R22 was introduced in the 1950’s and was designated by the EPA as HFCF-22. It quickly became the most widely used air conditioning refrigerant for commercial and residential applications. Ubiquitous, effective, and the least expensive refrigerant or freon will soon be phased out as the EPA, and other international bodies, has found that it depletes the ozone layer. The U.S. government has placed restrictions on the manufacture and use in repair, maintenance, or manufacture of air conditioning products using R22 freon.
The Montreal Protocol was signed on 26 August 1987 and is an international treaty designed to protect the ozone layer and ultimately phase out any substances that will deplete or contribute to the destruction of the ozone layer. That phase out will be complete in January 2020, R22 refrigerant will no longer be manufactured or imported into the US.
If your air conditioning unit is over ten years old it probably is still using R22. There should be a plate on your AC unit that has the date of manufacture for that particular unit. The only other way to figure out if your AC unit is using the R22 or the replacement R-410A is to call an HVAC technician and have them test it.
The options will be limited for residential or commercial property owners going forward. The price of R22 will continue to rise and soon it will become very expensive as the only R22 to be found will be recycled from AC units that have been replaced. If you own multiple properties that are using R22 then it may be wise to start replacing those units one at a time depending on your budget. There are some units that can be modified to work with the new R-410A but that would require an inspection of the equipment by an HVAC professional. However, modification could be costly and is only delaying the inevitable replacement. If you wait and have a breakdown after 2020 your only option will be to replace the equipment. The bottom line is that if any of your air conditioning or refrigeration equipment should breakdown after 2020 and is over ten years old and is using R22 then adding freon will become inordinately expensive and replacing the units may be the best or only option.