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Because of their unique physico-chemical properties, CNTs have attracted a great deal of research interest and have many promising industrial applications. However, this also increases the exposure potential for workers, raising the need to understand their hazard for an effective occupational health and safety management. CNTs can induce lung inflammation, granuloma formation, fibrosis and cancer in rodents; in particular, MWCNTs are known to induce in vitro markers of remodelling and fibrosis. CNTs greatly vary in length, thickness, rigidity, aspect ratio, surface defects and reactivity, with a remarkable contribution of synthesis methods and post-treatments. Thus, CNTs are not a single substance, but a heterogeneous family of materials that elicit different biological responses and, thus, are associated with different hazard levels not simply ascribable to the fibre paradigm.Cell models representative of the airway barrier were challenged with MWCNT preparations endowed with different physico-chemical properties, evaluating endpoints such as viability, expression of pro-inflammatory markers, nitric oxide production, epithelial barrier competence, clonogenic activity, genotoxicity. Epithelial-mesenchymal transition (EMT) was also assessed as an early event leading to fibrosis and, possibly, involved in neoplastic transformation.Only long MWCNTs promoted EMT and caused frustrated phagocytosis. On the other hand, MWCNT agglomeration led to contact-mediated focal epithelial damage and impaired barrier functionality in vitro. Functionalization with carboxyl or amino groups modified the quantity and type of proteins adsorbed and, hence, the interaction with cells.These findings may contribute to safe-by design manufacturing of MWCNT. Importantly, all the endpoints evaluated represent MIEs than can be combined to construct putative AOPs, associated with disease onset and progression. It is therefore concluded that the knowledge of the physico-chemical properties associated to the MIEs of different adverse outcomes is a pre-requisite for the toxicological profile of a MWCNT preparation.