Introduced to meet stringent environmental regulations, HFO-1234yf has replaced the older refrigerant HFC-134a, a potent greenhouse gas known for its contribution to climate change, now widely used in vehicle air-conditioning systems. However, findings published in Environmental Science & Technology Letters suggest that HFO-1234yf, which is in almost all new cars manufactured globally since 2017, is significantly contributing to increasing levels of the “forever chemical” trifluoroacetic acid (TFA) across Europe.
Both substances break down in the atmosphere and can form TFA, a highly persistent compound that belongs to the broader group of PFAS (‘forever chemicals’). TFA is increasingly being detected in water, plants, food, and even human samples.
To investigate how emissions of HFC-134a and HFO-1234yf break down in the atmosphere and contribute to the formation of TFA, the researchers used a global tropospheric chemistry transport model. The model simulates emissions of chemical species, their atmospheric transport, the chemical reactions that govern their fate and the loss processes, like deposition, that remove their products from the atmosphere.
Although emissions of the older HFC-134a are still much higher globally (around 22 times higher), the study found that the newer refrigerant HFO-1234yf may already be generating up to three-quarters as much TFA globally as HFC-134a and may already be responsible for greater TFA deposition across Europe. Specifically, the model predicts enhanced TFA deposition as a result of HFO-1234yf use, over Italy and parts of Austria, Germany, Switzerland, and France.
Using updated emissions estimates and atmospheric measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) network, the research team simulated present-day conditions and found that TFA produced from HFO-1234yf is concentrated near major emission regions, particularly across Europe.
The model predicts that TFA deposition resulting from HFO-1234yf degradation can be up to 3.6 times higher across parts of Europe than deposition associated with HFC-134a, even under conservative assumptions that maximise the TFA-producing potential of the older refrigerant.
Dr Rayne Holland, the study’s lead author from Bristol’s School of Chemistry, said: “There are concerns about what these chemicals turn into as they break down in the environment. When they degrade, they can form other compounds that are very powerful greenhouse gases (like HFC‑23 and CF₄) and long‑lasting pollutants such as TFA. Some refrigerants, including certain HFCs and many newer HFOs, can produce TFA when they react with substances in the atmosphere. Because HFOs react more quickly, they tend to create TFA faster and in larger amounts than HFCs.”
One of the key uncertainties is the amount of TFA formed when HFC-134a breaks down in the atmosphere. The study examined a range of plausible values and found that the lower the TFA yield from HFC-134a, the greater the relative contribution from HFO-1234yf. Under the lower-yield scenario, HFO-1234yf could already be generating around three-quarters as much TFA globally as HFC-134a despite emissions that are 22 times lower.
Rayne Holland added: “This study comes at a critical time. In June 2026, the European Chemicals Agency's Risk Assessment Committee concluded that TFA should be classified as Reproductive Toxicity Category 1B, meaning it may impair fertility and may damage the unborn child. The committee also supported classifying TFA as persistent, mobile and toxic (PMT) and very persistent and very mobile (vPvM). These developments have increased the urgency of understanding and quantifying the sources of TFA entering the environment.
“We were surprised by just how influential HFO-1234yf already appears to be. Even though emissions remain much lower than those of HFC-134a globally, its high TFA yield means that it may already be responsible for a comparable share of global TFA production. If the true TFA yield from HFC-134a lies toward the lower end of the currently accepted range, the relative importance of HFO-1234yf becomes substantially greater."
The researchers emphasise that the global transition from HFC-134a to HFO-1234yf remains in its early stages. As adoption of HFO-1234yf continues to expand and emissions increase, its contribution to TFA generation and environmental deposition is expected to grow further.
Dudley Shallcross, Professor of Atmospheric Chemistry from Bristol’s School of Chemistry and the study’s senior author, said: "Our results suggest that Europe may already be experiencing a greater influence from HFO-1234yf than from HFC-134a with respect to TFA deposition. As the transition away from HFCs accelerates, it is vital that we understand the full environmental consequences of replacement technologies and ensure that informed decisions can be made in both industrial and policy settings."
The study also highlights major uncertainties surrounding global HFO-1234yf emissions, especially as there are no current global international frameworks that monitor or control HFO production or emissions. Improved measurements of where and how much HFO-1234yf is being released will be essential for refining future estimates of TFA contamination and understanding how environmental burdens may change as the refrigerant transition progresses.
The research was funded by UK Research and Innovation (UKRI) (NERC InHALE Highlight Topic - Investigating HALocarbon impacts on 500 the global Environment, NE/X00452X/1).
Paper
‘TFA Generation and Deposition over Europe May Currently See a Greater Influence from HFO-1234yf than HFC-134a’ by R Holland et al. in Environmental Science & Technology Letters (open access)