Air pollution still remains one of the key environmental issues in the United States. Although it has seen incredible improvement since the 1970s, more than 4 in 10 Americans are still estimated to live in counties with poor air quality. Every year, air pollution kills more than 6 million people worldwide from heart attacks, stroke and diabetes. So just how clean is the air we breathe in the U.S.?

More more than 135 million Americans live with polluted air, placing their health and lives at risk, according to an American Lung Association report published Wednesday.

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