CFCs are a family of inert, nontoxic, and easily liquified chemicals used in refrigeration, air conditioning, packaging, insulation, or as solvents and aerosol propellants. At first they were thought to be harmless, due to their chemical inactivity but they are now known to be persistent in the upper atmosphere where they become active and destroy the ozone layer, they are also greenhouse gases. Until recently used extensively as aerosol propellants, solvents, refrigerants and in foam making. The signatories of the Montreal Protocol, set up under the auspices of the United Nations to control CFCs, have agreed to eliminate production of CFCs. Air conditioning and refrigeration equipment often contains CFCs or the related HCFCs.