Smog is up 65 percent in the western U.S., according to a new study.
Researchers say it's because of Asian countries' emission levels of nitrogen oxides, which have tripled since 1990.
The findings come from pollution data collected in cities and national parks across the western U.S.
The Asian-made gases travel across the ocean. They've offset the 50 percent cut in emissions of nitrogen oxides made by the U.S. over the past 25 years.
Researchers say a global strategy is necessary in order for the U.S. to meet air quality objectives.
High levels of these nitrogen oxides can be harmful to people and animals.