text: R-22 is often used as an alternative to the highly ozone-depleting CFC-11 and CFC-12, because of its relatively low ozone depletion potential of 0.055, among the lowest for chlorine-containing haloalkanes.  However, even this lower ozone depletion potential is no longer considered acceptable.
  As an additional environmental concern, R-22 is a powerful greenhouse gas with a GWP equal to 1810 (which indicates 1810 times as powerful as carbon dioxide).  Hydrofluorocarbons (HFCs) are often substituted for R-22 because of their lower ozone depletion potential, but these refrigerants often have a higher GWP. R-410A, for example, is often substituted, but has a GWP of 2088. Another substitute is R-404A with a GWP of 3900. Other substitute refrigerants are available with low GWP. Ammonia (R-717), with a GWP of <1, remains a popular substitute on fishing vessels and large industrial applications. Ammonia's toxicity in high concentrations limit its application in small-scale refrigeration applications.
  Propane (R-290) is another example, and has a GWP of 3. Propane was the de facto refrigerant in systems smaller than industrial scale before the introduction of CFCs. The reputation of propane refrigerators as a fire hazard kept delivered ice and the ice box the overwhelming consumer choice despite its inconvenience and higher cost until safe CFC systems overcame the negative perceptions of refrigerators. Illegal to use as a refrigerant in the US for decades, propane is now permitted for use in limited mass suitable for small refrigerators. It is not lawful to use in air conditioners or larger refrigerators because of its flammability and potential for explosion.