The study, based on 100,000 measurements over 25 years and a computer model tracking air-flow patterns, found that during the spring, ozone from Asia reaches Washington, Oregon, California and other states west of the Rocky Mountains.

For the first time, the study links ozone in the air above the United States with Asian pollution, said Dan Jaffe, a professor of atmospheric and environmental chemistry at the University of Washington-Bothell and one of the study's authors.

"It is possible that emissions from emerging economies like China, with relatively limited emissions controls, are outpacing reductions in the developing countries," the report concludes. It says that the Asian emissions may "hinder the USA's compliance with its own ozone air-quality standard."

Previous studies have detected such pollutants from Asia as mercury, soot and PCBs reaching the United States. A National Academy of Sciences study last year pointed to increasing unease among regulators about a growing problem.