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.