The American Lung Association's State of the Air report on America's most polluted cities is out. Here's  a summary (pointer via Planetizen).

Six out of ten Americans live in urban areas where air pollution can cause major health problems ... Despite America's growing "green" movement, the air in many cities became dirtier during the past 12 months. The research names Pittsburgh, Los Angeles and Bakersfield as the most polluted US cities. The report finds that air pollution hovers at unhealthy levels in almost every major city, threatening people's ability to breathe and placing lives at risk ...
Many cities, like Los Angeles, New York, Atlanta, Charlotte, Philadelphia, Washington, DC and Baltimore have made considerable improvements in their air quality over the past decade. People living in some of these cities however, are breathing even dirtier air than what was reported in the Lung Association's previous report. Only one city, Fargo, North Dakota, ranked among the cleanest in all three air pollution categories covered by the research.

Maps of the most polluted cities are here; the cleanest cities here.