The nation's 25 most smoggy cities improved air quality over the last year, but half the nation's residents still live with unhealthy levels of air pollution, according to an American Lung Association report released on Wednesday.

Weighing the pluses and minuses in U.S. air quality over the past year, the "State of the Air 2011" report concluded that the U.S. Clean Air Act, the federal law aimed at limiting pollution in the nation's skies, is working.

"The progress the nation has made cleaning up coal-fired power plants, diesel emissions and other pollution sources has drastically cut dangerous pollution from the air we breathe," Lung Association President Charles Connor said in a statement.

The most dramatic improvement has been controlling ozone, commonly known as smog. The report found all 25 cities most polluted by ozone had cleaner air than they did last year.

Still, the report found that 154.5 million people, just over half the nation's population, live in areas where the air is filled with dangerous levels of ozone and particle pollution, also known as soot.

Cities with the foulest air were broken down into three categories and the worst three in each were Los Angeles, Bakersfield and Visalia, all in California, as most ozone polluted; Bakersfield and Fresno, both in California, and Pittsburgh, Pennsylvania, as most polluted by short-term particle pollution; and Bakersfield, Los Angeles and Phoenix as most polluted by year-round particle pollution.

Cities with nation's best overall air quality were Honolulu and Santa Fe, New Mexico, the report said.