Regulating What We Cannot Yet Measure? A Scientific Perspective on Polymeric PFASs, Total Fluorine, and the PFAS Fluorine Gap

ABSTRACT Class‐based regulation of per‐ and polyfluoroalkyl substances (PFASs) requires analytical metrics that are both scientifically meaningful and operationally enforceable. The proposed EU REACH restriction uses total fluorine (TF), extractable or adsorbable organic fluorine (EOF/AOF), targeted PFAS analysis, and non‐target analysis as complementary lines of evidence. We argue that these approaches are valuable, but they do not measure the same chemical quantity. While TF is a useful conservative gatekeeper for fluorinated materials, it cannot distinguish between polymeric PFASs, inorganic fluorine, volatile precursors, or fluorinated substances that may or may not fall within broad PFAS definitions. This limitation is especially important for polymeric PFASs, including both backbone fluoropolymers and side‐chain fluorinated polymers, which can be non‐extractable, particulate, or transformable into mobile precursors such as fluorotelomer alcohols. We propose an analytical decision tree that separates bulk fluorine screening, extractable fractions, volatile and semi‐volatile PFASs, polymeric and particulate PFASs, and structurally confirmed molecular PFASs. Such an operational framework would preserve the regulatory value of fluorine screening while avoiding overinterpretation of any single metric as “PFAS content”. PFAS content should be assessed by what is there not by what can easily be measured.