Scientific considerations on the REACH RMOA for carbon nanotubes: Toward a tiered evaluation framework

The German Federal Institute for Occupational Safety and Health (BAuA), through an Annex XV Risk Management Option Analysis (RMOA), has proposed a precautionary regulatory framework for fibre-form substances, including carbon nanotubes (CNTs). This framework is intended to support evaluation and potential risk management considerations through an initial screening step informed primarily by morphological and dimensional parameters. This article evaluates the scientific basis of this approach from a regulatory toxicology perspective, focusing on in vivo hazard characteristics following inhalation exposure. While morphology-based screening may serve as a precautionary tool for hazard identification, reliance on such parameters alone for carcinogenic classification may not fully capture the biological complexity and heterogeneity of CNTs. International evaluation frameworks emphasize pulmonary retention (biopersistence), degradation and biological fate, and inflammation-mediated mechanisms as central determinants of hazard. Recent evidence further indicates that CNTs with similar morphologies may exhibit substantially different in vivo behavior and hazard potency. On this basis, a tiered evaluation framework is discussed that combines morphology-based initial screening, a weight-of-evidence assessment integrating existing in vivo and relevant non-animal data for hazard characterization, and risk-based regulatory decision-making. Such an approach may support scientifically robust, transparent, and proportionate evaluation under REACH.