Honolulu has been ranked among the cleanest cities in the country in the American Lung Association's State of the Air 2010 report.

The study examined fine particulate matter over 24-hour periods and as a year-round average.

Honolulu was No. 3 — behind No. 1 Cheyenne, Wyo., and Santa Fe-Espanola, N.M. — in year-round particulate levels and was one of 12 cities to receive an "A" rating for ozone pollution.

Bakersfield, Calif., had the nation's worst short-term particulate levels; Phoenix-Mesa-Scottsdale, Ariz., was worst in year-round particulate levels; and Los Angeles-Long Beach-Riverside, Calif., was worst in ozone pollution.

The Lung Association's report concludes that a decade of cleanup measures to reductions in emissions from coal-fired powered plants and the transition to cleaner diesel fuels and engines have paid off in cutting levels of particle and ozone pollution, especially in eastern and midwestern cities, including Atlanta, Cincinnati, Cleveland, New York City, Pittsburgh, St. Louis, and Washington, D.C.-Baltimore, Md.

But despite the progress, the report says that more than half the population of the United States still suffers pollution levels that are often dangerous to breathe.

The report also finds that some cities, mostly in California, had air that was more polluted than in the previous report.

The report estimates that nearly 30 million people live in areas with chronic levels of pollution so that even when levels are relatively low, people can be exposed to particles that will increase the risk of asthma, lung damage and premature death.

About 24 million people live in 18 counties with unhealthy levels of ozone, short-term particle pollution and year-round particle pollution, the report said, adding that new research shows the risk of health problems from pollution may be worse than once thought, especially for infants and children.

Freeways remain high-risk areas for everyone, the study said, increasing the risk of heart attack, allergies, premature births and infant deaths.

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The Associated Press contributed to this report

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On the Net:

The American Lung Association's State of the Air 2010 report: http://www.lungusa.org/about-us/our-impact/top-stories/americas-cities-show-success-fighting-for-air.html