Chinese exports are directly contributing to air pollution in the US, according to a study published yesterday in the Proceedings of the National Academy of Sciences (PNAS), a leading science journal. The country's massive export industry accounts for about 20 percent of its total pollution output, the authors write, in what is the first paper to quantify the impact of Chinese exports on US air quality.

The paper was authored by nine scientists from the US, the UK, and China, and was published this week after more than two years of research. The authors used data from 2006 to examine how air pollutants such as sulfur dioxide and black carbon are propelled by strong winds from China to the west coast of the US, as part of an effort to better understand the environmental dynamics of interlinked economies.

"The product doesn't contain the pollution, but creating it caused the pollution."

Chinese exports still account for a small percentage of overall air pollution in the US, compared to emissions from cars and refineries, though their effects vary by region. According to the study, China's export industries are responsible for between 12 and 24 percent of daily sulfate concentrations in the Western US, while outsourced manufacturing has actually improved air quality in the Eastern US. Every year, Los Angeles sees at least one extra day of elevated smog levels due to the manufacturing of products destined for Europe and the US.

The authors say their results prove that outsourcing manufacturing to China doesn't necessarily mean that the US and other countries are completely free of its environmental impacts.

"When you buy a product at Wal-Mart, it has to be manufactured somewhere," Steve Davis, a scientist at the University of California Irvine and one of the paper's co-authors, said in a statement. "The product doesn't contain the pollution, but creating it caused the pollution."

Davis and his co-authors also say their work could be used to help negotiate international emissions treaties, which have proven contentious in the past. "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," they said in a statement.