Per- and Polyfluoroalkyl Substances (PFAS)

Per- and polyfluoroalkyl substances (PFAS) are a large class of synthetic organic chemicals containing characteristic carbon groups bonded to fluorine. They are defined primarily by their molecular structure, rather than by a particular use, toxicity level or environmental concentration.

The structural feature that defines PFAS

Under the broad terminology recommended by the OECD, a PFAS contains at least one fully fluorinated methyl or methylene group, commonly represented as -CF3 or -CF2-. In these groups, the relevant carbon atom has no hydrogen, chlorine, bromine or iodine attached to it.

• In perfluoroalkyl substances, the alkyl carbon framework is fully fluorinated: fluorine replaces all hydrogen atoms attached to that framework.
• In polyfluoroalkyl substances, the alkyl framework is partially fluorinated, with a fully fluorinated portion and other carbon positions that are not fully fluorinated.
• PFAS therefore comprise many different molecular structures, including non-polymeric substances and certain polymers; they are not one chemical.

Why their chemistry matters

The strong carbon-fluorine bond helps explain the chemical stability of many PFAS. Fluorinated portions of their molecules can also confer useful resistance to water, oil and heat.

• Many PFAS resist environmental degradation, while some precursor substances can transform into other persistent PFAS.
• Their persistence has earned them the informal label forever chemicals, but this does not mean that every PFAS behaves identically.
• Water solubility, mobility, bioaccumulation and toxicity vary with molecular structure; these properties cannot be assumed to be uniform across the entire class.

Representative substances and environmental relevance

Well-known non-polymeric examples include perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). Polymeric members include polytetrafluoroethylene (PTFE), illustrating the breadth of the class.

• PFAS have been used in firefighting foams, water- and stain-resistant treatments, non-stick coatings and industrial processes because of their specialised chemical properties.
• Manufacturing, product use and waste disposal can release PFAS into air, soil and water.
• Environmental concern arises from the persistence and widespread distribution of many PFAS, together with adverse health effects associated with exposure to certain members of the class.
• Calling a substance a PFAS identifies its structural class; assessing its environmental risk additionally requires information about exposure and the properties of that particular substance.