More Americans Breathing Cleaner Air

But some cities have gotten worse

While more Americans are breathing cleaner air than in the past, many are living in cities that have worse air quality than a decade ago, Health Day News reported.

A new report from the American Lung Association (ALA) measured ozone and particle levels in the air in nearly 1,000 cities and counties in the United States between 2009 and 2011. Among the 25 most polluted cities in last year's report, about half had improved, and many displayed the cleanest air levels since the ALA began researching air quality trends in 2000.

"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."