Researchers at York University in Toronto, Canada, have found that the amount of trifluoroacetic acid (TFA) in the atmospheric declined during the COVID-19 pandemic in 2020 by a factor of 2–5, suggesting that sources of TFA in Toronto are mainly short-lived HFO precursors like HFO-123ryf.
The findings are described in a study – “Atmospheric Removal of Trifluoroacetic Acid by Dry and Wet Deposition: A Multiyear Analysis in Toronto” – published January 29 in the journal Environmental Science & Technology Letters.
TFA is regarded by scientists as an ultrashort-chain PFAS (per- and polyfluoroalkyl substance), or “forever chemical” that persists in the environment; it has two carbon atoms, one attached to three fluorine atoms. Longer-chain PFAS such as PFOA (perfluorooctanoic acid) and PFOS (perfluorooctanesulfonic acid) are linked to thyroid disease and to kidney and testicular cancer in the case of PFOA, among other adverse health outcomes.
While atmospheric deposition is regarded as a major source of TFA, it also generated as a degradation byproduct of certain pesticides, pharmaceuticals and other PFAS, as well as from industrial releases and sewage treatment.
The study took monthly measurements between 2018 and 2024 of TFA deposition from the atmosphere at an air-quality research station on the roof of York University’s Petrie Science and Engineering Building. The researchers linked the marked decline of TFA during the pandemic of 2020 to the reduction in usage of HFO-1234yf as a refrigerant, notably in car air-conditioning.
“When we turned off the tap, so to speak, and we all went home and stopped normal activities, we saw a really quick response, a dramatic reduction of TFA,” said Cora Young, a Professor and Atmospheric Chemist at York University and senior author of the study, in a press release.
HFO-1234yf that leaks from cars into the atmosphere converts entirely via photo-oxidation into TFA within about 12 days. In car air-conditioning HFO-1234yf, with a GWP of less than one, has largely replaced HFC-134a, which has a 100-year GWP of 1,530 and an atmospheric lifetime of 14 years, breaking down into TFA at a rate of 7–20%.
“I was so surprised when we saw that it had decreased during the pandemic, but I had to double- and triple-check the data because I didn’t believe it at first.”
Cora Young, Professor and Atmospheric Chemist at York University
TFA formed in the atmosphere is largely absorbed in rain or snow and deposited into the environment but some comes down as “dry” deposition, such as when gaseous TFA land on surfaces and forms a layer of dust. A 2025 study by North Carolina State and Duke University researchers found TFA in 89% of household dust samples linked to PFAS emissions from a fluorochemical plant.
Young said the “real surprise” in the study is that it points to TFA being formed from short-lived chemical precursors – primarily HFO-1234yf – emitted into the atmosphere. “I was so surprised when we saw that it had decreased during the pandemic, but I had to double- and triple-check the data because I didn’t believe it at first.”
She also noted in an email communication that it has been difficult previously to distinguish between TFA coming from the longer-lived HFC precursors and the short-lived HFO precursors. “I think this is compelling evidence that short-lived are already significant contributors.”
Peaking in the summer
As the COVID pandemic subsided and normal activities resumed, TFA levels in the atmosphere crept back up, the researchers found. The peak levels occur in the summer when there is more light, which triggers the transformation of f-gases into TFA. Last year, Young was part of a study that found the amount of TFA in the atmosphere is greatest in the summer and near urban areas, consistent with the leakage of HFO-1234yf from mobile air conditioners. Her group also determined in a 2024 study that the TFA in the atmosphere in Toronto was coming from atmospheric formation.
The new study looked at the relative contributions of wet, dry and total deposition of TFA from the atmosphere between 2018 and 2024. According to the study abstract, the researchers collected 103 total samples and 98 wet samples with TFA concentrations ranging from 0.07–4.55mcg/L and 0.09–3.19mcg/L, respectively. The median dry deposition flux (106mcg/m2 per year ) accounted for 0–94% (median 37%) of the total TFA deposition, higher and more variable than model estimates, the abstract said. “These results provide the first field-based quantification of TFA in dry deposition, demonstrating that it is a significant and previously underrepresented removal pathway,” it added.
Young saw a silver lining in the new study’s findings: “I see this as very important for framing regulation of these chemicals,” she said. “If short-lived precursors are already a dominant source, then regulation would be very effective.”
Some regions of the world are looking at regulating the emissions of HFO-1234yf and other HFOs that generate TFA, notably Canada, some U.S. states and especially the EU. On the other hand, the U.S. Environmental Protection Agency (EPA) last year approved pesticides known to break down into TFA.
Potential health effects
Young acknowledged that the health effects of long-term exposure to TFA on people and wildlife are still unknown. However, she added, its concentrations in the environment are “orders of magnitude” higher than that of bioaccumulative, cancer-linked PFAS like PFOA.
While TFA is not thought to bioaccumulate in humans, it’s “been found to accumulate in plants and can be found in the food people and wildlife eat. It has even been found in human blood,” said Young. “Our exposure could still be quite high, even if it’s not bioaccumulating.”
Research around the world is beginning to shed light on the potential toxicity of TFA. TFA has been found harmful in low concentrations to lab animals, sounding alarms about its potential effect on humans, especially over long-term exposure as TFA builds up in the environment. The European Chemicals Agency (ECHA) is evaluating a German proposal to classify TFA as reproductively toxic under the EU’s classification, labelling and packaging (CLP) regulation.
In the U.S., Duke University researchers are examining a potential link between firefighters’ exposure to PFAS, including TFA, and biomarkers of thyroid dysfunction and disease – one of the first studies to investigate human health outcomes related to TFA exposure. This study has found TFA in human blood serum, as have studies by North Carolina State University and Emory University.
In defending the use of TFA-producing refrigerants, (notably HFO-1234yf), the fluorochemical Industry points to the UN Environment Programme (UNEP) and its 2022 Environmental Effects Assessment Panel (EEAP) report. The report says that TFA is not bioaccumulative, is “not expected to pose significant risk to humans or the environment at the present time” and “is unlikely to cause adverse effects out to 2100.” The report does not comment on the contribution of pesticides and other precursors to TFA since they don’t fall under the purview of the Montreal Protocol, under which UNEP is an implementing agency.
