The use of carbon dioxide (CO2) as a refrigerant dates back more than a century, but CO2 fell out of favor in the air-conditioning and refrigeration industry with the development and commercialization of chlorofluorocarbons (CFCs) in the 1930s. Shortly thereafter, hydrochlorofluorocarbons (HCFCs), such as HCFC-22, were developed. HCFC-22 eventually became the primary refrigerant for stationary air-conditioning systems. When concerns grew about the depletion of the stratospheric ozone layer in the 1970s, countries enacted agreements to phase out CFCs and HCFCs.