• Many older air conditioners use chlorodifluoromethane as a refrigerant.window air conditioner image by Aaron Kohr from Fotolia.com
  
  Refrigerants must exhibit several properties, which greatly restricts the pool of suitable chemicals. They must have low boiling points; that is, they should be gases at room temperature and pressure. They should also possess high latent heats of vaporization, which means they should absorb significant amounts of heat as they convert from liquid to the gaseous state. Refrigerants should also exhibit low toxicity and be environmentally benign.
  
  

HFC-134A

• HFC-134A, chemical name 1,1,1,2-tetrafluoroethane, was developed as a substitute for CFC-12, also known as dichlorodifluoromethane or R-12, in automobiles and household refrigerators. The U.S. Environmental Protection Agency banned the manufacture of CFC-12 in 1995 due to its contribution to the depletion of the ozone layer. 1,1,1,2-Tetrafluoroethane exhibits zero ozone-depletion potential because it contains no chlorine.
  
  

HCFC-22

• HCFC-22, also known as chlorodifloromethane or R-22, serves as the refrigerant in numerous nonautomotive applications, particularly residential air conditioners and refrigerators. Its chlorine content, however, makes it a suspected ozone-depleting agent, and the EPA is currently phasing out its production and use. By January 1, 2020, all production and import of HCFC-22 will cease in the United States.
  
  

HFC-417A

• HFC-417A comprises a blend of other refrigerants, including HFC-134A, and serves as a direct replacement for HCFC-22 in both household and industrial cooling units because it exhibits compatibility with all existing compressor oils.
  
  

Carbon Dioxide

• Carbon dioxide's use as a refrigerant dates to the 1800s. Air-conditioning and refrigerator manufacturers never widely adopted it because it requires exceptionally high operating pressures. Modern manufacturing capabilities, however, make the use of carbon dioxide more practical, and it has been successfully implemented in numerous commercial refrigeration units in Europe.