<P> The inert nature of many halons, chlorofluorocarbons (CFC), and hydrochlorofluorocarbons (HCFC), particularly CFC - 11 and CFC - 12, made them preferred choices among refrigerants for many years because of their nonflammability and nontoxicity . However, their stability in the atmosphere and their corresponding global warming potential and ozone depletion potential raised concerns about their usage . This led to their replacement with HFCs and PFCs, especially HFC - 134a, which are not - ozone depleting, and have lesser global warming potentials . However, these refrigerants still have global warming potentials thousands of times greater than CO . Therefore, they are now being replaced in markets where leaks are likely, by using a third generation of refrigerants, most prominently HFO - 1234yf, which have global warming potentials much closer to that of CO . </P> <P> Some other refrigerants such as propane and ammonia are not inert, and are flammable or toxic if released . </P> <P> In order from the highest to the lowest potential of ozone depletion are: Bromochlorofluorocarbon, CFC then HCFC . </P> <P> New refrigerants were developed in the early 21st century that are safer for the environment, but their application has been held up due to concerns over toxicity and flammability . </P>

Which type of refrigerant is the most harmful