https://apnews.com/1d601528b4fe48dba03c69d066f44ef8

DENVER (AP) — Visitors appear to be steering clear of some U.S. national parks or cutting visits short because of pollution levels that are comparable to what's found in major cities, according to a study released Wednesday.

Researchers at Iowa State and Cornell universities looked at more than two decades of data on ozone pollution at 33 parks — from Shenandoah to Yellowstone, the Grand Canyon and Yosemite. They say visitor numbers dropped almost 2 percent when ozone levels went up even slightly and by at least 8 percent in months with three or more days of high ozone levels compared with months with fewer days of high ozone.