Our national parks are supposed to be places left in something like their original state. But the fact that so many people visit them, and in warm seasons of the year, means air pollution is on the rise.
A recent study found that ozone levels in national parks are on par with ozone levels in big U.S. cities.
Which raises some issues about whether visits to the parks should be capped, or if vehicle traffic, the main source of ozone, should be sharply curtailed.
David Keiser at Iowa State University, the lead author of the study, is our guest.