The American Lung Association measured levels of ozone and soot particles in the air in almost 1,000 U.S. cities and counties between 2009 and 2011, HealthDay News reported.
The study found about half of the 25 most polluted cities had improved since last year, and many were the cleanest they had been since the association began recording such information in 2000, but air quality in the other half of the 25 most polluted cities had declined to even worst levels.
"The long-term trend is positive and headed to much cleaner air," said report author Janice Nolen, assistant vice president of national policy and advocacy for the American Lung Association. "[However], there is an uptick in some areas that are a concern and some areas where the problem remains very, very serious."
Los Angeles, with the most ozone pollution, remained near the top of the most-polluted list while Bakersfield, Calif., had the highest level of particle pollution, the report said.
Other cities with high pollution levels included Houston, Dallas, Oklahoma City, Cincinnati, New York, Washington, Philadelphia and St. Louis.
The cleanest cities, which did not have a single day of unhealthy levels of ozone or particle pollution from 2009 to 2011, were Bismarck, N.D.; Rapid City, S.D.; and the Fort Meyers and Palm Beach areas of Florida, the study found.