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Identification and management of risks related to both established and innovative materials are central aims of occupational safety and health. Therefore, we propose a new grouping scheme to evaluate the risk of High Aspect Ratio Materials (HARM) according to hazard and exposure aspects.Our approach to measure the fibre rigidity (discussion) is based on frequency measurements at resonance conditions of single vibrating fibres by means of radio-frequency engineering and scanning electron microscopy; using Euler-Bernoulli’s beam theory to determine the young modulus.The new scheme considers both intrinsic material and handling process-related properties such as bio-durability, toxicity, respirability, HARM morphology/dimensions as well as grade of agglomeration and dust release propensity during/after processing. It is based on the results of extensive research regarding those properties and their scalability for risk assessment, most notably the dustiness. Its utilisation requires data for the mentioned intrinsic and process related properties. Especially with respect to aspects of dustiness, this requires data on HARM release propensities for different handling conditions. However this talk will focus in the fibre rigidity as new parameter.We propose to include the aspect of rigidity, more precisely the fibre flexural rigidity, as an extension to the fibre-toxicological paradigm as a new parameter for HARM toxicity assessment. Critical fibre rigidity is most probably the key to frustrated phagocytosis or HARM translocation and distinguishes HARM toxicology from that of granular bio-durable particle materials. The potential toxicity of HARMs is widely known and was also shown for CNTs/CNF in many recent studies. We believe that the toxicity should not be evaluated solely by the fibre dimensions in context with systematic animal testing, but propose to combine a fibre’s composition and diameter into the property flexural rigidity. For bio-durable HARM, rigidity has the potential to become an overarching, material independent assessment parameter.