10. Philadelphia
10. Philadelphia
Population: 6,533,683
Cardiovascular cases: 1,660,434
Rank in most ozone-polluted cities: 16

The City of Brotherly Love was once an industrial powerhouse, with heavy industries like steel, manufacturing, textiles and machinery.

Many of the factories are gone now and Philadelphia has much more of a service economy now. The industries that remain are subject to much stricter controls, according to Linda Rebarchak, a spokeswoman for the Pennsylvania Department of Environmental Protection.

Over the past few years, for example, continuous air monitors have been installed over many of the city's smokestacks to measure emissions. The monitors have helped to alert officials and factory managers to any problems in the systems, she said.

By Les Christie @CNNMoney - Last updated April 25 2012: 12:12 PM ET
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