Pollution doesn’t pay attention to national boundaries, so there’s nothing stopping China’s smog from drifting back across the Pacific Ocean to plague Los Angeles. And that’s just what’s happening, according to a new study published in the Proceedings of the National Academy of Sciences. Researchers from China, Britain, and the U.S. estimate that emissions from Chinese factories add up to an extra day of unhealthy air quality per year in the Hollywood Hills.

Over the past 30 years, many international companies have moved manufacturing operations—and much of the pollution that accompanies them—to East Asia. But that doesn’t mean factories far away are operating cleanly. “We’ve outsourced our manufacturing and much of our pollution,” study co-author and University of California at Irvine earth-systems scientist Steve Davis said in a statement. “But some of it is blowing back across the Pacific to haunt us.”

As much as a fourth of the sulfate pollution in the western U.S. derives from Chinese factories. But as Davis points out, it’s not entirely fair to wag fingers at China alone—after all, it’s Western consumers that fuel demand for China’s polluting export industries. “This paper shows that there may be plenty of blame to go around,” he says.

The U.S. is hardly alone in facing a China smog problem. Japan and South Korea regularly experience bouts of westerly winds bringing unwelcome particulate matter from their near neighbor. Korean media has even given a nickname to toxic clouds from China: “air raids.”

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