The study analyzed five CFCs with no or few current uses, starting from the point of their total global phase-out in 2010.
All five gases were at their highest abundance in 2020 since direct measurements began.
Those emissions have so far had only a minor effect on the ozone layer and a slightly larger climate impact, said co-author Luke Western of Bristol University and the Global Monitoring Laboratory.
They are equivalent to the 2020 CO2 emissions of Switzerland – about one percent of the total greenhouse gas emissions of the United States.
But if the rapid upward trend continues, their impact will only increase.
The researchers called their findings an “early warning” of a new way in which CFCs are threatening the ozone layer.
Emissions are likely due to processes that are not subject to the current ban and unrestricted uses.
The range of industrial aerosols developed to replace those banned by the Montreal Protocol is to be phased out over the next three decades under the most recent revision of the 1987 treaty.