LOS ANGELES, May 1 (Xinhua) -- For the first time, Pittsburgh, a city outside California, replaced Los Angeles to top one of three categories in the American Lung Association's annual report card ranking cities with the worst air pollution.

    But Los Angeles still placed atop the two other categories, the association announced in the report on Thursday.

    Pittsburgh beat out Los Angeles for the most-polluted city in the category of short-term particle pollution -- or soot -- but Los Angeles still ranked worst in the nation for the most year-round particle pollution and the most ozone pollution, or smog.

    Statewide, half of California's 52 counties with air quality monitoring stations received at least one failing grade on the report card, which uses letter grades to rate air quality, while 19 received at least one "A" grade.

    Several cities showed improvement in their rankings -- notably Fresno, Merced and Modesto, which experienced significantly fewer high-ozone days -- and Salinas even made it onto the list of top 25 cleanest cities nationwide.

    "We see improvements in some areas of the state, but the levels of ozone and particle pollution in California remain dangerously high," said Gwendolyn Young, board chair of the American Lung Association in California. "Improvements do not mean the problem is solved."

    Lung association officials warned that pollution can trigger breathing problems and other health complications, and can even lead to early death.

    "Particle pollution chokes our lungs, creating what's like a clogged home air filter that you haven't cleaned in years," said Dr. Tony Gerber, a lung association volunteer and assistant professor at University of California in San Francisco.