The 1987 Montreal Protocol established the global phasing-out of CFCs (chlorofluorocarbons), which damage the Earth's ozone layer. In 1992 the phasing-out of HCFCs (hydrochlorofluorocarbons) was also set.
For decades HCFC-22, also known as R-22, has been the main refrigerant gas used in air-con systems, and is also a greenhouse gas which contributes to global warming. As of January 1st 2010, systems using R-22 ceased production, and new models using ozone-friendly refrigerants introduced. R-410A is the most common of these, although it cannot be used in R-22 compatible systems, as it only performs under higher pressure. Instead, a drop-in substitute can be used, called R-407c, although this is sometimes reported as giving a diminished performance.
Therefore, if you have an older air-con system which uses R-22 gas, and the equipment is failing, it is usually more prudent to replace it with a new system. This is a worthwhile expense, nonetheless, as today's air conditioning is highly energy-efficient, offering significant on-going savings in running costs, far beyond the capabilities of older systems.