Researchers have published the first study to quantify just how much pollution invading the US West Coast from China is caused by the manufacturing of consumer items headed for places like the United States and Europe.

The answer, according to the scientists, should give complainers some pause.

"We've outsourced our manufacturing and much of our pollution, but some of it is blowing back across the Pacific to haunt us," said University of California, Irvine earth system scientist Steve Davis, a co-author. "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."

Powerful winds known as the "westerlies" are able to transport airborne chemicals across the Pacific Ocean in a matter of days, accumulating in the valleys and basins throughout California and other Western states. Of particular concern to researchers is black carbon, which is not easily removed from the atmosphere by rain, and has been linked to a myriad of health issues, such as asthma and cancer.

Published by the Proceedings of the National Academy of Sciences, the analysis found that every year Los Angeles experiences at least one extra day of smog exceeding federal ozone limits due to carbon monoxide and nitrogen oxides put out by Chinese factories producing goods for export. As much as a quarter of the sulfate pollution on the US West Coast on other days is because of Chinese exports, the study revealed.

"When you buy a product at Wal-Mart," said Davis, an assistant professor, "it has to be manufactured somewhere. The product doesn't contain the pollution, but creating it caused the pollution."

According to the scientists, the results could be used to better negotiate clean-air treaties.

"International cooperation to reduce transboundary transport of air pollution must confront the question of who is responsible for emissions in one country during production of goods to support consumption in another," the researchers concluded.