Some of the used fluids for heat transfer are of questionable sustainability and thus raise environmental concerns.
Earlier, there were concerns raised over the global warming impact of the R404a refrigerant fluid used in heat pumps called R404a. The hydrofluorocarbons in this refrigerant were orders of magnitude more potent than CO2 as greenhouse gases. While there are alternative refrigerants available today, the search is still on for high performance refrigerants that are also environmentally sustainable.