A newer ‘climate-friendly’ refrigerant, that refrigerant intended to benefit the planet, to help the planet, may be causing a new problem. Scientists at the University of Bristol say a gas called HFO-1234yf could be a major source of a “forever chemical” spreading across Europe.
Car air conditioners once relied on a gas called HFC-134a. It cooled cars well, but it was also a strong greenhouse gas. To fight climate change, regulators pushed carmakers to switch to a newer gas, HFO-1234yf. Since 2017, almost every new car built anywhere in the world uses it.
The switch aimed to benefit from being a win for the environment. But a new study, published in Environmental Science & Technology Letters, suggests it may come with a hidden cost. Both gases break down in the air over time. When they do, they can form a chemical called trifluoroacetic acid, or TFA. TFA belongs to a group of substances known as PFAS, often called “forever chemicals” because they barely break down once they exist. TFA is now turning up in rivers, crops, food, and even in people.
To understand how much TFA each gas produces, the researchers built a computer model. It tracked how the gases spread through the air, how they react with sunlight and other chemicals, and how their by-products eventually settle onto land and water.
The results were striking. Currently, far more HFC-134a is released into the air than HFO-1234yf, about 22 times more. Yet the model found that HFO-1234yf could already be creating up to three-quarters as much TFA worldwide as the older gas. That is because HFO-1234yf breaks down faster and turns into TFA more efficiently.
Across parts of Europe, including Italy, Austria, Germany, Switzerland, and France, the newer gas may already be dropping more TFA onto the ground than the older one. In some places, the model shows that TFA from HFO-1234yf could be as much as 3.6 times higher than TFA from HFC-134a, even when using careful estimates that favour the older gas.
Dr Rayne Holland, who led the study, explained that when refrigerants break down, they can form other harmful substances too, including powerful greenhouse gases. She said newer gases like HFO-1234yf react quickly in the atmosphere, so they make TFA faster and in bigger amounts than older gases do.
One big source of uncertainty is precisely how much TFA the older gas, HFC-134a, produces when it breaks down. Scientists have a range of possible answers. But interestingly, the lower that number turns out to be, the more important HFO-1234yf becomes as a source of TFA. Under that lower estimate, HFO-1234yf could already be responsible for roughly three-quarters as much TFA as HFC-134a, despite being emitted far less.
The timing of this research matters. In June 2026, a European Union scientific committee recommended that TFA be classified as toxic to reproduction, meaning it could harm fertility and unborn babies. The same committee also said TFA should be treated as a chemical that is both highly persistent and extremely mobile in the environment. That means it lasts a long time and spreads easily through soil and water.
Holland said the team was surprised at how much influence HFO-1234yf already has, given how new it is. As more cars switch to this gas, its role in producing TFA is expected to grow even further.
Professor Dudley Shallcross, who supervised the study, said Europe may already be feeling more impact from the newer gas than the older one when it comes to TFA pollution. He said it is important to fully understand the environmental effects of replacement technologies so that businesses and governments can make informed choices.
The researchers also point out a bigger problem. Unlike older refrigerants, there is currently no global system in place to track or limit how much HFO-1234yf is being released into the air. Without better monitoring, it will be challenging to know exactly how serious the TFA problem could become as more cars adopt this newer refrigerant.
