R-22 was the most common refrigerant used in HVAC systems for decades. However, R-22 is a Chlorofluorocarbon (CFC) which has been phased out under the Montreal Protocol since it is an ozone-depleting substance. In fact, starting in 2020, it is illegal to produce R-22 in the United States. Consumers can still operate existing R-22 systems using gas which was produced before 2020 (allowing them to replace aging A/C equipment in an appropriate manner), but R-22 supplies will not last forever. A variety of replacements provide alternatives that can be used without replacing R-22 systems.