Health

Updated: 5 October 2026.

Rain is hardly news in Britain. What it carries with it might be.

Follow a raindrop falling on Switzerland and, at first, there is nothing remarkable about its journey. It lands on an Alpine meadow, a vineyard, a city street or a field. Some of it runs into streams and lakes. Some simply disappears into the ground.

And that is where the interesting part begins.

As the water slowly makes its way through layers of soil and rock, it carries more than water with it. There are naturally occurring minerals, of course. But there are also traces of what we have released into the environment.

One of them has a rather unmemorable name and a remarkably memorable property.

Trifluoroacetic acid. TFA.

It is tiny, highly mobile and exceptionally persistent. Once it has entered the environment, getting it out again is anything but straightforward.

TFA is now widespread in Swiss groundwater. The Federal Office for the Environment (FOEN) detects it in precipitation virtually everywhere it looks. And in July 2026, the subject acquired a new significance when the European Food Safety Authority (EFSA) substantially revised its assessment of the potential health risks.

That sounds ominous.

It needn't be – at least not for the reason one might first assume.

Finding a substance in water is not the same thing as showing that the water is unsafe to drink. Concentration, exposure and duration matter enormously.

So, to understand what TFA actually means for drinking water, we need to look beyond the kitchen tap.

Quite a long way beyond it, in fact.

A molecule with a talent for travelling

TFA belongs to the broad family of PFAS. It is an exceptionally small, fully fluorinated molecule and dissolves readily in water.

None of those characteristics is particularly helpful if your aim is to keep it out of the water cycle.

TFA binds only weakly to soil and, under normal environmental conditions, barely degrades. While many other substances are trapped, transformed or broken down on their journey through the ground, TFA has an inconvenient habit of carrying on.

Where the water goes, TFA can go too.

Rain brings it to the surface. Water percolating through the soil carries it downwards. Eventually, it can reach groundwater.

Eventually is the important word.

According to FOEN, rainwater may take years – sometimes decades – to reach groundwater.

In other words, what we find beneath our feet today may partly be the chemical legacy of decisions made years ago.

And the reverse is equally important: substances released today may still be travelling towards our water resources long after we have stopped using them.

That is what makes TFA troublesome. Not a dramatic, immediate effect, but a particularly stubborn combination of persistence, mobility and continued environmental input.

Twenty-five tonnes a year – delivered by the weather

One figure gives a sense of the scale.

FOEN estimates that precipitation deposits around 25 tonnes of TFA on Switzerland every year.

Twenty-five tonnes.

The average concentration measured in precipitation is about 0.6 micrograms per litre, and it has roughly quadrupled since the 1980s.

So how does TFA end up in rain in the first place?

Part of the answer begins in the atmosphere. Certain fluorinated refrigerants and propellants can form TFA as they break down. Rain then brings it back to the ground.

But the atmosphere is only part of the story.

FOEN finds markedly higher concentrations beneath agricultural land. Some plant-protection products contain chemical structures capable of forming TFA as they degrade. FOEN currently lists 26 active substances authorised in Switzerland with this potential.

There are local industrial sources too. TFA can enter sewage treatment works in wastewater, only to encounter another awkward fact: conventional treatment was never designed to deal reliably with such a small and stable molecule.

Atmosphere. Agriculture. Industry.

Different routes, same destination: water.

So how much TFA is actually coming out of the tap?

This is where an important distinction tends to get lost.

In 2022 and 2023, FOEN looked for TFA at almost 550 sites in Switzerland's National Groundwater Monitoring programme, NAQUA. It found the substance across the country.

But the concentrations were far from uniform.

In Alpine areas above 1,000 metres, concentrations remained below 0.6 micrograms per litre. In catchments dominated by arable farming, values between 1 and 5 micrograms per litre were frequently measured.

At two nearby sites in north-western Switzerland affected by industrial inputs, concentrations reached 14 and 23 micrograms per litre respectively.

Those numbers deserve attention. They also need context.

Groundwater is not necessarily the water coming out of your tap.

A Swiss water supplier may draw from several groundwater abstraction points, springs or lakes. Water from different sources may be blended. Individual abstraction points may be taken out of service, and raw water may undergo treatment before distribution.

So if you want to know how much TFA is actually in a particular glass of tap water, the nearest NAQUA measurement is not enough.

The useful figure is the concentration measured in the drinking water actually supplied by the local water company.

What about bottled mineral water?

That is not an automatic escape route either. Mineral water also comes from natural water resources. Without an analysis of the particular source, there is no sound basis for assuming that it contains less TFA.

Then the benchmark moved

Until recently, TFA was discussed largely as an environmental problem.

On 22 July 2026, EFSA published a new health assessment.

And the numbers changed.

Its experts considered around 170 studies and scientific opinions, paying particular attention to changes in thyroid hormones observed in animal studies.

The most sensitive effect involved thyroxine, a hormone with important roles in metabolism, growth and development.

On the basis of that evidence, EFSA reduced the acceptable daily intake – the ADI – from 0.05 to 0.014 milligrams of TFA per kilogram of body weight per day.

It also established, for the first time, an acute reference dose of 0.07 milligrams per kilogram of body weight.

That is a substantial revision.

It is also very easy to misinterpret.

The ADI is not a drinking-water limit.

It is an estimate of the total amount a person can consume every day over a lifetime without an appreciable health risk being expected on the basis of current knowledge.

The important word there is total.

Because drinking water is not the only route by which TFA can reach us.

It can turn up on the dinner plate as well.

EFSA's new assessment does not therefore mean that Swiss tap water suddenly became unsafe on 22 July.

What it does mean is that there is now good reason to examine overall exposure more carefully than before.

The story does not end at the rim of the glass

TFA in water can be taken up by plants and enter the food chain. Switzerland's Federal Food Safety and Veterinary Office (FSVO) is currently investigating its occurrence in foods of plant origin.

That leaves an important piece of the puzzle still missing.

There is not yet a complete, up-to-date picture of how much TFA people in Switzerland consume in total through drinking water, other beverages and food.

A concentration measured in tap water therefore tells only part of the story.

From a health perspective, it is total exposure that matters.

There are scientific uncertainties too. EFSA considers it unlikely that TFA damages genetic material. However, important evidence gaps remain, including long-term studies on possible cancer development and sufficient data on effects on the developing immune system.

To account for these uncertainties, an additional safety factor of five was used when deriving the new ADI.

This matters, because scientific caution and newspaper drama are not quite the same thing.

The science is not saying:

“Nothing to worry about.”

Nor is it saying:

“Stop drinking the water.”

What it is saying is more measured – and more useful: new evidence indicates that the previous health-based benchmark was too permissive and needed to be revised.

Why not simply filter it out?

At this point, engineering appears to offer the obvious answer.

If there is something in the water, remove it.

If only TFA were quite so obliging.

Its small size and high solubility mean that many conventional water-treatment processes have little effect. Ordinary activated carbon is not a reliable barrier. UV treatment and ozonation do not provide a simple solution either.

Dense membrane processes such as reverse osmosis can, however, separate a substantial proportion of TFA from drinking water.

But separating a substance is not the same as making it disappear.

The TFA ends up concentrated in the rejected water stream.

Which rather neatly creates the next question: what do you do with that?

There are also energy requirements, capital costs, maintenance and the management of the concentrate itself.

For one household, that may be a technical choice.

For an entire public water supply, it becomes an infrastructure, economic and environmental question.

And, rather inconveniently, that brings us back to the beginning.

Once an extremely persistent substance has entered the water cycle on a large scale, removing it afterwards is considerably harder – and more expensive – than preventing it from getting there in the first place.

Perhaps we have been asking the wrong question

Not: how much filtration technology can we install?

But: how do we stop more TFA entering the environment?

Switzerland has begun to respond politically.

On 27 May 2026, the Federal Council opened a consultation on proposed changes to maximum PFAS levels in drinking water. The consultation closed on 18 September 2026 and includes closer alignment with European drinking-water regulation.

That does not, however, mean that Switzerland already has a specific maximum level for TFA in drinking water.

There has also been movement on plant-protection products. Approval for the active substances flufenacet and tritosulfuron was withdrawn in Switzerland with effect from 1 July 2025.

Other active substances capable of forming TFA nevertheless remain authorised.

And there is one last complication.

Even if every problematic input stopped tomorrow, TFA would not politely vanish from groundwater by Tuesday.

Groundwater moves slowly.

TFA stays around.

What, then, should we make of all this?

Perhaps the most useful conclusion is also the least sensational:

TFA does not lend itself to simple answers.

Its widespread presence in Swiss groundwater is not evidence that Swiss drinking water is generally unsafe.

Equally, the absence of evidence of a widespread health risk at concentrations measured so far is not a sensible reason to ignore a highly persistent substance that continues to enter the environment.

For anyone wanting to understand their own situation, there is a straightforward place to start: ask the local water supplier whether TFA is measured in the drinking water actually supplied, and what concentrations have been found.

But the larger lesson reaches beyond TFA.

Drinking water does not begin at the tap.

Its story starts much earlier: in the atmosphere, in the rain, across fields and industrial sites, and throughout the catchments feeding springs and groundwater.

Water then disappears beneath our feet.

Years later, sometimes decades later, part of it returns.

In a glass.

EFSA's new assessment is therefore not a reason for panic. It is something considerably more useful: an early warning.

Because with a chemical that persists for this long, the cleverest treatment technology is still second best.

The better solution is not to put it there in the first place.