean air."

Other cities in the top five for soot include Bakersfield, Calif.; Birmingham, Ala.; and Detroit, Mich.

The Allegheny County Health Department data for 2003-05 confirms the Lung Association findings, showing a slight upward trend for fine particle soot measurements at nine of its 11 monitors. But 2006 county measurements are much better, possibly reflecting the recent installation of emissions controls by some power plants to the west of Pittsburgh, said Guillermo Cole, a Health Department spokesman.

"The installation of the sulfur dioxide controls by power plants started a couple of years ago and is continuing. Sulfur dioxides are a key component of soot so those controls could be reflected in our lower readings," Mr. Cole said.

Airborne particles or soot pose a serious health risk because they are breathed deep into the lungs. Approximately 20,000 Americans die prematurely each year because of particle exposure -- 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 U.S. Environmental Protection Agency estimates that each ton of soot boosts health costs by $100,000 annually.

The fine airborne particles also play a big 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 planet's surface. That strong influence on cloud formation can alter the global climate.

Although the soot levels are a growing concern, the report contained good news on smog levels, which decreased nationwide from their 2002 peaks. That decline was especially noticeable in the East, where Pittsburgh and several other cities dropped off the list of the top 25 cities with the most smog. Last year, Pittsburgh was number 17.

To view the full Lung Association report online, go to www.lungusa.org.