Aspect ratio is the ratio of length to width of a particle (ISO, 1999). High aspect ratio nanoparticles (HARN) are nanoparticles with a length many times that of their width. Examples of HARN may include certain types of nanotubes, nanorods, and platelets, depending on their aspect ratio.
Based on toxicological research of asbestos and other industrial fibres, a ‘fibre paradigm’ has been developed in which fibres that are biopersistant in the lungs and longer than 15-20 µm with diameters less than 3 µm are considered to be potentially hazardous to human health. Indeed, a major review (Tran et al., 2008) identified many similarities between HARN and asbestos in terms of their physico-chemical properties and toxicological effects, concluding that ‘there is sufficient evidence to suggest that HARN which have the same characteristics (diameter, length and biopersistance) as pathogenic fibres are likely to have similar pathology’.
Although carbon nanotubes (CNT) are often described as HARN, they can actually vary greatly in length, diameter, structural properties, chemical composition (from residual metal catalysts or functionalisation) and other physico-chemical properties. For example, they can be found as highly curled, semi-spherical agglomerates such as those described by Pauluhn (2010), or straight/semi-straight high aspect ratio fibres as described by Poland et al. (2008). This means that, despite having the same name, two samples of CNT can vary substantially. Such variations in composition and size have added to the complexity in understanding the hazard and exposure potential of CNT, and other HARNs.