Forever Chemicals in Your Water: The Hidden Threat of PFOS & PFAS

Understanding PFASs: A Guide to Terminology, Classification, and Environmental Significance

Perfluoroalkyl and Polyfluoroalkyl Substances (PFASs) are a broad group of highly fluorinated aliphatic substances that have garnered significant global scientific, regulatory, and industrial attention due to their widespread presence in the environment, wildlife, and humans. A 2011 article by Buck et al. aimed to unify and harmonize communication about PFASs by providing clear, specific, and descriptive terminology, names, and acronyms, particularly emphasizing long-chain perfluoroalkyl acids, substances related to them, and their alternatives. This harmonization is crucial because inconsistencies in terminology have led to confusion in the scientific literature.

What are PFASs? PFASs are defined as organic compounds containing one or more carbon atoms where all hydrogen substituents (from nonfluorinated analogues) have been replaced by fluorine atoms, ensuring the presence of the perfluoroalkyl moiety CnF2n+1–. The article further breaks down PFASs into two main categories:

• Perfluoroalkyl substances: Aliphatic substances where all hydrogen atoms attached to carbon atoms in the notionally derived nonfluorinated substance have been replaced by fluorine, except those that would alter functional groups.

• Polyfluoroalkyl substances: Aliphatic substances where hydrogen atoms attached to at least one, but not all, carbon atoms have been replaced by fluorine, always containing the perfluoroalkyl moiety CnF2n+1–. These substances have the potential to transform into perfluoroalkyl substances.

  
  

Why are PFASs of concern? Since 1950, PFASs have been extensively used in various industrial and commercial applications due to the extreme strength and stability of the carbon-fluorine bond, which confers hydrophobic and lipophobic properties. Applications include textile stain and soil repellents, grease-proof food-contact paper, processing aids for fluoropolymer manufacturing, coatings, and aqueous film-forming foams (AFFFs) for firefighting.

However, their widespread use has led to broad environmental contamination, with substances like perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) detected globally in wildlife and human blood. The global regulatory community is particularly interested in "long-chain" PFASs, which are defined as:

• perfluoroalkyl carboxylic acids (PFCAs) with eight carbons and greater (i.e., with 7 or more perfluorinated carbons).

• perfluoroalkane sulfonic acids (PFSAs) with six carbons and greater (i.e., with 6 or more perfluorinated carbons). These "long-chain" compounds are of greater concern because they have been shown to be more bioaccumulative than their short-chain analogues.

  
  

Concerns about their environmental and toxicological impact have led to significant actions, including:

• Phase-out of PFOS and PFOA production by major manufacturers.

• Stewardship agreements to reduce and eliminate PFOA and related chemicals.

• Inclusion of PFOS in the Stockholm Convention on Persistent Organic Pollutants.

• Development of alternative PFASs intended to replace long-chain compounds.

  
  

Manufacturing Processes and Isomer Differences The sources identify two main production processes for introducing perfluoroalkyl moieties into organic compounds, which also determine the types of byproducts (isomers and homologues) formed:

• Electrochemical fluorination (ECF): This process involves electrolysis of an organic raw material, resulting in a mixture of linear (approximately 70–80%) and branched (20–30%) perfluorinated isomers and homologues. PFOS and PFOA historically originated from ECF.

• Telomerization: This process typically produces primarily or exclusively linear perfluoroalkyl chains when linear starting materials are used. Key raw materials produced include perfluoroalkyl iodides (PFAIs) and fluorotelomer iodides (FTIs), which are then used to create a family of fluorotelomer-based surfactant and polymer products.

  
  

Families of PFASs The sources categorize environmentally relevant PFASs into nonpolymers and polymers:

• Nonpolymer PFASs:

  • Perfluoroalkyl acids (PFAAs): This prominent family includes PFCAs (e.g., PFOA), PFSAs (e.g., PFOS), perfluoroalkane sulfinic acids (PFSIAs), and perfluoroalkyl phosphonic/phosphinic acids (PFPAs/PFPIAs). Many are highly persistent and are either directly emitted or formed from the degradation of precursor substances.

  • Fluorotelomer-based products: These originate from perfluoroalkyl iodides (PFAIs) and include compounds like fluorotelomer alcohols (FTOHs), fluorotelomer acrylates (FTACs), fluorotelomer methacrylates (FTMACs), and polyfluoroalkyl phosphoric acid esters (PAPs). Their degradation is a significant potential source of PFCAs in the environment.

  • Perfluoroalkane sulfonamido derivatives: These are derived from perfluoroalkane sulfonyl fluorides (PASFs) and include N-alkyl perfluoroalkane sulfonamides (FASAs) and sulfonamido ethanols (FASEs). Many of these can degrade to form PFOS.

• Fluorinated Polymers: These include:

  • Fluoropolymers: Polymers with a carbon-only backbone and fluorine atoms directly attached, like polytetrafluoroethylene (PTFE). Historically, their manufacture sometimes required PFASs like PFOA as processing aids.

  • Perfluoropolyethers (PFPEs): Polymers with an ether backbone and fluorine atoms directly attached, used as functional fluids, surfactants, and surface protection products.

  • Side-chain–fluorinated polymers: Polymers with nonfluorinated backbones but with polyfluoroalkyl (and sometimes perfluoroalkyl) side chains. These include fluorinated acrylate polymers and fluorinated urethane polymers, used for water, stain, and grease repellency. The extent to which these polymers break down to PFAAs in the environment over long periods is a subject of ongoing research.

    
    

The degradation of precursor substances, whether industrial, environmental, or metabolic, is a key pathway for the formation of long-chain perfluoroalkyl carboxylic or sulfonic acids. For instance, FTOHs can aerobically biodegrade into PFCAs, and perfluoroalkane sulfonamidoethanols can degrade to PFOS.

The overarching goal of the terminology provided in the sources is to promote a sound, unified understanding among all stakeholders—scientific, regulatory, and industrial—to facilitate meaningful and consistent communication regarding PFASs.