One of humanity’s big achievements when it comes to managing our environment has been the phasing-out of chlorofluorocarbons, or CFCs—the chemicals that were blasting a hole in Earth’s ozone layer.

That’s great news, because the layer protects us from the Sun’s ultraviolet radiation, which can cause skin cancer, premature aging of the skin, and even genetic damage. However, there may now be a new problem. Scientists from the U.S., U.K. and the Netherlands have discovered that someone, for some unknown reason, is continuing to produce CFCs, which were banned under the 1987 Montreal Protocol.

The ozone layer protects us from ultraviolet rays. Image: NASA/Goddard Space Flight Center Scientific Visualization Studio

The specific chemical in question is CFC-11, which was once widely used in appliances and foam building insulation. Still found in some old freezers, it was supposed to be completely out of production by 2010. But an analysis of long-term atmospheric measurements suggests it’s still being made somewhere in East Asia—and that means the concentrations of CFC-11 in the atmosphere are declining more slowly than they should be.

“We’re raising a flag to the global community to say, ‘This is what’s going on, and it is taking us away from timely recovery of the ozone layer,'” said Stephen Montzka of the U.S. National Oceanic and Atmospheric Administration (NOAA), the scientist who led the new research. “We don’t know why [someone is producing the CFC-11] and if it is being made for some specific purpose, or inadvertently as a side product of some other chemical process.”

Montreal Protocol expert Durwood Zaelke told The Washington Post that someone was clearly “cheating.”

“There’s some slight possibility there’s an unintentional release, but…they make it clear there’s strong evidence this is actually being produced,” Zaelke said.

The researchers found that emissions of CFC-11 were between 2014 and 2016 up by a quarter from the average between 2002 and 2012. If the source can be identified and controlled soon, they said, the damage to the ozone layer “should be minor.”