10. Pittsburgh/San Diego
During its industrial heyday, Pittsburgh was an environmental catastrophe, a steel-mill city where the soot was visible on residents' windowsills. Over the years, Pittsburgh has gone from a city of steel workers to one of middle managers — though, thankfully, the Steelers haven't gone away — and the air has improved. But pollution is still a major problem — an estimated 47,000 children in the greater Pittsburgh area suffer from asthma. Meanwhile, most Americans probably think of San Diego as a Southern California paradise. But heavy traffic on its highways — as well as localized pollution from the city's busy port — can turn the air brown, especially during rush hour. San Diego averages nearly 30 days of unhealthy ozone levels each year — though like Pittsburgh, its air has improved in recent years.