Options for an environmental risk assessment of intentional and unintentional chemical mixtures under REACH: the status and ways forward

It is acknowledged that a variety of chemicals enter the environment and may cause joint effects. Chemicals regulated under the European Chemicals Regulation REACH are often part of formulated mixtures and during their processing and use in various products they can be jointly released via sewage treatment plants or diffuse sources, and may combine in the environment. One can differentiate between intentional mixtures, and unintentional mixtures. In contrast to other substance-oriented legislations, REACH contains no explicit requirements for an assessment of combined effects, exposures and risks of several components. Still, it requires ensuring the safe use of substances on their own, in mixtures, and in articles. The available options to address intentional as well as unintentional mixtures are presented and discussed with respect to their feasibility under REACH, considering the responsibilities, communication tasks and information availability of the different actors (registrants, downstream-user and authorities). Specific mixture assessments via component-based approaches require a comprehensive knowledge on substances properties, uses, fate and behaviour, and the composition of the mixture under consideration. This information is often not available to the responsible actor. In principle, intentional mixtures of known composition can be assessed by the downstream-user. But approaches have to be improved to ensure a transparent communication and sound mixture assessment. In contrast, unintentional mixtures appear to be better addressable via generic approaches such as a mixture allocation factor during the chemical safety assessment, although questions on the magnitude, implementation and legal mandates remain. Authorities can conduct specific mixture risk assessments in well-defined and prioritized cases, followed by subsequent regulatory measures. In order to address intentional and unintentional mixtures within the current REACH framework, legal mandates together with guidance for the different actors are needed. Furthermore, further data on mixture compositions, uses and co-exposures need to be made accessible via shared databases.