According to a significant study, Chinese air pollution blowing across the Pacific Ocean is often caused by the manufacturing of goods for export to the US and Europe.
"We've outsourced our manufacturing and much of our pollution, but some of it is blowing back across the Pacific to haunt us," said Steve Davis, an earth system scientist at University of California, Irvine.
"Given the complaints about how Chinese pollution is corrupting other countries' air, this paper shows that there may be plenty of blame to go around," he added.
The study is the first to suggest how much pollution reaching the American West Coast is from the production in China of electronic and other consumer items.
Manufacturing of goods in China causes pollution in US
According to the study, Los Angeles experiences at least one extra day a year of smog that exceeds ozone limits because of nitrogen oxides and carbon monoxide emitted by Chinese factories making goods for export.
On other days, as much as a quarter of the sulfate pollution on the US West Coast is tied to Chinese exports, said the study that appeared in the Proceedings of the National Academy of Sciences.
All the contaminants tracked in the study are key ingredients in unhealthy smog and soot, it added.
"When you buy a product at Wal-Mart, it has to be manufactured somewhere. The product does not contain the pollution, but creating it caused the pollution," said Davis.
The powerful global winds known as 'westerlies' can push airborne chemicals across the ocean in days, particularly during the spring, causing dangerous spikes in contaminants.
Dust, ozone and carbon can accumulate in valleys and basins in California and other Western states.
Black carbon is a particular problem. Like other air pollutants, it's been linked to several health problems, from increased asthma to cancer, emphysema, and heart and lung disease, the study noted.
China's huge ramp-up of industrial activity in recent years, combined with poor pollution controls, has unleashed often fierce international debates.
To reduce trans-boundary transport of air pollution, said the study, we need to effectively negotiate clean-air treaties.