DENVER (AP) – A new study says visitors may be steering clear of some U.S. national parks or cutting their visits short because of air pollution. Researchers at Iowa State and Cornell universities looked at more than two decades of data on ozone pollution at 33 parks – from Yellowstone to the Grand Canyon. They say visitor numbers dropped almost 2 percent when ozone levels went up even slightly and at least 8 percent in some months with bad air quality. Study co-author Ivan Rudik says pollution warnings may be causing the visitation drop. Ozone, the main ingredient in smog, is formed by small particles of pollution and can cause respiratory problems. Levels have declined nationwide since regulators cracked down in the 1990s. Yet the researchers say many national parks still have pollution levels comparable to New York or Los Angeles. The study appeared Wednesday in the journal Science Advances.