According to a scientist at Harvard University, continued production of f-gases, including HFCs and HFOs, will lead to “greater burdens of TFA” (trifluoroacetic acid), an atmospheric byproduct of f-gases widely considered to fall into the category of environmentally persistent PFAS (per- and polyfluoroalyl substances).
“Left unchecked, concentrations [of TFA] will continue to increase,” owing to their durability in the environment, a characteristic of PFAS (known as “forever chemicals”), said the scientist, Heidi Pickard, a fourth-year Ph.D. student in the Department of Engineering Sciences at Harvard University, during a presentation at the ATMOsphere (ATMO) America Summit 2023. The event was held June 12–13 in Washington, D.C., and was hosted by ATMOsphere, publisher of R744.com.
A widely accepted definition of PFAS published in 2021 by the OECD (Organization for Economic Co-operation and Development) describes PFAS as fluorinated substances that contain at least one fully fluorinated methyl or methylene carbon atom. This definition encompasses certain f-gases, and TFA and is being followed by the EU in its consideration of a new request to regulate PFAS. This year refrigeration OEM Advansor posted on LinkedIn a statement connecting f-gas refrigerants with TFA and PFAS.
While earning a master’s degree in chemistry at Memorial University in Newfoundland, Canada, Pickard was part of a study published in 2020 that found three short-chain perfluoroalkylcarboxylic acids (scPFCAs, a subgroup of PFAS), including TFA, in Canadian Arctic ice cores. The increase in TFA concentrations in the ice cores since 1990 suggests that replacements of CFCs with HCFCs and HFCs (and later HFOs) per the Montreal Protocol contributed to the accumulation of TFA, the study found.
Certain f-gases are known to degrade via oxidation in the atmosphere into TFA, which then trickles down in rainwater and infiltrates the environment. In particular, up to 20% of HFC-134a converts over 14 years into TFA, while HFO-1234yf changes completely into TFA over a few weeks. HFC-134a’s long lifetime allows it to be transported in the atmosphere to regions like the high Arctic, far from where it was emitted or used, Pickard pointed out. On the other hand, because of its short-lived nature, the atmospheric breakdown of HFO-1234yf contributes to TFA accumulation “close to source regions,” said Pickard, though PFCAs like TFA can move through the ocean via marine aerosols.
Pickard noted that the TFA found in the Arctic is thought to come from human (anthropogenic) sources, pointing to a study by some of her colleagues in the earlier work disputing the notion that there are natural sources of TFA.
Last year, another study of Arctic ice samples in Svalbard (part of Norway) – “Levels and distribution profiles of Per- and Polyfluoroalkyl Substances (PFAS) in a high Arctic Svalbard ice core” – also found significant levels of TFA. TFA represented 71% of the total mass of shorter-chain PFCAs in the ice core “and had increasing temporal trends in deposition,” says the study, which points to HFCs and other CFC replacement compounds as TFA precursors.
While TFA is known to be toxic in low concentrations to green algae and has been found harmful to animals such as rats and rabbits, its toxicity for humans has not been fully established. “The data on TFA’s health effects are very limited,” said Pickard. “But based on the fact that short-chain PFCAs are very persistent, very mobile, will accumulate in aquatic ecosystems and are bioavailable and bioaccumulative – that alone warrants concerns about these chemicals that needs further evaluation and attention.”
Germany’s Environment Agency (UBA) has set a human health “orientation value” limit of 60 mcg/l for TFA in drinking water and a “precautionary measure” of 10 mcg/l. The concentration levels of TFA in the environment have begun to approach – or exceed – those levels in some studies.
Other recent studies
In her presentation, Pickard alluded to other recent studies showing the accumulation of TFA in the environment, including a 2022 study of surface waters in California and studies of plants in Germany. “Every study measuring short-chain PFCAs is seeing an increase in the environment across the globe over the last 10 to 30 years,” she said.
In the California study, TFA concentrations in streams increased by an average of six-fold between 1998 and 2021, resulting in a median concentration of 0.18mcg/l (with a range of 0.021–2.79).
A follow-up study on the accumulation of TFA in plants – “Long-term trends for trifluoroacetic in terrestrial environmental samples – Analysis of plant samples from the Federal Environmental Specimen Bank (UPB) for trifluoroacetic acid – Part 2” – was published in November 2022 by Finnian Freeling and Merle Käberich of the German Water Center.
The German researchers found a “statistically significant increase in TFA concentrations within the study period [1985–2022]” in conifer species, corresponding well with the results from a study of deciduous tree samples in the original study. “The presented results provide further evidence of a substantial increase in the atmospheric TFA deposition within the last decades in Germany.” The highest TFA concentrations – up to just under 1,000mcg/kg dry weight – were discovered in samples of Norway spruce.
This year, Chinese researchers published a study projecting HFO emissions – and the resulting TFA deposition – in China. The study estimated cumulative emissions of HFO-1234yf of 1.7Gt between 2024 and 2060, with accompanying deposition of TFA during that period of 0.4 to 1.0Gt.
The chemical industry addressed the environmental deposition of TFA in an October 2021 study funded by the Global Forum for Advanced Climate Technologies (globalFACT), which represents f-gas producers Chemours, Honeywell, Arkema and Koura (and equipment manufacturer Daikin). The study concluded that “with the current knowledge of the effects of TFA on humans and ecosystems, the projected emissions through 2040 would not be detrimental.” But the study also acknowledged that “the major uncertainty in the knowledge of the TFA concentrations and their spatial distributions is due to uncertainties in the future projected emissions.”
“Based on the fact that short-chain PFCAs are very persistent, very mobile, will accumulate in aquatic ecosystems and are bioavailable and bioaccumulative – that alone warrants concerns about these chemicals that needs further evaluation and attention.”Heidi Pickard, Harvard University