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Nov 02, 2023

Could the world go PFAS

XiaoZhi Lim is a freelance writer in Singapore. You can also search for this author in PubMed Google Scholar Illustration by Adrià Voltà You have full access to this article via your institution. This

XiaoZhi Lim is a freelance writer in Singapore.

You can also search for this author in PubMed Google Scholar

Illustration by Adrià Voltà

You have full access to this article via your institution.

This February, the European Chemicals Agency (ECHA) in Helsinki published a proposal that could lead to the world’s largest-ever clampdown on chemicals production. The plan, put forward by environmental agencies in five countries — Denmark, Germany, the Netherlands, Norway and Sweden — would heavily restrict the manufacture of more than 12,000 substances, collectively known as forever chemicals.

These chemicals, per- and poly-fluoroalkyl substances (PFASs), are all around us. They coat non-stick cookware, smartphone screens, weatherproof clothing and stain-resistant textiles. They are also used in microchips, jet engines, cars, batteries, medical devices and refrigeration systems (see ‘‘Forever chemicals’ in Europe’).

Source: ECHA

PFASs are extraordinarily useful. Their fluorine-swaddled carbon chains let grease and water slide off textiles, and they protect industrial equipment from corrosion and heat damage. But their strong carbon–fluorine bonds cannot be broken apart by natural processes. So after PFASs escape from factories, homes and vehicles into the environment1, they add to a forever-growing pollution problem. The February proposal estimates that tens of thousands of tonnes of these chemicals escape annually in Europe alone.

Several PFASs are now known to be toxic. They have been linked to cancers and damage to immune systems, and are now banned under national and international laws. Most PFASs, however, have not yet undergone toxicology assessments or been linked to health harms. But officials at the agencies that submitted the plan to the ECHA say their persistence means they will inevitably build up until as-yet unknown safe thresholds are crossed.

“We see that there is an unacceptable risk now,” says Richard Luit, a policy adviser at the Dutch National Institute for Public Health and the Environment in Bilthoven.

There’s no prospect of an instant ban. The ECHA is consulting on the idea before it takes a position. European legislators are unlikely to have a plan to vote on before 2025, and even the current proposal offers grace periods — of more than a decade in some cases — to allow manufacturers to develop alternative materials or systems. Several permanent exemptions are also offered (including for fluorinated drugs, such as Prozac, and for materials used to calibrate scientific instruments).

But taken as a whole, the idea is to shrink PFAS use to a minimum. “We are asking society to make quite a shift,” says Luit. “We are asking to reverse all of it, go back to the drawing table and invent alternative solutions.”

Change is already under way for consumer use of PFASs. The notoriety of the toxic examples has pushed more than 100 companies and brands, including Apple, to pledge to phase out PFASs, even before it’s clear whether other materials can do the same job.

For industrial users, however, the idea of life without PFASs is a more shocking prospect. So February’s proposal has ignited debate about which uses of fluorinated chemicals the world could leave behind — and which must stay.

A peculiarity with fluorinated compounds, researchers say, is that some kill, whereas others are safe enough for use in medical products. “Fluorine compounds are really, really, incredibly strange in this regard,” says Mark McLinden, a chemical engineer at the US National Institute of Standards and Technology in Boulder, Colorado. “Certain fluorine compounds are incredibly toxic. And then you have things like [the gas] R134a, which is benign enough that you’re shooting it directly into your lungs in asthma inhalers”.

Forever chemicals come in three distinct forms (see ‘Fluorinated world’). The notoriously toxic kinds are fluorosurfactants. These molecules resemble those in soap, made of two parts: carbon chains with fluorine atoms wrapped around them, that repel everything, and a water-loving portion at one end of the chains that allows the molecules to dissolve in water.

After some of these molecules were linked to serious health harms and widespread water pollution, individual substances were banned or severely restricted internationally: first PFOS (perfluorooctanesulfonic acid) in 2009, then PFOA (perfluorooctanoic acid) in 2019, and, last year, PFHxS (perfluorohexanesulfonic acid). Manufacturers have moved on to other fluorosurfactants, many of which lack toxicity studies.

The February proposal suggests phasing out all the fluorosurfactants at once to avoid “regrettable” substitutions, says Jona Schulze, a staff scientist at the German Environment Agency in Dessau-Roßlau.

But the proposal goes further than that. The five agencies behind it have adopted the Organisation for Economic Co-operation and Development’s definition of PFASs: any molecule with a carbon atom in a chain that’s bonded to two fluorine atoms (or, if at the end of the chain, three). Restrictions under this expansive definition cover the other two kinds of forever chemicals.

There are the fluoropolymers, the plastic-like form that most consumers encounter. The most famous example is Teflon, or polytetrafluoroethylene (PTFE), long carbon chains wrapped in fluorine atoms. A Teflon-based coating makes frying pans non-stick; in medical products, it helps catheters to glide through the body, safeguards implants from deterioration, and, coated on the inside of bottles and blister packs, prevents drugs from interacting with their glass or foil containers. Stain-resistant textiles use a variant of this structure, in which fluorine-wrapped side chains hang off a main carbon chain.

How the US will remove ‘forever chemicals’ from its drinking water

The third category of PFASs is made up of small, light fluorocarbon molecules that generally exist as gases or liquids. R134a, the asthma-inhaler propellant, is also a common refrigerant in refrigerators and mobile air-conditioning systems, for instance. Sensitive equipment that is prone to overheating, such as servers in a data centre, can be submerged in fluorocarbon fluids that cool the apparatus without shorting its circuits or running the risk of fire.

Although fluoropolymers and fluorocarbons haven’t been shown to harm consumers directly, the problems come when they’re produced and when their useful lives end. Fluoropolymers are created using toxic fluorosurfactants, which pollute water and soil around fluoropolymer plants worldwide. Some researchers also suspect that fluoropolymers might, during their long lifetimes, shed fragments small enough to be ingested, as is known to happen with microplastics (Nature 593, 22–25; 2021). As for the fluorocarbons, some are powerful greenhouse gases, and others break up into a small-molecule PFAS that is now accumulating in water.

“If no action is taken, at some point the societal costs due to continued use are likely to exceed the costs which are now associated with their restriction,” says Schulze.

To see all three forms of PFAS in one product, look no further than cars. Their air-conditioning systems use a fluorocarbon refrigerant, the hydraulic fluids usually contain fluorosurfactant additives that prevent corrosion, the painted chassis probably has a weatherproof fluoropolymer coating, and the seats are usually covered in a stain-resistant fluorinated textile.

Electric vehicles are even more reliant on fluoromaterials because of their lithium-ion batteries. These batteries get their high energy density, and therefore range, by operating at relatively high voltages, explains Gao Liu, a chemist at Lawrence Berkeley National Laboratory in Berkeley, California. The metallic content in their cathodes is usually a powder that must be bound together with a material that can withstand the high voltage. In the 1990s, that was PTFE; today, battery makers use a cheaper fluoropolymer called polyvinylidene fluoride (PVDF), containing half the fluorine.

A lithium-battery manufacturing plant in Huaibei, China.Credit: Li Xin/VCG via Getty

Smaller fluorinated molecules have become crucial, too. Adding them to battery electrolytes allows a protective layer of lithium fluoride to form on the electrodes, improving performance and extending lifetime by preventing cracks, says Cheng Zhang, a chemist at the University of Queensland in Brisbane, Australia. This area has become a battleground for battery manufacturers, who are developing cocktails of fluorinated additives.

Liu has developed a fluorine-free binder, but it works only for a lower-voltage battery such as one based on lithium iron phosphate. These batteries do have advantages: they last longer and don’t use critical minerals such as cobalt, nickel or manganese, important factors to consider as battery production ramps up in the fight against climate change, Liu says. But even though lithium iron phosphate batteries would work for stationary storage and already power half of Chinese electric vehicles, they might not be cost-effective for long-range vehicles.

“The whole field needs to look into better chemistries,” says Liu. “The reason we switch to batteries is to protect the environment. It doesn’t make sense to invent something that’s dirtier than before.”

The push for clean energy involves fluoromaterials on another front: building the hydrogen economy. Central to this effort are electrolysers that generate ‘green’ hydrogen by splitting water, powered by renewable electricity.

The fluctuations of wind and sun favour a type of electrolyser that uses a proton-exchange membrane system (PEM). Such systems can ramp up and down quickly, unlike an older, well-established electrolyser for splitting water. As the name suggests, PEMs involve membranes that control the movement of protons (that is, positively charged hydrogen ions) between electrodes. Fluorinated materials are favoured for the membrane because they can tolerate the acidic operating conditions.

How to destroy ‘forever chemicals’: cheap method breaks down PFAS

Seeking to enter green hydrogen production, the fluorochemicals manufacturer Chemours this January announced a US$200-million expansion in France to produce more of its fluorinated Nafion membrane. (Nafion is currently used for the valuable chlor-alkali process, which splits brine into chlorine and sodium hydroxide, products that in turn are used in half of all industrial chemical processes.)

But PFASs aren’t necessary for green hydrogen: an emerging alternative to PEMs involves systems that instead move negatively charged hydroxide ions across membranes in an alkaline environment, says Benjamin Britton, a chemist who co-founded the start-up Ionomr Innovations in Vancouver, Canada. Ionomr is among firms creating non-fluorinated membranes for such anion-exchange systems2.

It could prove harder to replace Nafion in the chlor-alkali process, however: there, fluorinated membranes are better than other materials at withstanding corrosive chlorine attack. Still, some researchers are studying whether this process can work without membranes at all.

By far the largest source of PFAS emissions comprises the light fluorocarbon gases. Their main application is as refrigerants. Although ammonia, an early refrigerant, is still used for industrial applications, it was fluorinated compounds, specifically chlorofluorocarbons (CFCs), that brought air conditioning and refrigeration to the masses. That’s because, unlike ammonia, they are not irritants and they are non-flammable, says McLinden.

Air conditioning units in Mumbai, India.Credit: Kuni Takahashi/Getty

CFCs were phased out because they deplete atmospheric ozone, and were replaced by hydrofluorocarbons such as R134a. But these are greenhouse gases — and so there is an ongoing switch to hydrofluoroolefins (HFOs)3. These contain a double bond between two carbon atoms, a link that’s susceptible to attack by atmospheric compounds, which helps these molecules to break apart in weeks.

Problem solved? Not exactly. Environmental scientists and officials are now advocating the phasing out of HFOs because those molecules break up in the atmosphere to form a PFAS called trifluoroacetic acid or TFA. Karsten Nödler, an analytical chemist at the German Water Centre in Karlsruhe, says that although TFA has not been linked to any health issues, its accumulation warrants concern because it is extraordinarily difficult to remove from water. Should the time come when a clean-up is required, the only option will be reverse osmosis, an expensive technique of last resort.

Other than ammonia, the fluorine-free refrigerant options are hydrocarbons, which are flammable, or carbon dioxide, which suffers efficiency losses, especially in hot weather when cooling is needed most, McLinden says. European refrigerators already use hydrocarbons, but these substances might pose too great a fire risk in large air-conditioning systems, for example. Air conditioners for small residences have become safe enough for hydrocarbons, argues Audun Heggelund, a senior adviser to the Norwegian Environmental Agency in Oslo. The February proposal gives the air-conditioning industry 12 years to switch to hydrocarbons, but it grants a permanent exemption where safety codes prohibit the use of flammable refrigerants.

McLinden suggests that a common-sense approach is to crack down on leaks. Refrigerants operate in a closed loop — in that if they leak, the device doesn’t work. So if manufacturers could assure no leaks, any refrigerant would be fine, he argues.

The simplest but most pervasive uses of PFASs in machinery — from engines to chemical reactors — are at the interfaces between parts. Fluoropolymer greases lubricate moving surfaces, and fluoroelastomer O-rings, gaskets and seals join parts together. (Elastomers are polymers that regain their shape after being deformed.) Fluoromaterials are the only flexible ones that can resist aggressive chemical corrosion, very high temperatures and, in some applications, ultraviolet radiation, says Michael Eason, a materials engineer at James Walker, a company headquartered in Woking, UK, that manufactures high-performance sealing products. Fluoroelastomer seals are also usefully non-stick when equipment is disassembled for maintenance.

Fluoromaterials’ resistance to heat alone sets them apart from other soft materials: PTFE, for instance, can withstand a constant temperature of 260 °C for 10 years while losing only 1% of its mass, says Barbara Henry, a materials scientist at W. L. Gore, a materials-science company based in Newark, Delaware. This allows seals to last the lifetime of their equipment, for instance in an oil-well head, minimizing maintenance and therefore worker exposure to occupational hazards. It also allows machinery such as jet engines to operate at higher temperatures, and therefore more efficiently. “Because fluorinated polymers exist, every piece of equipment that’s followed a capitalist process, trying to get faster, quicker, more efficient, has adopted fluorinated materials,” says Eason.

A technician inspecting seals on an aircraft engine, which use PFASs.Credit: Operation 2021/Alamy

PTFE also protects workers in heavy industries. A thin internal layer of PTFE in multilayered textiles allows garments to remain light and breathable while providing enough heat resistance to withstand arc flashes, the explosive electrical discharges that can melt textiles on to skin. Gore has developed fluorine-free weatherproof outerwear for consumers (using expanded polyethylene), but high-performance gear still demands PTFE, says Henry.

Aware of the push to ban PFASs, however, Eason and Chaoying Wan, a materials scientist at the University of Warwick, UK, are starting a collaboration to find alternatives. A replacement that has all the properties of PTFE would be “almost impossible” to find, Eason says. But substitutes could emerge for applications where just one or two properties of PTFE are needed, although this would complicate supply chains. Eason expects that the outcome might be dozens of specialized products, whereas now a handful of fluoropolymers meet the needs of industries ranging from aerospace to pharmaceuticals to semiconductors.

Fluorochemical producers are also buoyed by the world’s race for semiconductor dominance. Last September, Chemours announced an expansion at its North Carolina facility to support domestic semiconductor production. And this year, Asahi Glass Company, a chemicals and glass manufacturer in Tokyo, also cited strong demand from the semiconductor industry when it announced a ¥35-billion ($250-million) expansion in fluorochemicals production.

PFASs are used in many ways to make computer chips. In one crucial step, manufacturers coat a silicon wafer’s surface with a ‘photoresist’ material containing PFASs: when the photoresist is illuminated, those PFASs generate strong acids that eat away at portions of the material, leaving a carefully patterned gap. In a second step, the exposed parts of the wafer are etched away — and in ‘dry etching’, a mixture of gases is used, usually containing some fluorocarbons. (Fluoropolymers are also used in a variety of microchip coatings.)

PFASs are used to help manufacture electronic components on microchips.Credit: Qilai Shen/Bloomberg via Getty

It is not easy to find alternatives to the strong acids or the etching gases. Fluorine atoms impart the necessary acidity, and fluorocarbon gases are prized for their precision in etching. The Semiconductor Research Corporation, a consortium based in Durham, North Carolina, is promoting research into ways to limit PFAS emissions and to find alternatives in the microchip industry.

In one case, companies have managed to ditch a small use of fluorosurfactants in ‘wet etching’ — processes that involve chemicals in solution. Here, fluorosurfactants helped the solutions to spread over the surfaces to be etched, says Christopher Christuk, president of electronic chemicals supplier Transene in Danvers, Massachusetts. Transene is now using fluorine-free surfactants that were identified by researchers at the University of Massachusetts Lowell (UML)4. Key support for this switch came from the Massachusetts Toxics Use Reduction Institute, a state agency funded by fees levied on businesses that use toxic chemicals, which set up the partnership between Transgene and UML and funded the research project, Christuk says.

Industries that have known nothing but fluorine chemistry need to break away from believing in its magic, says Martin Scheringer, an environmental scientist at the Swiss Federal Institute of Technology in Zurich (ETHZ). “PFASs are a block to innovation,” he says, pointing to the example of firefighting foams. Despite making foams from PFOS for decades, the multinational technology company 3M managed to create fluorine-free firefighting foam in 2002, but only after PFOS became a high-profile pollutant. Many other industries now need to make similar breakthroughs. “We need lots of materials that have not been invented that are fluorine-free,” Scheringer says.

In December, 3M announced it would stop making all its fluorochemical products — including fluoropolymers and fluorocarbon gases and liquids — by 2025, but did not say what would take their place. This June, it reached a $10-billion settlement to pay to clean fluorosurfactants from drinking water in parts of the United States, although it faces other unresolved lawsuits.

Tainted water: the scientists tracing thousands of fluorinated chemicals in our environment

For the moment, most of the funding granted to PFAS topics relates to cleaning up pollution, and neither of the huge government-funded European Union or US programmes to boost clean energy or the manufacture of semiconductor chips specify the need to find alternatives to PFASs. “We should channel more of the funding to the research that will find new solutions,” says Jonatan Kleimark, an adviser at ChemSec, a non-profit organization based in Gothenburg, Sweden, that advocates for safer chemicals.

Eason and Wan are trying to find ways to manufacture fluoropolymers without using toxic fluorosurfactants. If that can be achieved, Eason argues, it should be fine to continue using fluoropolymers where they cannot be substituted, provided that recycling at the end of their life is also resolved. But Eason recognizes the problem of persistence with fluoropolymers. “The ECHA proposal has made everyone realize they have to do something different,” he says. “In my view, a responsible company should be looking to minimize the use of fluorinated materials.”

The officials who proposed the ban say that they welcome proposals from manufacturers to extend producer responsibility and develop closed-loop systems for recycling fluorochemicals. “They have to provide the information and step forward,” says Heggelund. But he is highly sceptical, noting the low rates of plastic recycling. And if fluoropolymers could be made without toxic surfactants, then manufacturers should have done it from the start instead of reacting to regulation, he says.

The ECHA is collecting feedback on the proposal until the end of September. After that, it will revise the plan and carry out a techno-economic assessment to evaluate the costs and benefits for society.

The agency is the only one in the world contemplating such comprehensive PFAS restrictions. But enacting a ban would send a signal to the rest of the world about the acceptability of the chemicals. Zhanyun Wang, an environmental scientist at ETHZ, thinks that the proposal will spur innovative research for applications that don’t have obvious alternatives to fluorinated chemicals. And for those that do, Wang hopes the proposal and market changes that follow could act as a “lighthouse”, as he puts it: showing industries around the world how to ditch forever chemicals for good.

Nature 620, 24-27 (2023)

doi: https://doi.org/10.1038/d41586-023-02444-5

Correction 01 August 2023: The graphic ‘Fluorinated world’ incorrectly stated that TFA is a greenhouse gas. It isn’t. The graphic has now been updated.

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