PITTSBURGH -- One week after topping the rankings of America's most livable cities, Pittsburgh finds itself on another high-profile list -- one it would probably rather not be on.

The American Lung Association's 2007 "State of the Air" report lists the Pittsburgh area as the second-most polluted metropolitan area in America, behind only the Los Angeles metropolitan area.

The rankings are based on the ozone pollution levels produced when heat and sunlight come into contact with pollutants from power plants, cars, refineries and other sources.

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Full Report: See The Air-Quality Stats For Your County

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The group also studied particle pollution levels emitted from these sources, which are made up of a mix of tiny solid and liquid particles in the air.

Such pollution can contribute to heart disease, lung cancer and asthma attacks, the association said. Those especially vulnerable to polluted air are children, senior citizens, people who work or exercise outdoors and people with asthma or chronic obstructive pulmonary disease.

Statistics from 2003 through 2005 were used in the rankings.

Nearly half of the U.S. population lives in counties that still have unhealthy levels of ozone or particle pollution, even though there appeared to be less ozone in many counties than previous years, the study found.

This isn't the first time the American Lung Association ranked Pittsburgh second in dirty air.

It happened in 2005, which spawned an award-winning report by Team 4 investigative reporter Jim Parsons.

Parsons tracked local sources of PM2.5 or small particle pollution and compared it to the communities with the highest levels of heart and respiratory disease.

Parson's story showed that communities where people were dying of heart and lung disease were the same communities that had major local sources of particle pollution.

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