Millions of vacationers escape to national parks each summer to take in the fresh air and scenic vistas.
But a report released Tuesday by a conservation group finds that some of the nation’s most treasured landscapes are plagued with polluted air and hazy skies -- and remain decades behind schedule in restoring visibility.
The report by the National Parks Conservation Assn. flunked four national parks in California -- Sequoia, Kings Canyon, Joshua Tree and Yosemite -- giving them F grades for having levels of ozone that are unhealthy for visitors during the busy summer season.