A little bit of chemistry
When you look at the periodic table of elements, you may be overwhelmed by all the information presented. To simplify things a little, it may help to think about general trends in chemical properties. One of these trends is what chemists call electronegativity, which defines how well an atom can attract electrons towards itself, more generally known as the ‘pulling power’. Electronegativities increase from left to right across a period and decrease from top to bottom of a group. Therefore, leaving the noble gases aside, the most electronegative element is fluorine.
This pulling power is the reason that the carbon fluoride (C-F) bond is one of the strongest in nature and becomes even stronger when a carbon atom is partially (poly-) or fully (per-) fluorinated. PFAS chemicals are exactly that: polyfluorinated or perfluorinated substances. As a result of these strong bonds, PFAS chemicals do not readily degrade in the environment.
The PFAS universe
In 2018, the Organisation for Economic Co-operation and Development (OECD) identified nearly 5,000 PFAS-related CAS numbers1 as part of a global database2. However, it is likely that there are many more PFAS than were identified at that time. In a recent webinar held by European Chemicals Agency (ECHA), the agency declared that, based on their own data, around 6,300 PFAS substances have been identified3.
Given this high number of individual substances, the PFAS family tree is complex. In 2017, Wang et al. published a comprehensive overview of the individual family members4. Firstly, it is important to distinguish between fluorotelomers, which may be a source of environmentally persistent PFAS (such as PFOA and PFOS), and fluoropolymers (like PTFE) are considered to be “polymers of low concern” due to certain properties, (e.g. insolubility in water, inability to cross the cell membrane) (Henry et al. (2018)5. Thus, there is an ongoing discussion if grouping fluoropolymers with other classes of PFAS is scientifically appropriate (Henry et al. (2018)5, Lohman et al. (2020)6).
The tricky part of PFAS is that, like in a real family, the members are related to each other. Therefore, some PFAS have been used to facilitate the production of others (in the early days, for example, PFOA was used in the production of PTFE) or that some PFAS degrade over time into other PFAS.
PFAS contain fully fluorinated carbon chains of various lengths attached to a functional group, like carboxylic or sulfonic acids. The most well-known PFAS, PFOA and PFOS, both contain 8 carbon atoms but have different functional groups, as shown below. Recently, other types of PFAS, including perfluoroethers (PFPEs), have also come under greater scrutiny.
PFAS have been used since the 1940s in a variety of industries, from aerospace to clothing, due to their unique physical and chemical properties. PFAS are costly to produce (for example fluorosurfactants are up to 1,000 times more expensive compared with analogous hydrocarbon- surfactants, for example) and are often used where high performance is required, such as in extreme conditions or where non-reactivity is needed.
Many PFAS applications have made our everyday lives more convenient, for example non-stick pans, anti-stain for carpets and clothing, and water repellent items like outdoor jackets.
A recent publication by Gluege et al. (2020)7 identified 21 industry branches with more than 200 uses. Looking at these, it would be reasonable to assume that the majority of industries use PFAS in one way or another: