R-22a is a different name for the hydrocarbon refrigerant known as R-290. (Propane) R-290 has seen widespread usage throughout the world except in the United States. Earlier this year (2015) the EPA approved Propane to be used in NEW machines manufactured this year. I empathize new as there have been instances of companies marketing R-22a or R-290 as an alternative to the R-22 refrigerant that has been phased out.
R-22 machines made prior to 2010 are not meant to take R-22a/Propane. Yet, these companies who are selling R-22a are advertizing it as a drop in replacement, no retrofitting needed. This can be dangerous. Not only is it bad for your air conditioning unit but since you are dealing with propane the chance of explosion is extremely high. I would highly avoid purchasing R-22a to replace your current R-22 Refrigerant.