New acid in the rain: what is TFU and should we be afraid of it

Rain and snow increasingly bring traces of the same molecule to Earth: trifluoroacetic acid (TFA). Over the past decades, it has been found in rivers and lakes, groundwater, tree leaves, beer and bottled water, and in human blood and urine samples. Where long-term measurements have been made, concentrations are rising. Scientists debate how dangerous this is, with some regulators calling for tighter controls, while others see minimal risk at current levels. At stake are the refrigeration, insulation, agrochemical, and pharmaceutical industries, where TFA is either produced as a byproduct or used as a building block.

Background of the study

Trifluoroacetic acid (TFA) is an ultra-short-chain "organofluorine" that is found literally everywhere today: in rain and snow, lakes and rivers, groundwater, tree leaves, beer and bottled water, as well as in human blood and urine. Where long-term series of measurements are taken, a steady increase in concentrations is recorded. This global trend and the accompanying debate about the risks is analyzed in the Nature article.

There are several sources of TFC. These include direct emissions from chemical production and the decay of "predecessors" - some pesticides, pharmaceutical compounds and PFAS polymers. But the main contribution to precipitation is made by fluorinated refrigeration and insulation gases (F-gases), in particular modern freon substitutes from the HFC/HFO families: during atmospheric oxidation, they turn into TFC and "return" to the earth with rain. This formation pathway is analyzed in detail in UNEP profile assessments (including updates on HFO-1234yf) and reviews of the environmental effects of F-gases.

The accumulation of TFA is confirmed by independent archivists. In Denmark, a retrospective of 113 monitoring wells showed a 60-year increase in TFA in groundwater: in “old” (pre-1960) water, TFA was not detected, and as the inflow “rejuvenates,” levels increase. Ice cores in the Canadian Arctic and new records from Antarctica show an increase in the deposition of short-chain perfluoroalkyl acids, with TFA providing the lion's share of the signal and increasing with the change in refrigerant generations after the Montreal Protocol. These data point to long-range atmospheric transport and the contribution of replacement CFCs to the “acid footprint” of the 21st century.

The risks are a subject of active discussion. Unlike the "long" PFAS, TFA is very water-soluble and, according to current data, is quickly eliminated from humans, i.e. it does not demonstrate any pronounced bioaccumulation. At the same time, toxicological assessments in animals at high doses have given German authorities grounds to submit a proposal to ECHA for a harmonized classification of TFA as a substance "toxic for reproduction, cat. 1B", as well as PMT/vPvM (persistent, mobile and toxic). Regulators emphasize: this is a hazard classification, not a statement of the current risk to the population - the actual levels of exposure remain decisive. In parallel, UNEP is updating its findings on the background growth of TFA and potential ecosystem effects.

The biggest "blind spots" are the source balance and the ocean budget of TFC: estimates hint at large reserves in seawater that are not fully explained by known emissions, and at possible additional atmospheric precursors. But for practice, something else is more important: land levels are rising and are linked to human activity, and removing TFC from water using standard methods is difficult. Therefore, the consensus course now is broad monitoring, targeted "closing of taps" (of F-gas leaks and precursor decay), and reassessment of standards where growth is especially noticeable.

Where does TFU come from: a short map of sources

TFC enters the environment in several ways. On the ground, it comes from direct emissions from chemical plants and the breakdown of precursors - some pesticides, drugs and PFAS polymers in landfills and wastewater. And in atmospheric precipitation, TFC comes mainly from fluorinated refrigerant gases and heat-insulating gases (the so-called F-gas), which are destroyed in the lower atmosphere, forming TFC; leaks occur during the operation and disposal of equipment and materials. The history of interest in the molecule began after the Montreal Protocol: "replacements" of ozone-depleting CFCs unexpectedly provided a chemical path to TFC.

Evidence of accumulation: where and how much growth is noticeable

Independent recordings in different environments show that TFU does indeed accumulate:

• Forests of Germany - fivefold to tenfold growth in needles and leaves of individual species in ~40 years.
• Groundwater in Denmark - a retrospective of 113 monitoring wells shows a steady increase in TFC over 60 years, associated, among other things, with the contribution of F-gases.
• Canadian Arctic ice cores show increases since the late 1960s; TFCs appear there before HFC/HFO refrigerants became widespread, suggesting additional atmospheric precursors (e.g., inhalational anesthetics). Similar records are now being published for Antarctica.

Is TFU dangerous to health and ecosystems?

Classic PFAS are of concern because of their bioaccumulation and long-lived CF bonds. TFA is an ultra-short-chain variant: it is very water-soluble and is quickly excreted in urine, so it does not accumulate in the human body as much as the “long” PFAS. Early toxicology studies indicated low acute toxicity, but new data are ambiguous: laboratory studies on animals used by European agencies have linked high doses of TFA with reproductive toxicity (smaller fruit weights and defects), although the levels at which this was shown are orders of magnitude higher than those currently found in drinking water. In parallel, ecotoxicologists are concerned about plants: TFA is easily absorbed by roots and remains in tissues without evaporating with moisture. The bottom line today is that the risk to humans at current concentrations is assessed as low, but ecosystem effects and potential background growth require monitoring.

Why is TFU controversial: is it a PFAS or a “special case”

Some scientists and regulators call TFA a PFAS in essence (CF skeleton and stability), others object: the molecule is too small, is quickly eliminated and does not accumulate in tissues - therefore, it cannot be measured by the same ruler as long-chain "forever chemicals". The dispute is not academic: if TFA is recognized as a "regular PFAS", it will fall under strict control regimes. In 2024-2025, Germany sent a dossier to the European Chemicals Agency (ECHA) proposing to classify TFA as a reprotoxic substance (Cat. 1B) and as vPvM/PMT (very persistent/very mobile; persistent/mobile and toxic). ECHA opened public comments; the result will affect the entire chain - from refrigerants to pesticides and pharmaceutical intermediates.

What do international assessments say?

The UNEP panel, which has been tracking the effects of the “coolant switch” since the 1990s, had long considered the risk from TFCs to be minimal, at least until 2100, but member states requested a reassessment in 2024-25, taking into account new trends. The updates emphasize that even if some TFCs are naturally present in the oceans, this does not justify adding anthropogenic amounts, and on land, the increase in concentrations is already difficult to deny.

"Blank Spots" of TFU Chemistry: Where Does So Much of It Come From?

The paradox of the 2000s: measurements in the Atlantic and Southern Oceans hinted at very large reserves of TFC in seawater that are poorly explained by known emissions. Hence the hypothesis of “natural” oceanic TFC – but no convincing mechanism for its natural formation has yet been proposed, and extrapolations from a few points to the entire ocean are subject to criticism. The practical conclusion remains the same: land levels are rising due to human activity, and they are the ones that need to be monitored and limited, regardless of the “ocean mystery.”

What to do: a roadmap of actions

For regulators and industry:

• Close the "holes" in the F-gas cycle - fewer leaks during operation and disposal of equipment; accelerated replacement of gases most prone to producing TFU during decay.
• Precursor accounting - review of registrations for pesticides and pharmaceutical compounds that degrade to TFCs, with realistic assessment of rates and contributions.
• Water standards and monitoring - clarification of thresholds in drinking water and regular 'long series' of observations in vulnerable regions.

For science:

• Source balance - quantitatively separate the contributions of F-gases, pesticides, pharmaceuticals and waste.
• Ecosystem effects - chronic tests on plants/soils at realistic concentrations; transport and transformation in agricultural landscapes.
• Removal methods - TFA is poorly captured by classic filters; technologies aimed at ultra-short PFAS are needed.

What everyone can do:

• Properly dispose of equipment with refrigerants (air conditioners, refrigerators, heat pumps) and insulation materials - do not throw them away "as is".
• Monitor the water source and local quality reports; if necessary, use certified filters and update cartridges according to regulations.
• Maintain air conditioning equipment repair/refilling at licensed service centers to reduce leaks.

Where caution is appropriate - and where it is not

It is important to distinguish: the growth of TFU in the environment is a fact, but this does not equal an immediate danger to the health of a specific person. Most of the measured levels today are far from the doses where reproductive effects began in animal experiments. The alarm signal is different: if sources are not closed, the background will grow, and ecosystem consequences (water-soil-plants) may manifest themselves earlier than individual medical risks. This is why some scientists are calling for "insurance" in a regulatory and source-based manner.

What we don't know yet (but are already studying)

• How much TFU do pesticides and pharmaceuticals "make" relative to F-gases? Real kinetics of decay in the environment are needed.
• Is there a “natural” oceanic TFC and what is its contribution? Even if so, anthropogenic growth on land does not negate it.
• What is the ecological "pain threshold" for plant communities at chronic background levels? This is where the least data is available.

The main conclusion

TFA is a small, stubborn molecule with strong CF bonds that is expanding in our water cycle. The current scientific consensus: vigilant monitoring + targeted source control is a smart strategy in the face of growing trends and an incomplete risk picture.

Source: Nature review “There's a new acid in our rain - should we be worried?” (July 23, 2025). doi: https://doi.org/10.1038/d41586-025-02259-6