7. Background

7.1 History of F-gases – from CFCs to HFOs

Hydrofluoroolefins (HFO) are the fourth generation of synthetic refrigerants, succeeding the generations of natural refrigerants, CFCs, HCFCs and HFCs. This section briefly describes the three earlier generations of fluorinated refrigerants developed and used before HFOs came on the market. This section also briefly explains why CFCs, HCFCs and HFCs have been or are in the process of being phased down or out. The major global agreements that are the driving force behind the phase-outs or phasedowns are described in Chapter 6.

The first generation of refrigerants (1830s–1930s)

Before the introduction of halogenated refrigerants, multiple different substances with good thermodynamic characteristics were used, several of which are still in use today, including ammonia, propane, and carbon dioxide. Many of the first available refrigerants, including ammonia, methyl chloride, and sulphur dioxide, are toxic and/or flammable. When incorrectly handled, severe health and safety risks are associated with their use, and several fatal accidents have occurred.

McLinden, M. O. & Huber, M. L. (2020)

Chlorofluorocarbons (1930s–1990s) and hydrochlorofluorocarbons (1940s–2010s)

CFCs are fully halogenated chlorofluorocarbons; CFCs were introduced in the late 1920s under the popular name of freon. CFCs have several properties that make them especially suitable for various applications: They are non-flammable, non-toxic and inert in the lower atmosphere with an atmospheric lifetime of 100 years.

Elkins (1999)

However, CFCs have a very high ozone-depleting potential (ODP) and a very high global warming potential (GWP). In the 1940s, hydrochlorofluorocarbons (HCFCs) were introduced. HCFCs are partly halogenated substances with a lower ODP than CFCs and an approximate atmospheric lifetime of 12 years.

NOAA (n.d.)

The discovery of the ozone layer depletion caused by chlorinated refrigerants led to the establishment of the Montreal Protocol, a global joint initiative to phase out CFCs and HCFCs to protect the ozone layer. It was signed in 1987 and came into force in 1989.

Ozone Secretariat (2018a)

By the 1980s, CFCs and HCFCs were widely used as refrigerants and foam-blowing agents in aerosol sprays and cleaning solvents. The production of CFCs was discontinued in the early 1990s,

vanLoon & Duffy (2011)

but there are still several CFC banks in building foams, etc.

Papst (2020)

In the EU, the use of reclaimed or recycled HCFCs was allowed until 2015; globally, a total global phase-out is scheduled for 2040.

Ozone Secretariat (2018a)

Hydrofluorocarbons (1990es–2030)

HFCs were developed to substitute CFCs and HCFCs following the global agreement to phase out these substances due to their impact on the ozone layer. HFCs first entered the market in the 1990s. HFCs are widely used as refrigerants, blowing agents in foams and propellants in aerosol sprays. HFCs do not contain any chlorine and are not ozone-depleting substances (ODS), but HFCs are very potent greenhouse gases with a high GWP. For instance, R-134a, used in mobile air-conditioning systems (MACs), has a GWP of 1,430. The GWP for certain HFCs is as high as 14,800 (HFC-23). Because of their potency as greenhouse gases, it was decided to initiate a global HFC phasedown with the Kigali Amendment to the Montreal Protocol in 2016.

Ozone Secretariat (2018a)

The next primary refrigerant?

Figure 1 Historical development of refrigerants, adapted from the report ‘Legislation and practices for End-of-life Management of refrigerants and other F-gases in Norway and the EU’ by Asphjell et al. (2023)

There exist multiple alternatives that can replace most HFCs in different appliances, both the so-called natural refrigerants and a new generation of fluorinated refrigerants. No 'one size fits all' solution exists due to the wide array of applications where HFCs are in use and the alternatives' different operating pressures, safety properties, and thermodynamic properties. Alternatives to HFCs include:

Natural refrigerants
Hydrofluoroolefins (HFOs)
HFC-HFO Blends

Many low GWP alternatives present other challenges, environmental, safety and cost implications, many natural refrigerants are flammable (hydrocarbons), and some are toxic (ammonia). Some HFO substances are mildly flammable, and several HFOs break down to TFA in the environment; some partly and others completely (see section 6.6). Many refrigerants will likely be blends of currently used refrigerants.

A3	B3	Higher Flammability
A2	B2	Flammable
A2L	B2L	Lower Flammability
A1	B1	No Flame Propagation
Lower toxicity	Higher Toxicity

Figure 2 Refrigerant safety classification ISO 817 (ISO 817:2014)

7.2 Hydrofluoroolefins

Hydrofluoroolefins (HFOs) are, together with hydrochlorofluoroolefins (HCFOs), considered the fourth generation of fluorinated refrigerant gases. Hydrofluoroolefins are unsaturated hydrofluorocarbons composed of hydrogen, fluorine, and carbon. HCFOs contain chlorine as well. The carbon-carbon double bond greatly reduces their atmospheric lifetime. Many HFOs are low-pressure fluids with a high boiling point.

Ozone Secretariat (2018b)

HFOs have neither ozone-depleting properties nor a high Global Warming Potential (GWP). Some HFOs are mildly flammable, and their safety classification is A2L (ISO 817:2014).

Mota-Babiloni et al (2015)

Box 1: Nomenclature for Hydrofluoroolefins:

Exemplified with 1234yf.

HFO- 1234yf

HFC-1234yf

R1234yf

uHFC- 1234yf

HFOs are not a recently developed novelty. The first synthesis of HFO-1234yf was reported in 1946. But when the EU adopted the MAC directive in 2006, which prohibits refrigerants with a GWP higher than 150 in mobile air-conditioning of new vehicles, HFO-1234yf was found to be a suitable alternative for mobile air-conditioning systems.

McLinden, M. O. & Huber, M. L. (2020)

HFOs entered the market in 2008. Several different HFOs are now on the market, and their use and applications have increased in response to earlier generations of F-gases being phased out or down. HFOs are in use as refrigerants, foam-blowing agents, and aerosol propellants.

HFOs are mainly produced in the US, China, Japan, and India. There is no manufacturing of these gases in the EU,

Behringer et al. (2021)

but the following HFO substances are commercially available on the EU market.

Behringer et al (2021) & A-GAS (n.d.)

They are all listed in Annex II of the EU Regulation No 517/2014.

Regulation (EU) No. 517/2014

Substance name	Main use	GWP 100
HFO-1234yf	Refrigerant (esp. mobile air conditioning)	0.501
HFO-1234ze	Refrigerant, foam blowing agent, aerosol propellant	1.37
HFO-1336mzz	Refrigerant, foam blowing agent	17.9

Table 1 HFOs listed in annex II of the F-gas Regulation 517/2014/EU, GWP values Based on the Sixth Assessment Report adopted by the Intergovernmental Panel on Climate Change.

The World Meteorological Organisation frequently publishes a scientific assessment of ozone depletion. The most recent assessment (2022) concludes that there are no comprehensive global datasets on the production or consumption of HFOs.

Regulation (EU) No. 517/2014

According to data from the EC, the total EU import of synthetic alternatives H(C)FOs increased from 1,900 tonnes in 2014 to 21,763 tonnes in 2019.

European Commission (2022a)

However, the import of HFOs, especially in products (e.g., passenger cars), is currently underreported due to the threshold of 500 t CO₂-eq for reporting obligations when putting products containing F gases on the market.

There are also indications of regional concentration increases of HFOs measured in the environment. European atmospheric observations from the two observatories, Dubendorf and Jungfraujoch, have registered increases in the background concentration of some HFOs. Jungfraujoch documented increases from less than 0.01 ppm in 2016 to annual median levels of 0.10 for HFO-1234yf and 0.14 ppt for HFO-1234ze(E) in 2020.

European Commission (2022a)

7.2.1 HFO/HFC Blends

There are several different substance blends of HFOs and HFCs on the market. These blends are manufactured to lower the flammability of the substance or, in other words, enhance their performance while lowering the GWP of the blend.

The gases often used in blends are:

HFC refrigerants: R32, R125, R152a and R134a
HFO refrigerants: R1234yf and R1234ze(E)
And/or natural refrigerants such as R-290, R-600a and R-744

There is a need for further research since there are some uncertainties concerning these blends. There are studies that investigate the stability of blends to see whether they maintain their expected original compositions when in use or try to establish the most reliable route to recovering these gases, including an ongoing Swedish-funded project.

KTH (n.d.)

Mota-Babiloni et al (2015)

The following table provides an overview of some of the blends that have been commercially available in the EU for some years. The market is rapidly evolving, and numerous new blends have likely entered the market. One of the commercial benefits of blends is that they technically can serve as a drop-in in existing systems designed for HFCs, meaning no or little modification is required.

European Commission (n.d.)

Table 2 Common market-available blends of HFCs and HFOs (European commission n.d.a). GWP based on the Fourth Assessment Report adopted by the Intergovernmental Panel on Climate Change.

Substance	GWP	Composition	Safety Group	Replacement for	Suitable for
R448A	1387	R32/125/1234yf/1234ze(E)/134a	A1	R404A	Centralised systems for commercial refrigeration, condensing units, and refrigerated vehicles
R449A	1397	R32/125 /1234yf/134a	A1	R404A	Centralised systems for commercial refrigeration, condensing units, industrial refrigeration, refrigerated vehicles
R452A	2140	R32/125/1234yf	A1	R404A	Refrigerated vehicles, refrigerated containers
R454C	148	R32/1234yf	A2L	R410A	Heat pumps and chillers
R455A	148	R32/1234yf/CO₂	A2L	R404A	Chiller
R513A	631	R1234yf/134a	A1	R134a	Condensing units, industrial refrigeration, heat pumps, chillers, refrigerated containers
R515B	299	R1234ze/R227ea		R134a, R450A, R513A, R227ea, R124	Heat pumps, chillers

7.3 Natural Refrigerants

The use and application of natural refrigerants have come a long way since the early 1900s, and many of the previous challenges, such as flammability, have been addressed by lowering the amounts of refrigerants and optimising system designs, making them safer to use. There are often additional technical training requirements when working with natural refrigerants to ensure safe use and proper handling. Switching from HFCs to natural refrigerants in RACHP applications requires entirely different systems than switching to fluorinated blends that can be used as drop-ins.

The most commonly used natural refrigerants are carbon dioxide (R744), hydrocarbons, ammonia (R717) and dimethyl ether (R-E170).

7.3.1 Carbon dioxide (R744)

CO₂is a non-flammable and non-toxic refrigerant that operates at a higher pressure than other refrigerants, both fluorinated and natural.

The Natural Voice Magazine (2016)

CO₂has been used since the end of the 19^th century, and historically the higher pressure has given some technical challenges. Today, these are largely solved.

Ozone Secretariat (2018b).

CO₂ is not considered applicable for split systems because of the high-pressure requirement and the lower efficiency in transcritical operation. Furthermore, high-pressure requirements impose extra costs to ensure a safe design.

European Commission (2020).

In Europe, CO₂ is widely used as a refrigerant in supermarket cooling systems.

Ozone Secretariat (2022a).

The application of CO₂has previously been limited to regions with lower temperatures, but currently technical progress allowing CO₂ to operate in high ambient temperature climates is being made.

Ozone Secretariat (2018b).

7.3.2 Hydrocarbons

Numerous different hydrocarbons are used as refrigerants and foam-blowing agents. Some of the most common are listed below. Hydrocarbons have similar thermodynamic properties to fluorinated refrigerants, but hydrocarbons are flammable and, therefore, have higher safety requirements.

Ozone Secretariat (2018b).

Propane (R290) is a classified A3 refrigerant, meaning there are some limitations from product standards and/or building codes.
Isobutane (R600a) Isobutane is widely used in low-charge hermetically sealed applications such as refrigerators and freezers.
Copenhagen School of Marine Engineering and Technology Management (2023)
Propylene (R1270) is also classified as an A3 refrigerant, so there are some limitations from product standards and/or building codes. Propylene is mainly used in chillers today.
Pentane (R601), cyclopentane, and isopentane are applied as foam-blowing agents.
European Commission (n.d.)

7.3.3 Ammonia (R717)

Ammonia has been used for over a century. Ammonia has great thermodynamic properties but is toxic and flammable in certain conditions, so additional safety measures are required. Ammonia can be used for both cooling and heating. Due to the toxicity of ammonia, it is often used in conjunction with other refrigerants, such as CO_2,in cascade systems to make it safer. Ammonia is used in appliances at an industrial scale.

Ozone Secretariat (2018b).

7.3.4 Dimethyl ether (R-E170)

Dimethyl ether (DME) was one of the first refrigerants and was first used in the late 1800s. Today, DME is used as an aerosol propellant, a (co-)blowing agent for foam, and in refrigerant blends. The application of dimethyl ether is projected to increase in the future. DME is both highly flammable and explosive. DME is often more expensive than other low-GWP non-fluorinated refrigerants since it is chemically synthesised.

Ozone Secretariat (2018b); (2018c); (2018d)

7.4 Environmental Concerns

Market stakeholders often highlight in their marketing that HFOs have little to no adverse impacts on the environment. Therefore, the HFOs are promoted as an environmentally friendly alternative to HFCs. This claim is based on the short atmospheric lifetime of HFOs, low GWP, and zero ODP. However, there are concerns about the environmental impact of HFOs, not least in relation to the current increase in usage and the expected future increase. One of the major environmental concerns is the persistence, aqueous mobility and toxicity of HFO breakdown products, especially trifluoroacetic acid. Moreover, even though HFOs are listed as non-toxic (toxicity level A), some HFO feedstock substances are toxic, have a high GWP or are ODSs.

WMO (2022)

Molar yield = The amount of a substance obtained in a chemical reaction expressed in moles (SI unit for amount of substance)

7.4.1 Trifluoroacetic Acid (TFA)

HFOs have an approximate lifetime of days in the atmosphere before being degraded.

Behringer et al (2021)

One of the breakdown products of some HFO substances is trifluoroacetic acid (TFA). According to the OECD definition for PFAS, TFA can be considered an ultrashort per- and polyfluoroalkyl substance. Furthermore, there is a new class of very persistent and very mobile substances (vPvM), another major cause for concern. TFA fulfils the criteria for this classification as well.

Miljødirektoratet (2023)

The OECD’s PFAS definition

PFAS are defined as fluorinated substances that contain at least one fully fluorinated methyl or methylene carbon atom (without any H/Cl/Br/I atom attached to it) i.e., with a few noted exceptions, any chemical with a least a perfluorinated methyl group (-CF₃) or a perfluorinated methylene group (-CF₂-) is a PFAS (OECD, 2021)

TFA is formed when HFOs are emitted into the atmosphere, where they oxidise. It is uncertain to what extent TFA naturally occur in the environment.

WMO (2022)

There has generally been a widespread consensus that there are natural sources of TFA in the deep sea, but this assumption has recently been challenged.

Joudan et al (2021)

However, the steeply increasing concentrations of TFA in freshwater bodies over the last few decades can only be explained by anthropogenic sources. Significant anthropogenic sources of TFA exist. Other sources aside from some fluorinated refrigerants include pesticides and pharmaceuticals, and substantial amounts of TFA are now found in inland waterbodies.

WMO (2022)

TFA is also detected in crops and food.

German Environment Agency (2021)

Figure 3 Structure of Trifluoroacetic Acid (TFA)

TFA has a relatively short lifetime in the atmosphere of approximately four months.

Holland et al (2021)

However, within the lifetime, TFA is often washed out of the atmosphere by precipitation and thus enters the soil and water bodies. TFA is highly persistent in waterbodies where it accumulates.

Behringer et al (2021)

A Swedish study estimated an overall yearly input of 170 kg TFA into the Swedish lake Vättern, with 98 kg originating from atmospheric deposition.

Björnsdotter et al (2022)

Measurements of TFA concentrations in German rivers show an increase since 1996 from concentrations of 0.04–0.3 to above 1, although varying greatly during time and between locations.

Brunn et al (2023)

For instance, in 2018, a maximum of 12.8 mg/l was measured where the river Elbe exits Hamburg harbour, but at Schmilka on the Upper Elbe 1.8 mg/l was measured.

German Environment Agency (2021)

In 2021, the Danish Environmental Protection Agency reported findings of TFA in 219 out of 247 groundwater wells (89% of all samples). TFA was also found in some drinking water supplies; the concentration was lower than 1mg/L for most findings.

Danish Environment Agency (2021)

HFO-1234yf completely breaks down to TFA, and with a continued substitution of HFC-134a with HFO-1234yf, the total amount of TFA deposited from the atmospheric degradation of fluorinated substances is projected to increase by more than 300% in 2050 (compared to 2018). This results in a projected annual increase of 49,718 tonnes of TFA by 2050 from EU-28; 96% will come from the atmospheric degradation of HFO-1234yf.

Behringer et al (2021)

Increasing TFA concentrations also pose a health concern, especially since TFA is persistent and accumulates in water bodies. According to current knowledge, toxicological and ecotoxicological effects are only observed at very high concentrations. However, according to the background report ‘Reducing the input of chemicals into waters: trifluoroacetate (TFA) as a persistent and mobile substance with many sources’ from the German Environmental Agency in 2021, the long-term impacts of TFA are still very uncertain.

German Environment Agency (2021)

In drinking water production, no practicable and economical method exists for its removal.

Umweltbundesamt (2021)

Germany has put a threshold on allowable TFA concentrations in freshwater, and it is currently set to 60μg/l with recommendations of not exceeding 10μg/l in drinking water. The threshold value is based on the no observed effect concentration threshold (NOEC) of 30 ppm for humans, corresponding to 1.8 mg/kg body weight.

Atmosphere (2022)

Denmark has also implemented a threshold value for TFA in drinking water at 9μg/l.

Drikkevandsbekendtgørelsen (2021)

Molar yield of TFA from different HFOs:

1234yf 100%

1234ze(E) <10%

1336mzz(Z) <20%

1225ye(E) 100%

1225ye(Z) 100%

(Behringer et al 2021)

TFA is already widely present in the environment and in freshwater reservoirs. There is a need for further research to better understand the atmospheric and hydrospheric cycle of TFA and to clarify some of the current uncertainties concerning the lifecycle of TFA as well as long-term impacts.

7.4.2 HFC-23

HFC-23 (Trifluoromethane (CHCl₃)) is a potent GHG with a GWP₁₀₀ of 12400.

Myhre et al (2013)

HFC-23 is produced as a by-product of HCFC-22 production. HCFC-22 is a widely used feedstock for several refrigerants, including HFO-1234yf, meaning the production of HFO-1234yf and any blends containing HFO-1234yf indirectly leads to a by-production of HFC-23. HCFC-22 production is still allowed for feedstock use under the Montreal Protocol. However, the Kigali Amendment initiated reporting requirements for HFC-23.

WMO (2022)

Although this study focuses on the end-of-life treatments of HFOs, it is important to note some of the environmental concerns related to the HFO production feedstock since they pose a potential risk for counteracting results obtained due to the Montreal Protocol and the Kigali Amendment, as well as EU strategies.

7.5 Health and Safety

When exposed to high temperatures or high doses of UV light combined with heat, HFOs like HFCs will decompose to toxic substances, including hydrofluoric acids and carbonyl fluoride, raising concerns over potential toxicity hazards in the workplace. In high concentrations, HFOs are asphyxiant, and contact with evaporating liquid can lead to frostbite. Therefore, proper education of practitioners and adequate safety measures in the workplace are crucial. According to a Norwegian study from 2017, there is a lack of publicly available information on HFOs' effect on health.

Ozone Secretariat (2018b)

Fleet et al (2017)

Since the MAC Directive was adopted in 2006, the use of HFO-1234yf as a replacement for HFC-134a in MAC systems has raised some debate that continued up through the 2010s due to safety concerns. Consequently, numerous tests were conducted in the same period.

European Commission (2014b)

One of the main concerns was if the MAC system was disrupted in a car accident, it could leak toxic gases with a potentially fatal outcome for passengers due to the flammability of HFO-1234yf.

BAM (2010)

However, the risk was small, and HFO-1234yf is considered safe to use in MAC systems.

European Commission (2014a)

HFO foam-blowing agents have similar toxicity exposure limits to HFCs, and exposure concerns include frostbite and oxygen deprivation when large amounts are released in an enclosed space. Several studies have been conducted to determine when foam-blowing agents have degassed sufficiently and when it is safe to re-enter the area in question.

Ozone Secretariat (2023)