Dustiness testing probes for the propensity of a powdery material to release dust particles following agitation. For high aspect-ratio materials like nanotubes, the most important dust fraction is that of potentially carcinogenic fibres (WHO-fibres). We developed the fluidizer particularly for fibres that makes use of vibro-fluidization in order to effectively disentangle single fibres and agglomerates of multi-walled carbon nanotube powders. Counting rules for morphological characterization of collected particles by means of electron microscopy were established, allowing quantifying the WHO-fibre fraction. Dustiness tests on 15 different multi-walled carbon nanotubes were performed using two different levels of energy input for each material. The fluidizer accomplished bubbling fluidization for 13 out of the 15 multi-walled carbon nanotubes, resulting in continuous aerosol generation with stable particle number concentration and a high fraction of single fibres. Dustiness measurands like average particle number concentrations varied by three orders of magnitude. Morphological characterization of particles on aerosol samples proved to be essential to overcome the limitations of the applied aerosol instruments in quantifying the WHO-fibre fraction, therefore allowing material ranking based on associated risk. The materials showed strong ordering discrepancy when ranked based on total dustiness and WHO-fibre dustiness. Several multi walled carbon nanotubes showed WHO-fibre concentrations high enough to potentially cause workplace exposure at hazardous concentration levels in case powders are handled carelessly. (C) 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.