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Despite being banned under the Montreal Protocol, the five chlorofluorocarbons (CFCs) measured increased rapidly in the atmosphere from 2010 to 2020, reaching record-high levels in 2020, according to a study published in the journal Nature Geoscience. It said the increase was probably due to leakage while producing chemicals meant to replace CFCs, including hydrofluorocarbons (HFOs). Although they do not threaten the ozone layer's recovery at current levels, they contribute to a different threat, joining other emissions in heating the atmosphere. Ozone 'early warning' - The study analysed five CFCs with no or few current uses, beginning at the point of their total global phase-out in 2010. The study said further research was needed to know the precise source of the recent rise in CFC emissions.