Derivation of guidance values under the EU Drinking Water Directive : the case study of trifluoroacetic acid.

Trifluoroacetic acid (TFA) is a metabolite of e.g., pesticides, refrigerants and propellants, and it is considered a 'non-relevant metabolite' of pesticides under the EU Drinking Water Directive (DWD) as current exposures have not been found to provide a consumer health risk. Since an EU-harmonised concept for the derivation of drinking water guidance values is unavailable, various guidance values have been proposed or implemented on the Member State level for TFA in drinking water. This causes uncertainty with respect to the adequacy of such values. To address this uncertainty, the scientific rationale underlying the different guidance values is reviewed here. It is further shown that TFA should not be included in the DWD definitions for per- and polyfluoroalkyl substances; this is relevant for one of the suggested guidance values. Finally, a science-based approach for deriving guidance values for substances in drinking water is proposed. Application of this approach to TFA as an example yields a health-based guidance value of 294 µg/L TFA in drinking water. To foster harmonisation and acceptance across agencies, this guidance value should be complemented with appropriate risk management measures (such as introduction of an upper limit for hygienic reasons); however, such further considerations are outside the scope of the present article. All stakeholders are encouraged to align on an EU-harmonised approach to derive scientifically justified and robust guidance values for substances in drinking water to adequately protect consumers.