EEAP 2022: How should TFA be regulated
Extracts from EEAP 2022 Chapter 6: [1] More recently, a majority of a panel of experts agreed that “all PFAS should not be grouped together, persistence alone is not sufficient for grouping PFAS for the purposes of assessing human health risk, and that the definition of appropriate subgroups can only be defined on a case-by-case manner.”[2]. In addition, the majority opinion with respect to toxicology was that “it is inappropriate to assume equal toxicity/potency across the diverse class of PFAS” [2].
This same argument applies to the inclusion of TFA, with a two-carbon chain and a single CF3 group, into a class with longer chain PFAS. These longer chain PFAS have key chemical, physical, and biological properties that become quite different with increasing length of the carbon-chain. For example, log KOW (a measure of partitioning between lipids in organisms and water); Henry’s Law Constant (a measure of partitioning between water and air); KOC (a measure of adsorption to soil and sediment); and the half-life in humans (related to chronic exposure and chronic toxicity) all vary with changes in the length of the carbon chain. These relationships are well recognised as they are important drivers of adsorption, distribution, and excretion in animals, which are major determinants of adverse effects.
Extracts from EEAP 2022 Chapter 6 Table 2 (page 278): Comparing key physical properties of the linear perfluorinated carboxylic acids TFA (2 carbons) and PFOA (8 carbons).
According to EEAP 2022, there has been considerable discussion as to the inclusion of TFA in the class PFAS for regulatory purposes. Regulatory agencies in North America acknowledge the physical, chemical, and biological properties of chemicals in the class of PFAS and, in particular, the influence of chain length on these properties. A sound assessment of the environmental impact of TFA needs to consider the relevant physical, chemical, and toxicological data and realistic environmental concentrations. We are of the opinion that the properties of TFA indicate that it should not be included in this class for the purposes of generic regulatory risk assessment.
Environmental persistence is one of the criteria used to identify persistent organic pollutants (POPs), such as those regulated under the purview of the Stockholm Convention. Persistence alone has been suggested to be a criterion for regulatory action. The suggested threshold for this classification is a degradation half-life > 6 months. The stability of TFA and its salts indicates a half-life >> 6 months, but our opinion [EEAP] is that persistence should only be considered as a regulatory criterion for substances that are moderately or highly toxic and/or are bioaccumulative in organisms and/or undergo trophic magnification. TFA does not bioaccumulate nor is it toxic at the low to moderate exposures currently measured in the environment or those predicted in the distant future.
TFA is a perfluorinated acid that has been included in the class of per- and polyfluoroalkyl substances (PFAS). This class of chemicals contains 4730 substances, of which about 256 are in commercial use. Even in the subclass of perfluorinated alkanoic acids, the physical, chemical, and biological properties of these substances differ widely, mostly in relation to length of the alkyl chain. EEAP 2022 states that to regulate these substances as a class (as has been suggested) is not scientifically defensible and TFA should be treated as a unique chemical for the purposes of regulation.
References
[1] Montreal Protocol on Substances that Deplete the Ozone Layer UNEP 2022 Assessment Report of the Environmental Effects Assessment Panel Chapter 6, pages 278-279 available at http://ozone.unep.org/science/eeap. All the text is reproduced from chapter 6, except for the table which uses selected data from EEAP2022 and adds difference factors and table notes.
[2] Anderson, J. K., Brecher, R. W., Cousins, I. T., DeWitt, J., Fiedler, H., Kannan, K., Kirman, C. R., Lipscomb, J., Priestly, B., Schoeny, R., Seed, J., Verner, M., & Hays, S. M. (2022). Grouping of PFAS for human health risk assessment: Findings from an independent panel of experts. Regulatory Toxicology and Pharmacology, 105226. https://doi.org/10.1016/j.yrtph.2022.105226