The Pros and Cons of Hydrofluoroolefins: Solving Climate Change but Raising Environmental Concerns

Dr Bob Symons, Regional Technical Manager, Eurofins Environment Testing Australia & New Zealand

Hydrofluoroolefins (HFOs) are unsaturated organic compounds composed of hydrogen, fluorine and carbon and contain one or more double bonds in their molecular structure

– see Figure 1. They are of interest as refrigerants because they have zero ozone depletion potential and low global warming potential. They are considered the fourth generation of fluorinated materials used as refrigerants, blowing agents, propellants, and solvents. HFOs are designed explicitly as alternatives to hydrofluorocarbons (HFCs) and were developed to address the concerns associated with HFCs, which are potent greenhouse gases contributing to climate change. One of the critical advantages of HFOs is their significantly lower global warming potential (GWP) compared to HFCs. GWP is a measure of how much heat a greenhouse gas traps in the atmosphere relative to carbon dioxide (CO2) over a specific time period. HFOs have GWPs typically less than 1, whereas some HFCs have GWPs in the thousands, viz., HFC-23 GWP100 = 14,800 and methane’s GWP100 = 25. This means that HFOs have a much smaller impact on global warming when released into the atmosphere. It is important to note that while HFOs offer environmental benefits over HFCs in terms of climate impact, as mentioned earlier, there are ongoing discussions and studies regarding potential concerns related to their degradation products, such as trifluoroacetic acid (TFA) – see Figure 2.

Figure 1 HFO-1234yf (2,3,3,3-Tetrafluoropropene) with the formula CH2=CFCF3. Source: Wikipedia

Figure 2 Trifluoroacetic acid Source: Wikipedia

HFO-1234yf (Figure 1 & Figure 3) is a type of HFO that is used as a refrigerant in automobile air conditioners. It has a low global warming potential (GWP) of less than 1, but it also has some environmental concerns related to its breakdown products. One of these products is trifluoroacetic acid (TFA), which is a type of perfluoroalkyl substance (PFAS). PFAS are a group of synthetic chemicals that are persistent, bioaccumulative, and potentially toxic to humans and wildlife. TFA is a strong acid that can accumulate in water bodies and affect aquatic ecosystems. Another breakdown product of HFO-1234yf is HFC-23 or CHF3, which is a very potent greenhouse gas. HFC-23 can form from HFO-1234yf in the open atmosphere, adding to the secondary GWP of HFO-1234yf. The amount of HFC-23 that may form from HFO-1234yf is still uncertain and depends on various factors, such as atmospheric conditions and emissions. Therefore, the environmental impact of HFO-1234yf and its breakdown products is still being studied and evaluated by researchers and regulators.

Figure 3: Honeywell R1234YF AC Refrigerant for Mobile Systems Solstice HFO-1234YF Source: Walmart.com

One such study by Björnsdotter et. al detected the ultrashort-chain perfluoroalkyl acids (PFAAs), perfluoropropanoic acid (PFPrA), perfluorobutanoic acid (PFBA) as well as TFA in Lake Vättern that is one of Sweden’s largest lakes and is an important source for drinking water. Ultrashort-chain PFAAs are highly polar, which generally leads to a lower potential for bioaccumulation in organisms. Still, accumulation in plants has been observed, indicating some degree of uptake and retention. There is limited data available on human exposure to ultrashort-chain PFAAs, but TFA has been detected in human blood. The combination of high persistence and mobility in the environment makes ultrashort-chain PFAAs a potential threat to drinking water. Once contamination occurs, it can be challenging and costly to remove these substances from water sources, unlike their longer-chain counterparts, where conventional treatment systems are less effective.

Figure 4: from Björnsdotter et. al. https://pubs.acs.org/doi/10.1021/acs.est.1c04472

In February 2023, the European Chemical Agency (ECHA) published its proposals for bans of fluorinated substances under the REACH regulations. As proposed, fluorinated refrigerants, namely HFOs and HFCs, would be banned from manufacture and sale where suitable alternatives exist. ECHA’s scientific committees have begun their scientific evaluation of the proposal and if adopted, they will be sent to the European Commission, who, together with the EU Member States, will then decide on the potential restriction. The projected timeline is outlined below.

Figure 5: ECHA’s proposed timeline below the next steps

The analysis of these ultrashort PFAS is undertaken at Eurofins with a recent study of drinking water samples taken in Norway and Sweden showing levels of TFA ranging from 70 ng/L - 720 ng/L with trifluoromethanesulfonic acid (TFMS) being detected at seven sites where TFA concentrations were highest. The mobility and persistence of these ultrashort PFAS make them a candidate for inclusion in the EU drinking water directive (DWD) to be adopted in EU Member States whereby drinking water quality standards for a defined sum of 20 PFAS at 100 ng/L or for total PFAS at 500 ng/L.