Air pollution is still a "pervasive public health threat" in the U.S. — and while many of the nation's most polluted cities have cleaned up their acts, a number are still choked by ozone and particle pollution, according to a new study from the American Lung Association, NBC News reported. The five cities with the dirtiest air are all in California, where heat and drought have exacerbated a climate already "ripe" for pollution, the report found. But the worst cities — like Los Angeles, Pittsburgh, and Birmingham — are starting to make significant steps toward fresher air.