A couple months back, scientists from around the world revealed a new mystery: somebody in East Asia was again producing banned chlorofluorocarbons (CFCs)—the chemical that put a hole in the ozone layer—and it wasn’t clear who was responsible.

This is a big issue because the ozone layer, without which we’re more likely to get skin cancer and premature aging of the skin, is recovering thanks to the global effort to get rid of CFCs. And now we know who’s responsible for threatening that recovery.

It’s the Chinese manufacturers of plastic foam, according to an investigation by an NGO called the Environmental Investigation Agency (EIA.) The group said Monday it has evidence that 18 Chinese makers of polyurethane foam insulation, which is generally used in construction, regularly use CFC-11 in the manufacturing process.

“EIA’s calculations show that emission estimates associated with the level of use reported by these companies can explain the majority of emissions identified in the atmospheric study. In addition there is significant potential for illegal international trade in CFC-11 containing pre-formulated polyols for foam manufacturing in other countries,” the group said.

CFCs are banned under the 1987 Montreal Protocol, to which China is a party. Even developing countries are supposed to have stopped using CFC-11 (aka trichlorofluoromethane) in 2010, despite its historic utility in making appliances and foam building insulation.

The EIA said China should now clamp down on what turns out to be a pervasive practice in the country. According to the NGO, companies it spoke to “acknowledged the illegality of their actions and explained that it was used because it was cheaper and made more effective foams.”

“We didn’t know what on Earth someone would be using CFC-11 for,” Steve Montzka of the U.S. National Oceanic and Atmospheric Administration, which revealed the rise in CFC-11 levels, told the Guardian. “Well, here’s one answer and that’s a surprise.”

Montzka added that it was possible that someone else is also still producing CFC-11. “If this one issue is targeted within China, we want to be sure that will take care of the problem,” he said.