LOS ANGELES — Six in 10 Americans – about 175 million people – are living in places where air pollution often reaches dangerous levels, despite progress in reducing particle pollution, the American Lung Association said in a report released Wednesday.

The Los Angeles area had the nation’s worst ozone pollution.

The report examined fine particulate matter over 24-hour periods and as a year-round average. Bakersfield, Calif., had the worst short-term particle pollution, and the Phoenix-Mesa-Scottsdale area of Arizona had the worst year-round particle pollution.

The U.S. cities with the cleanest air were Fargo, N.D., Wahpeton, N.D., and Lincoln, Neb.

Maine was found to be less healthy in the ozone pollution category, but more healthy in the particle pollution category.

The complete report can be found at www.stateoftheair.org.

The report is accurate but doesn’t show how far California has come, said Dimitri Stanich, a spokesman for the California Air Resources Board.

“More than 45 percent of the days in the 1990 ozone season were considered very unhealthy (in the South Coast area). Today, 45 percent of the days are clean, more than double the number of clean days during 1990.”

In Arizona, Benjamin H. Grumbles, the state’s environmental quality director, issued a statement objecting to the methodology of the report highlighting the Phoenix area’s levels of particulates such as dust.

“This finding came about because of one lonely air quality monitor near the cowtown area of western Pinal County, nearly 40 miles and across the mountains from downtown Phoenix,” he said. He also called the report, based on 2006-08 figures, outdated, saying pollution levels have improved since then.

He said the state recognizes that the Phoenix area has significant air pollution problems, and “we’re making some progress on dust and ozone in the Phoenix area, but not enough and not as quickly as we’d like.”

The Lung Association credited cleaner diesel engines and controls on coal-fired power plants for decreasing pollution such as soot and dust.

However, 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.