Scientists have detected an unexpected rise in atmospheric levels of CFC-11, a chlorofluorocarbon (CFC) highly damaging to the ozone layer.
Banned by the Montreal Protocol in 1987, CFC-11 was seen to be declining as expected but that fall has slowed down by 50% since 2012.
Researchers say their evidence shows it's likely that new, illegal emissions of CFC-11 are coming from East Asia.
These could hamper the recovery of the ozone hole and worsen climate change.
CFC-11 is also known as trichlorofluoromethane, and is one of a number of CFCs that were initially developed as refrigerants during the 1930s.
They were also used as propellants in aerosol sprays and in solvents.
However, it took many decades for scientists to discover that when CFCs break down in the atmosphere, they release chlorine atoms that are able to rapidly destroy ozone molecules.