In nanotoxicology the question arises whether high aspect ratio materials should be regarded as potentially pathogenic like asbestos, merely on the base of their biopersistence and length to diameter ratio. A higher pathogenicity of long asbestos fibers is associated to their slower clearance and frustrated phagocytosis. In the past decades, two amosite fibers were prepared and studied to confirm the role of fiber length in asbestos toxicity. Long fiber amosite (LFA) and short fiber amosite (SFA) have here been revisited, to check differences in their surface properties, known to modulate the biological responses elicited. We report: (i) micromorphology (abundance of exposed cylindrical vs. truncated surfaces; (ii) surface reactivity (oxidation and coordination state of surface iron, free radical generation and oxidizing potential); (iii) activation of nitric oxide (NO) synthase in lung epithelial cells, as representative of an inflammatory cell response. LFA shows a higher free radical yield, stimulates, more than SFA, NO production by cells and reacts with ascorbic acid, thus depriving the lung lining layer of its antioxidant defenses. The higher activity of LFA than SFA is ascribed to the presence of Fe2+ ions poorly coordinated to the surface. SFA shows only a large number of loosely bound Fe3+ ions, pristine Fe2+ ions having been oxidized during the grinding process converting LFA into SFA. Several factors determine a higher toxicity of LFA than SFA, beside length. The lesson from asbestos indicates that other features besides aspect ratio contribute to the pathogenic potential of a fiber type. All these aspects should be considered when predicting the possible hazard associated to any new fibrous material proposed to the market, let alone nanofibers.