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RIVM on Advanced Materials, October 2024

Environment

Several OECD guidelines for testing chemicals have been updated to include nanospecific considerations, which are generally effective. Despite these efforts, challenges remain in standardising the testing of nanomaterials in water, as particles tend to clump together, leading to unstable dispersions and irreproducible results. Additionally, the nanospecific test guidelines may not always be suitable for advanced materials, as demonstrated by issues encountered when testing functionalised cellulose nanofibers. Consequently, guidelines may need case-by-case adjustments for different materials.

Harmonised testing of chemicals

To enable internationally harmonised risk assessment of chemicals, research groups and regulatory bodies have spent many years creating and approving guidelines for standardised testing of chemicals, such as the ISO standards and OECD guidelines. For example, the OECD Test Guidelines form an important basis of the Mutual Acceptance of Data which reduces duplication of testing.

Updating guidelines for nanomaterials

In recent years, work has been done to update the chemical testing guidelines to include specific considerations for nanoparticles. This is necessary because nanomaterials behave differently in water and soil than soluble substances. An example can be found in OECD Guidance Document 317, which provides instructions for nanospecific testing in aqueous and sediment systems. This nanospecific guidance intends to improve the testing of particles.

Standardised testing of nanomaterials in water remains challenging

Despite the significant work on nanospecific testing guidelines, experts have recently reported several challenges in standardised testing of (nano)particles in water. They found that despite the updated guidance stable dispersions of particles are difficult to maintain in test systems. Particles tend to clump together in an aqueous environment, or may (partly) dissolve, leading to an unstable dispersion. Testing the toxicity of such a dispersion inevitably leads to irreproducible results, which the regulatory authorities may not accept. The authors, therefore, propose quantifying the particle behaviour during exposure. However, this approach requires additional effort and cost.

Nanospecific guidelines are not always suitable for advanced materials

Another group of researchers recently reported that specific problems also occur when testing advanced materials, in this case a functionalised cellulose nanofiber. The presence of algae made it difficult to determine the nanofiber concentration. Although the authors found a solution, this example highlights the limitations of indiscriminately applying nanospecific OECD test guidelines to advanced materials.

Reflections by RIVM

Various caveats still hinder a proper standardisation of nanomaterials’ fate and toxicity testing. The publications indicate that for every type of particle being tested, whether a nanomaterial or an advanced material, the test setup in guidelines needs to be evaluated to see if any adjustments are required. As indicated in Guidance Document 317 and the papers described here, this may require additional controls and/or monitoring the particle behaviour during exposure, which will require additional effort and cost. Currently, OECD is also working on a revision of Guidance Document 317 (Project 2.71(PDF)) to provide further nanospecific guidance for the acute testing of algae (Test Guideline 201), daphnia (Test Guideline 202), and fish (Test Guideline 203)