Aquatic toxicity and bioaccumulation of sparingly soluble manufactured particulate substances

Background

In 1996 ECETOC published Monograph No. 26 which reviewed the difficulties of sparingly soluble substances in aquatic toxicity assays. This work helped establish the current paradigm in aquatic environmental assessment, accepted by authorities globally, that the dissolved molecule represents the most relevant exposure condition for aquatic toxicity testing and that testing above the solubility limit does not help to inform environmental risk. The increasing focus on nanomaterials and microplastics in the environment over the last decade is challenging this paradigm and has inspired debate regarding the adequacy of existing aquatic testing frameworks for substances, which are now realised to have potential for emission and transmission in the aquatic environment in varied undissolved / particulate states. The majority of published research that has investigated the potential ecotoxicity of nanomaterials has employed procedures involving exposure to substances above the solubility limit and in the presence of undissolved substance. Relatively little attention is given to defining the physical states associated with known or expected exposure pathways, and the distinction between intrinsic toxicity and physical effects associated with those relevant physical states. The latter is a fundamental requirement in regulatory aquatic toxicology studies.

The need for guidance on when and how to test particulates in aquatic toxicity tests has re-emerged because of the apparent bioavailability of some nanomaterials. Several current activities at the OECD are focused on developing guidance for aquatic hazard and bioaccumulation testing of nanomaterials. Industry is sparsely represented in these fora and there is a risk that this guidance will be inconsistent with currently accepted procedures for "conventional” substances. This inconsistency can have major consequences for the chemical industry because there is no single regulatory definition of a nanomaterial, distinguishing it from a conventional particulate substance, which is globally accepted. In Europe, the EC has issued a risk-neutral definition of nanomaterial of such a wide scope that it could encompass most solid particulate substances, regardless of manufacturing intent. If the OECD adopts new test strategies and recommendations for aquatic tests with nanomaterials, there is a risk that existing studies for poorly soluble substances, deemed nanomaterials by some subsequently applied definition, may be considered insufficient for characterising exposure and risk; and new studies following nanomaterial testing recommendations would be requested.