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CHEMISTRY PILL

    1. Chemical features and structure, molecules and differences

    PFAS are a controversial group of chemicals that are found in a wide variety of products.

    In recent years, PFAS gained public attention due to their potential harm to the health and the environment. The danger associated with PFAS is still an active matter of studying.

    But what are the features that make PFAS both extremely useful and harmful at the same time?

    PFAS are organic aliphatic compounds; meaning that their molecule is made up of a linear carbon chain. Furthermore, PFAS are either perfluoroalkyl or polyfluoroalkyl, meaning that at least one of the C atoms of the carbonic chain they are made of is completely bonded to F atoms instead of H atoms (that means that at least one C atom only bonds with either another C or F, but not H).

    Lastly, at the end of the molecule there is a functional group that can vary according to the specific compound.

    In perfluoroalkyl substances all H atoms are replaced by F atoms, aside for those located at the end of the group.

    In this case, when the linear chain consists solely of C and F, the C atoms (those not involved in functional groups) are bound to F atoms in a molar ratio of CnF2n+1.

    On the other hand, in polyfluoroalkyl substances, at least one of the C atoms retains some of the H atoms while still being bonded to at least one F.

    PFAS are commonly divided in three classes:

    • PerFASs: perfluoroalkyl substances;
    • PoliFASs: polyfluoroalkyl substances;
    • Fluorinated polymers.

    These classes comprehend a wide number of different molecules, and can be furthermore divided.

    PFAS can be beyond classified as “long chain” and “short chain”.

    The Organisation for Economic Co-operation and Development defines that long chain must be attributed to:

    • PFCA perfluoroalkyl carboxylic acids, made up of eight or more C atoms (seven or more atoms of perfluorinated C atoms);
    • PFSA perfluorate sulphonates, made up of six or more C atoms (six or more perfluorinated C atoms).

    2. How are PFAS synthesized?

    Two main processes are involved in the synthesis of compounds that are made of perfluoroalkyl chains: electrochemical fluorination and telomerization.

    3. How are PFAS employed?

    PFAS are costly to produce, which is why they are used when other substances are unable to archive the same performances or when they can be used in much smaller quantities while maintaining the same level of performances of other non fluorinated substances.

    The authors of the paper An overview of the uses of per- and polyfluoroalkyl substances (PFAS) were able to identify at least 300 functions associated to PFASs

    There are a numerous reasons why PFAS are highly versatile and widely used; to mention a few:

    • Lowering of water surface tension;
    • Hydrophobicity;
    • Oil repellency;
    • Flame retardants propriety;
    • High stability;
    • Extremely low reactivity;
    • Electric conductivity;
    • Works at high temperatures;
    • Radiation impermeability.

    All these properties make PFAS highly employable. The same paper was able to demonstrate their application in a wide range of sectors: 87 uses in 21 industrial branches and 43 more categories.

    Among these, for example, we can find the aerospace industry, biotechnologies, construction industry, energy sector, food industry, textile industry, pharmaceutical and many more.

    Between the many uses we can cite automotive, cleaning products, paints, sport equipment, paper and packaging.

    For instance, the hydrophobic and oil-repellent properties of PFAS’ perfluorocarbonic chains make them highly effective surfactants and surface protectors.

    Why are PFAS “forever chemicals”?

    PFAS are referred to as “forever chemicals” due to their complex degradation process: to this day, PFAS can only be effectively degraded by incineration, plasma arc technology, electron transfer and chemical oxidants and reductants.

    However, these methods are non-specific, costly and, above all, imply the synthesis of toxic and reactive side products.

    The biodegradation of fluorinated substances is indeed a complex process as well: taking weeks to months, the range of molecules degraded is limited and, to date, only a small number of enzymes are known to be able to biodegrade PFAS.

    One of the reasons behind the rare PFAS biodegradation by microorganisms in nature (compared to, for example, chloride-organic substances), may be attributed to the high toxicity of the F– ions to cells.

    In fact, when the C-F bond is broken, F– is released.

    Fluoride ion cell’s toxicity depends on a variety of reasons: oxidative stress, interfere with redox homeostasis, alter genetic expression and induce apoptosis.

    Bacteria cell’s damages caused by the fluoride ions seems to be associated with its enzymes’ inhibition capacity (both essential and non-essential enzymes). For this reason, the way bacteria reacts to fluoride was extensively studied in the context of oral cavity’s bacteria, which is why fluorides are part of toothpaste formulation.

    5. Why are PFAS dangerous?

    PFAS binds with high affinity to albumin and, generally, protein which binds to fat acids, leading to tissues-specific distribution in the body.

    Additionally, PFAS ubiquitously distributes in the environment, and have the ability to magnify along the food chain.

    The bioaccumulation of PFAS varies among different organisms and species and depends on the structural properties of the molecule such as linear or branched chains, chain length and functional groups. The elimination rate also depends on the structure of the PFAS molecule.

    In biota PFOS (eight fluoro-carbons) is usually the most prevalent PFAS, and its distribution increases  along the food chain (high bioaccumulation propriety).

    On the other hand, PFOA (seven fluoro-carbon) has lower bioaccumulation properties and behaves similarly across all trophic levels. Its low bioaccumulation propriety might be related to its shorter C chain and its functional groups, that differs to PFOS’

    Human exposition to PFAS is widespread but variable. People are most likely to be exposed to PFAS by contaminated food or water consumption, use of products containing PFAS or, finally, by inhaling contaminated air.

    Due to the slow PFAS degradation, individuals or animals with repeated exposure are subject to PFAS accumulation, leading to the increase of (some) PFAS levels in the blood.

    Authors: Alice Bettio and Laura Tonolo

    Bibliography