Among the earliest refrigerants used were air, water and brine, though these act as secondary refrigerants, requiring prior cooling and circulation to achieve any significant effect. On their own, they perform poorly as refrigerants. Ammonia (R717) is one example of a naturally occurring primary refrigerant. Though flammable and highly toxic, it is still used for large-scale industrial applications. The earliest refrigerators used ammonia and sulfur dioxide, another naturally occurring and toxic refrigerant, but these were soon replaced in favor of safer alternatives.
The 1920s ushered in the use of halocarbon refrigerants -- synthetic gases developed under the trade name Freon. Composed primarily of carbon and the halogens fluorine and chlorine, these colorless, odorless, nonflammable and nontoxic gases replaced the toxic refrigerants used in earlier cooling units. The downside to these gases, particularly chlorofluorocarbons (CFCs), was their high ozone-depleting potential. Hydrochlorofluorocarbons (HCFCs) cause much less ozone depletion, and hydrofluorocarbons (HFCs) virtually none. Examples of halocarbon refrigerants include R12, R22 and R134a.
This class consists of various blends of chemical refrigerants in varying proportions. The combination of three different refrigerants that maintain their individual properties despite being in a mixture forms a zeotropic refrigerant. One example is R404a, a mixture of 44-percent pentafluoroethane (R125), 52-percent trifluoroethane (R143a) and 4-percent tetrafluoroethane (R134a). Azeotropes, on the other hand, are stable blends of two refrigerants which act as one when combined. For instance, R410a, an azeotrope composed of R125 and difluoromethane (R32) in equal proportions, behaves differently from its component refrigerants.
This group of refrigerants has been used successfully since the 1880s in domestic and industrial applications. Hydrocarbon refrigerants have an edge over all other refrigerants in terms of their high energy efficiency and zero ozone-depleting potential. Although highly flammable, they give off only carbon and steam upon burning and emit no toxic fumes, unlike chemical refrigerants. While this makes them excellent substitutes for halocarbon refrigerants, their use is highly regulated because of the inherent risks associated with their flammability. Examples include methane (R50), ethane (R170), propane (R290), butane (R600), isobutane (R600a), their non-halogenated derivatives and various blends.