In a new study, researchers found that goods made in China have much higher carbon dioxide emissions than the same products manufactured in other countries. That’s because the factories making them rely on antiquated technologies and processes and source much of their energy from heavy carbon-emitting coal-fired power plants.

One of the worst-polluting culprits identified was petrochemical plants, which produce propylene—the main ingredient in most plastic products—to feed growing demand from first-world countries such as the U.S.

“To make a pound of propylene in China is 21 times more carbon-intensive than to produce it in the EU,” said Steven Davis, an energy scientist at the University of California, Irvine.

He, along with collaborators at Harvard University and the University of Maryland, looked at data from China, reviewed the life-cycle assessment of various products, and researched how Chinese industries exchanged goods and services. They then calculated how much energy the different industries use and how much carbon dioxide is generated from that energy. Their findings were published in the journal Nature Climate Change.