Ozone Levels in Many U.S. National Parks Are Similar to Those in Large Cities

Even in the awe-inspiring canyons of Yellowstone and mountains of Yosemite, the fresh air may not be so fresh: Concentrations of ozone in many U.S. national parks are similar to levels in America’s largest metropolitan areas, according to a new study in Science Advances by researchers at Iowa State University and Cornell University.

Back in 1990, the biggest U.S. metro areas had higher average ozone concentrations and more exceedance days (when the EPA deems ozone levels unhealthy for sensitive groups) than national parks. But by the early 2000s, the study notes, ozone concentrations were “nearly identical” in national parks and large metro areas. In cities, average summer ozone levels decreased by more than 13 percent from 1990 to 2014. The same metric for national parks increased from 1990 to the early 2000s, but declined to 1990 levels by 2014.