The most commonly used gas in air conditioning systems is a refrigerant called R-410A (also known as Puron). R-410A is a hydrofluorocarbon (HFC) refrigerant that is used in place of older refrigerants, such as R-22 (Freon), which have been phased out due to their negative impact on the environment. R-410A is considered more environmentally friendly and is widely used in modern air conditioning systems, especially in residential and commercial applications.