"It's late summer, and Americans are flocking to the country's national parks for some recreation and fresh air.

But a study released this week by the National Parks Conservation Association found that air in some of the country's most popular parks is not so fresh—and it's potentially hazardous. The report rated the country's 48 parks in three categories: levels of ozone (a pollutant that can irritate or damage lungs), haziness, and the impacts of climate change on the park. Here are the 12 worst contenders:"

Sequoia NP (CA), Kings Canyon NP (CA), Everglades NP (FL), Joshua Tree NP (CA), Carlsbad Caverns NP (NM), Acadia NP (ME), Yosemite NP (CA), Guadalupe Mountains NP (TX), Big Bend NP (TX), Mammoth Cave NP (KY), Great Smoky Mountains NP (TN/NC), and Saguaro NP (AZ).

Julia Lurie reports for Mother Jones July 29, 2015. [2]