Are short-chain polyfluoroalkyl substances (PFAS) safe substitutes for long-chains?

On 24 February and 26 February 2016 I gave lectures in Australia with the title: “Are the short-chain polyfluoroalkyl substances (PFAS) safe alternatives?” at seminars, respectively, at the CAPIM, the University of Melbourne, and at the NSW Office of Environment and Heritage in Sydney.

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PFAS, such as perfluorooctane sulfonate (PFOS), perfluorooctanoate (PFOA) and their C8-precursors, are well-known chemical hazards, and such chemicals are either already banned or close to be.

The most used alternatives are polyfluoroalkyl substances with shorter fluoroalkyl chain than C8. The industry claims that these chemical are safe alternatives, however, the report concluded that these chemicals also are persistent in the environment, accumulate long-term in human tissues and have various toxic effects as already explained shortly in a previous News article from 19 September 2015. https://nipsect.dk/new-reports-on-polyfluorinated-chemicals/

The most important short-chain perfluoroalkyl sulfonic acids are perfluorobutane sulfonic acid (C4, PFBS) and perfluorohexane sulfonic acid (C6, PFHxS). These short-chain alternatives have hundreds of precursors, such as e.g. the more complex molecules N-methyl perfluorobutane sulfonamidoethanol (MeFBSE) and N-methyl perfluorohexane sulfonamidoethyl acrylate.

The most important short-chain perfluorocarboxylic acids (PFCAs) are perfluorobutanoic acid (PFBA) and perfluorohexanoic acid (PFHxA) and their salts and precursors; including the short-chain fluorotelomers such as 4:2 FTOH and 6:2 FTOH.

Short chain PFAS

PFAS are almost completely absorbed orally and by inhalation but skin absorption is negligible. Both the short- and long-chain perfluoroalkyl chain is considered being metabolically inert; however, fluorotelomers are biotransformed into perfluorocarboxylic acids.  The excretion through the kidney is small because of resorption, especially in humans.

PFAS have contrary to other persistent organic pollutants (POPs) a low affinity to lipids, and in the blood PFAS are bound to serum proteins. The serum elimination half-lives are shorter for all short-chain PFAS but in humans PFHxS has a longer one than PFOS and PFOA. See Table:

PFAS serum elimination half-lives

When the chemicals leave the blood in humans they are not excreted but transferred to other body compartments. In the body PFAS are mainly associated to cell membrane surfaces and distributed in well-perfused tissues such as the lung, liver, kidney and spleen but also in the bone, testes and brain.

A human study from Spain (Pérez F, Nadal M, Navarro-Ortega A, Fàbrega F, Domingo JL, Barceló D, Farré M. Accumulation of perfluoroalkyl substances in human tissues. Environ Int. 2013; 59: 354-362) revealed both individual differences between 99 deceased people and differences between the tissue distribution of the PFAS, and relatively high concentrations of short-chain PFAS, especially PFBA. See Table:

Body levels

1Perfluorohexylethanoic acid – metabolite of 6:2 FTOH

In animal experiments the acute toxicity of short-chain PFAS is low. The toxicity in humans for PFHxS is close to PFOS. For the other short-chain PFAS  the available data is insufficient.

In the environment short-chain analogue has the same high persistence as the long-chain but has lower bioaccumulation potential. Increasing usage of short-chain PFAS mean increasing environmental concentrations. PFOS/PFOA and other long-chain PFAS are generally more ecotoxic than the short-chain analogues, and sulfonic acids tend to be more toxic than carboxylic acids.

 

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