A refrigerant is a working fluid used in the refrigeration cycle of air conditioning systems and heat pumps where in most cases they undergo a repeated phase transition from a liquid to a gas and back again. Refrigerants are heavily regulated due to their toxicity, flammability and the contribution of CFC and HCFC refrigerants to ozone depletion and that of HFC refrigerants to climate change.
Refrigerants are used in a direct expansion (DX) system to transfer energy from one environment to another, typically from inside a building to outside (or vice versa) commonly known as an "air conditioner" or "heat pump". Refrigerants can carry per kg 10 times more energy than water and 50 times more than air. 
In some countries, refrigerants are controlled substances due to high pressures (700–1,000 kPa (100–150 psi)), extreme temperatures (−50 °C [−58 °F] to over 100 °C [212 °F]), flammability (A1 class non-flammable, A2/A2L class flammable and A3 class extremely flammable/explosive) and toxicity (B1-low, B2-medium & B3-high), as classified by ISO 817 & ASHRAE 34. 
Refrigerants must only be handled by qualified/certified engineers to the relevant classes of refrigerant (in the UK,  C&G 2079 for A1-class, amd C&G 6187-2 for A2/A2L & A3 class refrigerants).

History
The first air conditioners and refrigerators employed toxic or flammable gases, such as ammonia, sulfur dioxide, methyl chloride, or propane, that could result in fatal accidents when they leaked.In 1928 Thomas Midgley Jr. created the first non-flammable, non-toxic chlorofluorocarbon gas, Freon (R-12). The name is a trademark name owned by DuPont (now Chemours) for any chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), or hydrofluorocarbon (HFC) refrigerant. Following the discovery of better synthesis methods, CFCs such as R-11, R-12, R-123 and R-502 dominated the market.

Phasing out of CFCs
In the early 1980s, scientists discovered that CFCs were causing major damage to the ozone layer that protects the earth from ultraviolet radiation, and to the ozone holes over polar regions. This led to the signing of the Montreal Protocol in 1987 which aimed to phase out CFCs and HCFC but did not address the contributions that HFCs made to climate change. The adoption of HCFCs such as R-22, and R-123 was accelerated and so were used in most U.S. homes in air conditioners and in chillers from the 1980s as they have a dramatically lower Ozone Depletion Potential (ODP) than CFCs, but their ODP was still not zero which led to their eventual phase-out.
Hydrofluorocarbons (HFCs) such as R-134a, R-143a, R-407a, R-407c, R-404a and R-410a (a 50/50 blend of R-125/R-32) were promoted as replacements for CFCs and HCFCs in the 1990s and 2000s. HFCs were not ozone-depleting but did have global warming potentials (GWPs) thousands of times greater than CO2 with atmospheric lifetimes that can extend for decades. This in turn, starting from the 2010s, led to the adoption in new equipment of Hydrocarbon and HFO (hydrofluoroolefin) refrigerants R-32, R-290, R-600a, R-454B, R-1234yf, R-514A, R-744 (CO2), R-1234ze and R-1233zd, which have both an ODP of zero and a lower GWP. Hydrocarbons and CO2 are sometimes called natural refrigerants because they can be found in nature.
The environmental organization Greenpeace provided funding to a former East German refrigerator company to research alternative ozone- and climate-safe refrigerants in 1992. The company developed hydrocarbon mixes such as isopentane and isobutane, propane and isobutane, or pure isobutane, called "Greenfreeze", but as a condition of the contract with Greenpeace could not patent the technology, which led to their widespread adoption by other firms. Policy and political influence by corporate executives resisted change however, citing the flammability and explosive properties of the refrigerants, and DuPont together with other companies blocked them in the U.S. with the U.S. EPA.Beginning on 14 November 1994, the U.S. Environmental Protection Agency restricted the sale, possession and use of refrigerants to only licensed technicians, per rules under sections 608 and 609 of the Clean Air Act. In 1995, Germany made CFC refrigerators illegal.In 1996 Eurammon, a European non-profit initiative for natural refrigerants, was established and comprises European companies, institutions, and industry experts.In 1997, FCs and HFCs were included in the Kyoto Protocol to the Framework Convention on Climate Change.
In 2000 in the UK, the Ozone Regulations came into force which banned the use of ozone-depleting HCFC refrigerants such as R22 in new systems. The Regulation banned the use of R22 as a "top-up" fluid for maintenance from 2010 for virgin fluid and from 2015 for recycled fluid.

Addressing greenhouse gases
With growing interest in natural refrigerants as alternatives to synthetic refrigerants such as CFCs, HCFCs and HFCs, in 2004, Greenpeace worked with multinational corporations like Coca-Cola and Unilever, and later Pepsico and others, to create a corporate coalition called Refrigerants Naturally!. Four years later, Ben & Jerry's of Unilever and General Electric began to take steps to support production and use in the U.S. It is estimated that almost 75 percent of the refrigeration and air conditioning sector has the potential to be converted to natural refrigerants.In 2006, the EU adopted a Regulation on fluorinated greenhouse gases (FCs and HFCs) to encourage to transition to natural refrigerants (such as hydrocarbons). It was reported in 2010 that some refrigerants are being used as recreational drugs, leading to an extremely dangerous phenomenon known as inhalant abuse.From 2011 the European Union started to phase out refrigerants with a global warming potential (GWP) of more than 150 in automotive air conditioning (GWP = 100-year warming potential of one kilogram of a gas relative to one kilogram of CO2) such as the refrigerant HFC-134a (known as R-134a in North America) which has a GWP of 1526. In the same year the EPA decided in favour of the ozone- and climate-safe refrigerant for U.S. manufacture.A 2018 study by the nonprofit organization "Drawdown" put proper refrigerant management and disposal at the very top of the list of climate impact solutions, with an impact equivalent to eliminating over 17 years of US carbon dioxide emissions.In 2019 it was estimated that CFCs, HCFCs, and HFCs were responsible for about 10% of direct radiative forcing from all long-lived anthropogenic greenhouse gases. and in the same year the UNEP published new voluntary guidelines, however many countries have not yet ratified the Kigali Amendment.
From early 2020 HFCs (including R-404a, R-134a and R-410a) are being superseded: Residential air-conditioning systems and heat pumps are increasingly using R-32. This still has a GWP of more than 600. Progressive devices use refrigerants with almost no climate impact, namely R-290 (propane), R-600 (isobutane) or R-1234yf (less flammable, in cars). In commercial refrigeration also CO2 (R-744) can be used.

Requirements and desirable properties
A refrigerant needs to have: a boiling point that is somewhat below the target temperature (although boiling point can be adjusted by adjusting the pressure appropriately), a high heat of vaporization, a moderate density in liquid form, a relatively high density in gaseous form (which can also be adjusted by setting pressure appropriately), and a high critical temperature. Extremely high pressures should be avoided.The ideal refrigerant would be: non-corrosive, non-toxic, non-flammable, with no ozone depletion and global warming potential. It should preferably be natural with well-studied and low environmental impact. Newer refrigerants address the issue of the damage that CFCs caused to the ozone layer and the contribution that HCFCs make to climate change, but some do raise issues relating to toxicity and/or flammability.

Common refrigerants
Refrigerants with very low climate impact
With increasing regulations, refrigerants with a very low global warming potential are expected to play a dominant role in the 21st century, in particular, R-290 and R-1234yf. Starting from almost no market share in 2018, low GWPO devices are gaining market share in 2022.

Most used
Banned / Phased out
Other
Refrigerant reclamation and disposal
Coolant and refrigerants are found throughout the industrialized world, in homes, offices, and factories, in devices such as refrigerators, air conditioners, central air conditioning systems (HVAC), freezers, and dehumidifiers. When these units are serviced, there is a risk that refrigerant gas will be vented into the atmosphere either accidentally or intentionally, hence the creation of technician training and certification programs in order to ensure that the material is conserved and managed safely. Mistreatment of these gases has been shown to deplete the ozone layer and is suspected to contribute to global warming.With the exception of isobutane and propane (R600a, R441a and R290), ammonia and CO2 under Section 608 of the United States' Clean Air Act it is illegal to knowingly release any refrigerants into the atmosphere.Refrigerant reclamation is the act of processing used refrigerant gas which has previously been used in some type of refrigeration loop such that it meets specifications for new refrigerant gas. In the United States, the Clean Air Act of 1990 requires that used refrigerant be processed by a certified reclaimer, which must be licensed by the United States Environmental Protection Agency (EPA), and the material must be recovered and delivered to the reclaimer by EPA-certified technicians.

Classification of refrigerants
Refrigerants may be divided into three classes according to their manner of absorption or extraction of heat from the substances to be refrigerated:
Class 1: This class includes refrigerants that cool by phase change (typically boiling), using the refrigerant's latent heat.
Class 2: These refrigerants cool by temperature change or 'sensible heat', the quantity of heat being the specific heat capacity x the temperature change. They are air, calcium chloride brine, sodium chloride brine, alcohol, and similar nonfreezing solutions. The purpose of Class 2 refrigerants is to receive a reduction of temperature from Class 1 refrigerants and convey this lower temperature to the area to be cooled.
Class 3: This group consists of solutions that contain absorbed vapors of liquefiable agents or refrigerating media. These solutions function by nature of their ability to carry liquefiable vapors, which produce a cooling effect by the absorption of their heat of solution. They can also be classified into many categories.

R numbering system
The R- numbering system was developed by DuPont (which owned the Freon trademark), and systematically identifies the molecular structure of refrigerants made with a single halogenated hydrocarbon. ASHRAE has since set guidelines for the numbering system as follows:R-X1X2X3X4

X1 = Number of unsaturated carbon-carbon bonds (omit if zero)
X2 = Number of carbon atoms minus 1 (omit if zero)
X3 = Number of hydrogen atoms plus 1
X4 = Number of fluorine atoms

Series
R-xx	Methane Series
R-1xx	Ethane Series
R-2xx	Propane Series
R-4xx	Zeotropic blend
R-5xx	Azeotropic blend
R-6xx	Saturated hydrocarbons (except for propane which is R-290)
R-7xx	Inorganic Compounds with a molar mass < 100
R-7xxx	Inorganic Compounds with a molar mass ≥ 100

Ethane Derived Chains
Number Only	Most symmetrical isomer
Lower Case Suffix (a,b,c,etc.)	indicates increasingly unsymmetrical isomers

Propane Derived Chains
Number Only	Most symmetrical isomer
2nd Lower Case Suffix (a,b,c,etc.)	Indicates increasingly unsymmetrical isomers
a Suffix	Cl2 central carbon substitution
b Suffix	Cl2F central carbon substitution
c Suffix	F2 central carbon substitution
d Suffix	Cl, H central carbon substitution
e Suffix	F, H central carbon substitution
f Suffix	H2 central carbon substitution

HFOs
x Suffix	Cl substitution on central atom
y Suffix	F substitution on central atom
z Suffix	H substitution on central atom
a Suffix	=CCl2 methylene substitution
b Suffix	=CClF methylene substitution
c Suffix	=CF2 methylene substitution
d Suffix	=CHCl methylene substitution
e Suffix	=CHF methylene substitution
f Suffix	=CH2 methylene substitution

Blends
Upper Case Suffix (A,B,C,etc.)	Same blend with different compositions of refrigerants

Miscellaneous
R-Cxxx Cyclic compound
R-Exxx Ether group is present
R-CExxx	Cyclic compound with an ether group
R-4xx/5xx + Upper Case Suffix (A,B,C,etc.)	Same blend with different composition of refrigerants
R-6xx + Lower Case Letter	Indicates increasingly unsymmetrical isomers
7xx/7xxx + Upper Case Letter	Same molar mass, different compound
R-xxxxB#	Bromine is present with the number after B indicating how many bromine atoms
R-xxxxI#	Iodine is present with the number after I indicating how many iodine atoms
R-xxx(E)	Trans Molecule
R-xxx(Z)	Cis MoleculeFor example, R-134a has 2 carbon atoms, 2 hydrogen atoms, and 4 fluorine atoms, an empirical formula of tetrafluoroethane. The "a" suffix indicates that the isomer is unbalanced by one atom, giving 1,1,1,2-Tetrafluoroethane. R-134 (without the "a" suffix) would have a molecular structure of 1,1,2,2-Tetrafluoroethane.
The same numbers are used with an R- prefix for generic refrigerants, with a "Propellant" prefix (e.g., "Propellant 12") for the same chemical used as a propellant for an aerosol spray, and with trade names for the compounds, such as "Freon 12". Recently, a practice of using abbreviations HFC- for hydrofluorocarbons, CFC- for chlorofluorocarbons, and HCFC- for hydrochlorofluorocarbons has arisen, because of the regulatory differences among these groups.

Refrigerant safety
ASHRAE Standard 34, Designation and Safety Classification of Refrigerants, assigns safety classifications to refrigerants based upon toxicity and flammability.
Using safety information provided by producers, ASHRAE assigns a capital letter to indicate toxicity and a number to indicate flammability.  The letter "A" is the least toxic and the number 1 is the least flammable.

See also
Brine (Refrigerant)
Section 608
List of Refrigerants

References
Sources
IPCC reports
IPCC (2013). Stocker, T. F.; Qin, D.; Plattner, G.-K.; Tignor, M.; et al. (eds.). Climate Change 2013: The Physical Science Basis (PDF). Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press. ISBN 978-1-107-05799-9. (pb: 978-1-107-66182-0). Fifth Assessment Report - Climate Change 2013
Myhre, G.; Shindell, D.; Bréon, F.-M.; Collins, W.; et al. (2013). "Chapter 8: Anthropogenic and Natural Radiative Forcing" (PDF). Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. pp. 659–740.
IPCC (2021). Masson-Delmotte, V.; Zhai, P.; Pirani, A.; Connors, S. L.; et al. (eds.). Climate Change 2021: The Physical Science Basis (PDF). Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press (In Press).
Forster, Piers; Storelvmo, Trude (2021). "Chapter 7: The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity" (PDF). IPCC AR6 WG1 2021.

Other
"High GWP refrigerants". California Air Resources Board. Retrieved 13 February 2022.
"BSRIA's view on refrigerant trends in AC and Heat Pump segments". 2020. Retrieved 2022-02-14.
Yadav, Saurabh; Liu, Jie; Kim, Sung Chul (2022). "A comprehensive study on 21st-century refrigerants - R290 and R1234yf: A review". International Journal of Heat and Mass Transfer. 122: 121947. doi:10.1016/j.ijheatmasstransfer.2021.121947. S2CID 240534198.

External links
US Environmental Protection Agency page on the GWPs of various substances
Green Cooling Initiative on alternative natural refrigerants cooling technologies
International Institute of Refrigeration Archived 2018-09-25 at the Wayback Machine