The nation's worst ozone pollution can be found in Los Angeles. Bismarck, North Dakota has the least ozone exposure.
The study by the American Lung Association says a decade of efforts to reduce air pollution in the U.S. has improved air quality in many cities in the Northeast and Midwest, but 175 million people are still exposed to dangerous levels of smog and soot.
The study credits a considerable overall drop in air pollution nationwide to a shift towards cleaner fuels and engines.
Along with recent industrial clean up efforts, the result has been a drop in both particle pollution and ozone pollution.
Cook County received an ozone grade of F, while DuPage County got a B.