International efforts to standardize nanomaterial toxicity testing have become increasingly critical as nanotechnology advances and its applications expand across industries. The Organisation for Economic Co-operation and Development (OECD) and the International Organization for Standardization (ISO) have taken leading roles in developing frameworks to ensure consistency, reliability, and comparability in nanotoxicology assessments. These efforts focus on harmonizing methodologies for dose metrics, dispersion protocols, and reference materials, which are fundamental to accurate risk evaluation.

The OECD’s Working Party on Manufactured Nanomaterials (WPMN) has been instrumental in coordinating international collaboration to address the unique challenges posed by nanomaterials. A key achievement is the OECD Testing Programme, which evaluates the applicability of existing toxicity testing guidelines to nanomaterials. The program has identified gaps and proposed modifications to traditional protocols to account for the distinctive physicochemical properties of nanomaterials, such as size, surface area, and aggregation state. For instance, traditional mass-based dose metrics may not be sufficient for nanomaterials, as their biological interactions often depend on particle number, surface area, or surface reactivity. The OECD recommends a multi-metric approach, incorporating these parameters to better reflect nanomaterial behavior in biological systems.

Similarly, ISO has developed standards under Technical Committee 229 (Nanotechnologies), focusing on terminology, characterization, and safety evaluation of nanomaterials. ISO/TR 13014 provides guidance on physicochemical characterization for toxicological testing, emphasizing the need for thorough material characterization before toxicity assessments. ISO/TR 16197 outlines a screening strategy for manufactured nanomaterials, advocating for tiered testing approaches to prioritize resources efficiently. Both OECD and ISO stress the importance of standardized dispersion protocols, as nanomaterials tend to agglomerate in biological media, altering their bioavailability and toxicity. ISO/TS 12025 addresses this by specifying methods for generating nanoparticle dispersions, including sonication energy, dispersant selection, and stability monitoring. These protocols aim to minimize variability between laboratories and ensure reproducibility.

Reference materials play a pivotal role in validating toxicity testing methods. The OECD and ISO have collaborated with institutions like the National Institute of Standards and Technology (NIST) and the Joint Research Centre (JRC) to develop certified reference materials (CRMs) for nanomaterials. Examples include NIST’s gold nanoparticles and JRC’s silica nanoparticles, which are characterized for size, shape, and surface properties. These CRMs serve as benchmarks for inter-laboratory comparisons and method validation. The OECD’s Sponsorship Programme for the Testing of Manufactured Nanomaterials has utilized such materials to assess the reliability of toxicity tests across multiple countries, revealing both consistencies and discrepancies in outcomes due to methodological differences.

Dose metrics remain a contentious issue in nanotoxicology. While mass concentration is a conventional metric, it may not correlate with biological effects for nanomaterials. The OECD recommends complementary metrics such as particle number concentration, specific surface area, or surface charge, depending on the hypothesized mechanism of action. For example, in vitro studies often use surface area dose metrics for high-aspect-ratio materials like carbon nanotubes, as their toxicity has been linked to surface reactivity. ISO/TS 12901-2 further elaborates on exposure assessment, advocating for real-time monitoring techniques to capture dynamic changes in nanoparticle behavior in environmental or biological matrices.

Dispersion protocols are another critical area of standardization. The OECD’s guidance document No. 318 emphasizes the need for detailed reporting of dispersion methods, including sonication parameters, medium composition, and stabilization techniques. Poor dispersion can lead to artifactual toxicity due to large agglomerates, while excessive sonication may alter particle properties. ISO/TS 20787 provides a standardized approach for preparing nanomaterial dispersions in biological media, specifying parameters such as energy input, temperature control, and duration. These efforts aim to reduce variability and improve the comparability of toxicity data across studies.

Despite progress, challenges persist in aligning international standards. Differences in regulatory requirements, testing capacities, and resource availability across countries can hinder uniform implementation. The OECD and ISO continue to address these issues through stakeholder engagement, capacity-building initiatives, and iterative updates to guidelines based on emerging science. For instance, the OECD’s Series on Nanomaterials includes ongoing revisions to reflect advances in characterization techniques and dose-response modeling. Similarly, ISO regularly updates its technical reports to incorporate new evidence and technological developments.

The harmonization of nanomaterial toxicity testing is a dynamic and collaborative process. By establishing consensus on dose metrics, dispersion protocols, and reference materials, the OECD and ISO provide a foundation for reliable risk assessment. These efforts not only enhance scientific rigor but also support regulatory decision-making, ensuring the safe development and deployment of nanomaterials globally. Continued international cooperation and adaptive standardization will be essential to keep pace with the evolving landscape of nanotechnology.