It might seem counterintuitive, but there are a number of U.S. national parks that have high levels of pollution. CBS News travel editor Peter Greenberg has more.

(Photo by Mario Tama/Getty Images)

<p>It might seem counterintuitive, but there are a number of U.S. national parks that have high levels of pollution. CBS News travel editor Peter Greenberg has more.</p>
<p>(Photo by Mario Tama/Getty Images)</p>

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