Bioaccumulation assessment of nanomaterials cannot be done using the equilibrium partitioning method commonly used for organic chemicals. Therefore, the current guidance on bioaccumulation assessment is not suitable for nanomaterials. To address this issue, the OECD has proposed a tiered approach designed specifically for nanomaterials. This approach is tailored for metallic and metal oxide particles but can likely be extended to other advanced (nano)materials as well.

Challenges of bioaccumulation testing of nanomaterials

Bioaccumulation testing is required for regulatory chemical risk assessment, including for nanomaterials. When chemicals accumulate in an organism, it may lead to chronic effects and accumulation in the food chain. Typically, bioaccumulation is tested in fish or, more recently, in invertebrates. The octanol-water partitioning coefficient (KOW) is a valuable indicator for quantifying the accumulation potential of organic chemicals. However, nanomaterials do not accumulate according to this basic principle of equilibrium, and there is no straightforward alternative. This often leaves the fish test as the only option for regulatory compliance for nanomaterials.

An alternative approach to test bioaccumulation of nanomaterials

Risk assessors within the OECD proposed a tiered approach for testing the bioaccumulation of nanomaterials. This approach starts with collecting information on the physicochemical properties of the nanomaterial and its dissolution behaviour. By integrating data and models from testing with cell cultures, invertebrate testing, and computer modelling, it aims to reduce the need for testing on vertebrates. Moreover, bioaccumulation tests with earthworms have been found to be a good predictor of bioaccumulation in fish.

Reflections by RIVM

The proposed OECD’s tiered approach for bioaccumulation assessment is highly relevant for the regulatory risk assessment of nanomaterials. It provides essential guidance for assessing the bioaccumulation of nanomaterials. By effectively combining different types of information, the tiered approach allows for optimal use of available data.

It should be noted that the majority of current information on bioaccumulation is related to metallic and metal oxide nanomaterials. Consequently, the tiered approach is tailored to these types of nanomaterials. As more data become available, this approach can likely be adapted for organic-based and advanced (nano)materials.