Refrigerant chemicals have a history of being problematic. CFCs were originally introduced as "safer", alternative refrigerants to ammonia, sulfur dioxide and methyl chloride, until it was discovered that CFCs were volatile enough to reach the stratosphere and release Cl radicals that lead to the decomposition of stratospheric ozone, particularly over Antarctica . In this manner, CFCs were a planetary boundary threat associated with severe impacts . To protect the ozone layer from poorly reversible depletion, the Montreal Protocol required CFCs used in refrigeration and air conditioning systems to be substituted with HCFCs and HFCs 79 or natural refrigerants. HCFCs have lower ozone depletion potential than CFCs, and HFCs have no ozone depletion potential; however, HFCs have a high global warming potential (GWP) .