A liquid with a low boiling point must be used to make practical use of the heat transfer that occurs when a liquid boils. Refrigerant-12 (R-12) is the refrigerant that was universally used in automotive air conditioning systems. At normal temperatures, it is a colorless, odorless gas that is slightly heavier than air. Its boiling point at atmospheric pressure is -21.7°F (minus 6°C). If liquid R-12 is spilled into the open air, it would be seen for a brief period as a rapidly boiling, clear liquid.
R-12 was nearly an ideal refrigerant. It operated at low pressure and condenses easily at the temperature ranges found in automotive air conditioning systems. It is also non-corrosive, non-toxic (except when exposed to an open flame), and nonflammable. However, due to its low boiling point and the fact that it is stored under pressure, certain safety measures must be observed when working around the air conditioning system. Unfortunately it was discovered the carbo-floro-carbons (CFC's) which were chemicals in the same group as dichlorodifluoromenthane, which you know as R-12 or Freon were depleting the ozone layer of the atmosphere.
On December 31, 1995, CFC-12 production essentially ended in the U.S. However to avoid release into the atmosphere it is still legal to use the existing stockpiles of CFC-12.
The replacement for CFC-12 has been a non-CFC refrigerant R-134a. This has been used since the 1994 model year. Some of the older R-12 systems are being changed over to R-134a but this can be a costly and complicated process on some vehicles.
There is a third category for refrigerants that substitute CFC-12, these also contain ozone depleting HCFC's such as R-22, R142b, and R-124.
There are strict governmental regulations enforced by the clean air act of 1990 section 609. These include specific regulations for the use and handling of each of the three types of refrigerants.