Scientific rationale for the development of an OECD test guideline on engineered nanomaterial stability

Standardized methods for assessing the behaviour of engineered nanomaterials (ENMs) under relevant environmental conditions are an important part of ENM risk assessment. Existing assays, often developed for traditional chemicals, are frequently not applicable to ENMs, which present special challenges due to their particulate nature and complex intrinsic and extrinsic properties. Here we present the development of the novel OECD test guideline (TG) No. 318 for studying the "dispersion stability of nanomaterials in simulated environmental media". We discuss the rationalization of the test design and required simplifications to develop a test, which can be executed in standard laboratories on a routine basis at reasonable costs. The relevance of the test for capturing ENM stability in surface waters is ensured by a strategic selection of adequate test media and testing scheme. As an example, we present data of a full test performed according to the new OECD TG using NM 105 as a representative TiO2 ENM. Limitations of the test in terms of scarce kinetic information and a focus on homo-instead of heteroaggregation are discussed. The developed OECD Test No. 318 represents the first standardized assay for ENMs using an operationally defined testing scheme capable of systematically comparing different ENMs in terms of their dispersion stability under environmentally relevant conditions. It will provide crucial data to inform risk assessment and regulation and can be adapted to different types of test media if needed.