PFAS – per- and polyfluoroalkyl substances – are often referred to as “forever pollutants.” They are man-made chemical compounds used in thousands of everyday products: from non-stick coatings to waterproof fabrics, from food packaging to cosmetics, up to industrial and medical applications. Their strength is also their problem: they are extremely stable and persistent.
This persistence causes PFAS to accumulate in the environment, water, soil and the human body. Numerous epidemiological studies associate exposure to these substances with elevated cholesterol, reduced response to vaccines and an increased risk of certain tumors, particularly kidney and testicular, in the most exposed populations.
The PFAS family is very vast: tens of thousands of theoretical chemical structures are estimated, while those actually produced and used are still several thousand. And it is precisely this breadth that makes an apparently technical question central: what exactly do we define as PFAS?
The answer is not neutral. It depends on which substances will be monitored, regulated and reclaimed. And which ones, however, risk remaining off the radar.
The OECD definition: a scientific fixed point
In 2021, the Organization for Economic Co-operation and Development (OECD) adopted a definition of PFAS based solely on chemical structure. A work that is the result of a transparent process that involved university researchers, regulatory authorities and representatives of the chemical industry.
According to the OECD definition, PFAS is any substance that contains at least one fully fluorinated methyl or methylene carbon atom, i.e. free of hydrogen or other halogens. In practical terms, PFAS includes any molecule that contains at least one group: –CF₃ (perfluorinated methyl); –CF₂– (perfluorinated methylene).
This is a clear, structural and easily applicable criterion, designed precisely to avoid the ambiguities that in the past had excluded some fluorinated substances purely due to definitional limits.
The attempt to narrow the field
A group of 20 international scientists, with specific expertise in chemistry and regulation, recently expressed strong concern about attempts to introduce an alternative and more restrictive definition of PFAS, also discussed in fields related to IUPAC, the International Union of Pure and Applied Chemistry.
According to the authors, these attempts are not motivated by new scientific evidence, but by political and economic considerations. The risk is that a narrower definition, if endorsed by a reference scientific body, could provide a sort of technical legitimacy to less stringent regulatory policies.
The categories that some proposals would aim to exclude include: fluorinated gases (F-gas), trifluoroacetic acid (TFA), fluorinated polymers. According to the OECD definition, however, all these substances are PFAS.
Because these exclusions are controversial
Many F-gases contain at least one fully fluorinated carbon and can persist in the environment or degrade to form TFA. TFA, in turn, is one of the smallest and most persistent PFAS, now widespread in waters.
Fluorinated polymers, often excluded from the regulatory debate due to the lack of evidence of toxicity during use, still remain PFAS from a chemical point of view: the classification does not depend on the immediate danger, but on the molecular structure.
For the signatory scientists, excluding these categories is not a scientific choice, but a political decision.
Chemical definition and regulatory choices
A central point of the debate concerns the distinction between definition and regulation. The OECD definition only establishes what a PFAS is from a chemical point of view; it does not say how these substances should be regulated.
It is up to governments and institutions to decide on any exemptions, derogations or specific areas of application, as already happens in various regulatory contexts. But, the authors point out, bending the chemical definition to justify political choices is misleading.
The risk of generating confusion
The introduction of alternative definitions could lead to confusion at an international level, with inconsistent and contradictory regulations between countries. A lack of harmonization would also make monitoring activities more difficult, such as the use of total PFAS analysis methods, which are now essential for a more complete assessment of contamination. Regulators, businesses, citizens and the environment would be affected.
According to scientists, there is no evidence that the OECD definition is wrong or problematic. On the contrary, it represents a solid and shared basis for tackling a complex problem.
