Final answer:
R-438A and R-407C are two HFC-based refrigerant blends used to retrofit R-22 air-conditioning systems. These refrigerants are chosen for their non-ozone depleting characteristics, though they still contribute to global warming.
Step-by-step explanation:
Two HFC-based refrigerant blends that are being used to retrofit R-22 air-conditioning systems today include R-438A and R-407C. R-438A (also known by its trade name Freon™ MO99) is a retrofit option for R-22 that can be used in a wide range of applications, including residential and commercial air conditioning systems. R-407C is another common retrofit replacement for R-22, which closely matches the operating characteristics of R-22 and is non-ozone depleting.
R-22, also known as hydrochlorofluorocarbon-22 (HCFC-22), has been phased out due to its ozone-depleting potential and contribution to climate change. Therefore, HFC-based refrigerant blends, which do not contain chlorine, are increasingly used as alternatives. However, while safer for the ozone layer, HFCs still have high global warming potentials, and work continues on developing more environmentally friendly refrigerants.