You already know that air pollution is bad for your lungs, heart and sinuses. But as new research reveals, it may also be responsible for your expanding waistline.

In a study published in the Journal of the Federation of American Societies for Experimental Biology, scientists reported that lab rats who breathed Beijing’s highly polluted air for three to eight weeks gained significantly more weight than a control group, and developed a slew of other obesity-related health problems as well.

Beijing, which is surrounded by mountains that help to trap emissions from its factories, automobiles and the coal-fired boilers that many homes still have, is notorious for its dirty air. Between 2008 and 2015, the Chinese capital averaged a daily Air Quality Index of 100, four times above the healthy limit.  In December, the pollution grew so severe that officials called Beijing’s first-ever 72-hour pollution alert, in which vehicle traffic was restricted.

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Previous research already points to the chronic smog as a major public health problem. A 2014 study by Harvard researchers, for example, found that the high exposure to particulates significantly reduced lung function in the city’s 22 million residents.

But the new study, which was led by Duke University professor of global and environmental health Junfeng “Jim” Zhang, indicates that long-term exposure to polluted air may also cause metabolic and inflammatory changes that lead to obesity. At 8 weeks old, female and male rats who lived in a chamber filled with polluted Beijing air were 10 percent and 18 percent heavier, respectively, than those exposed to clean air.

The negative physical changes were more pronounced at 8 weeks than at 3 weeks, which suggests that longer-term exposure is necessary to lead to weight problems.

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“If translated and verified in humans, these findings will support the urgent need to reduce air pollution, given the growing burden of obesity in today’s highly polluted world,” Zhang said in a press release.

2013 study concluded that obesity is a rising problem for young adults in China, affecting about 11 percent of the population between ages 20 and 39. Even so, the Chinese aren’t gaining as much weight as Americans. More than a third of U.S. adults are dangerously overweight, according to the U.S. Centers for Disease Control and Prevention.