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. Study co-author Ivan Rudik said air quality warnings issued by parks and other government agencies may be causing the visitation drop. That's consistent with previous research on so-called avoidance behavior in response to pollution alerts in other settings. The study sought to control for seasonal variations and daily changes in the weather.