A new study is finding that ozone concentrations in national parks are similar to those of our country's major metropolitan areas.

Researchers from Cornell and Iowa State Universities looked into data at 33 national parks, including Grand Canyon National Park. They found that on days with higher ozone levels, visitor numbers dropped.

Ivan Rudik, an assistant professor at Cornell and one of the authors of the study, said that the elderly, children and those with lung disease are at risk and are affected by higher ozone levels.

But, in Arizona, Rudik said that the Grand Canyon and Saguaro National Parks have significantly fewer days with risks than in the Valley.

"There might be two to three weeks a year where ozone might be elevated at the Grand Canyon," Rudik said, "whereas you can compare that to several months out of the year in Phoenix."

Ozone is caused by traffic and other emissions and is spread by the wind, which carries it into parks.