Emissions of the potent greenhouse gas perfluorocyclobutane (c-C4 F8 , PFC-318, octafluorocyclobutane) into the global atmosphere inferred from atmospheric measurements have been increasing sharply since the early 2000s. We find that these inferred emissions are highly correlated with the production of hydrochlorofluorocarbon-22 (HCFC-22, CHClF2 ) for feedstock (FS) uses, because almost all HCFC-22 FS is pyrolyzed to produce (poly)tetrafluoroethylene ((P)TFE) and hexafluoropropylene (HFP), a process in which c-C4 F8 is a known by-product, causing a significant fraction of global c-C4 F8 emissions. We find a global emission factor of ∼ 0.003 kg c-C4 F8 per kilogram of HCFC-22 FS pyrolyzed. Mitigation of these c-C4 F8 emissions, e.g., through process optimization, abatement, or different manufacturing processes, such as refined methods of electrochemical fluorination and waste recycling, could reduce the climate impact of this industry. While it has been shown that c-C4 F8 emissions from developing countries dominate global emissions, more atmospheric measurements and/or detailed process statistics are needed to quantify c-C4 F8 emissions at country to facility levels.