17 Dustiness of carbon nanotubes as a prerequisite for inhalation toxicology studies: influence of morphology and surface functionalization

Abstract Handling of carbon nanotube (CNT) powders represents a plausible exposure scenario throughout their life cycle. To broaden their industrial and technological applications, CNTs are often functionalized with hydroxyl or carboxyl groups to improve hydrophilicity, dispersibility, and processability. However, due to the diversity of experimental models, it remains uncertain whether functionalized CNTs are more, less, or equally toxic than pristine materials. Before conducting inhalation toxicity studies, it is therefore necessary to evaluate whether such surface modifications influence their ability to generate airborne particles (“dustiness”), a key parameter for aerosol generation and exposure assessment. Twelve multi-walled CNT powders were characterized using the Vortex Shaker method (EN 17199-5, 2019), which simulates aerosol generation through mechanical agitation with controlled airflow. Four morphologies (short/thin, long/thin, short/thick, long/thick) were investigated, each available in pristine, hydroxylated, and carboxylated forms. Mass- and number-based dustiness indices were determined for the respirable fraction, together with particle size distributions. Mass-based dustiness indices were of similar magnitude across all samples (2.5 to 7 × 10³ mg/kg). Number-based indices were slightly higher for CNTs with larger diameters (>10⁵ #/mg versus ∼5 × 10⁴ #/mg), whereas surface functionalization had no significant effect. Aerosols generated from short CNTs were dominated by micrometric particles (mode ≈ 2 µm), while longer CNTs produced more submicrometric particles (mode ≈ 0.1 µm). These findings indicate that CNT morphology, rather than surface chemistry, primarily governs dustiness. They provide critical input for exposure assessment and for generating aerosols meeting OECD inhalation testing criteria, which require a mass median aerodynamic diameter ≤ 2 µm; a condition particularly challenging for thin CNTs.