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Pittsburgh overtook Los Angeles in the category that measures short-term particle pollution or soot. Los Angeles, the country's longtime soot and smog leader, has enacted aggressive measures to tackle sources of pollution, resulting in a substantial drop in particle pollution levels, said Janice Nolen, the [American Lung Association]'s assistant vice president of national policy and advocacy.

"It's not that Pittsburgh has gotten worse; it's that Los Angeles has gotten better," Nolen said. "If the trend continues, Pittsburgh will top two lists, and L.A. will only be leading the nation in ozone."

Still, Los Angeles held its own in two other categories measuring year-round soot levels and smog. And statewide, 26 of California's 52 counties with air quality monitoring stations got failing grades for having either high ozone days or particle pollution days.

The association's "State of the Air: 2008" report, being released today, was based on air quality measurements reported to the U.S. Environmental Protection Agency by state and local agencies between 2004 and 2006. The study looks at three key pollution measures.

The eight metropolitan areas considered to be the nation's most polluted by every measure were Los Angeles, Bakersfield, Fresno, Visalia-Porterfield and Hanford-Corcoran, all in California; Washington-Baltimore; St. Louis; and Birmingham, Ala.

The cleanest cities were Fargo, N.D., and Salinas, Calif.