High aspect ratio materials (HARM) like carbon nanotubes (CNT) show material properties that enable innovative applications but also raise concerns about harmful effects to humans due to their asbestos-like morphology. A banding approach for HARM with hazard- and exposure-related parameters has been developed as a promising way to enable risk assessment and risk mitigation for an important family of advanced materials. It also provides guidance for a safer design of HARM and corresponding products.
Our scheme attributes HARM to three risk levels: low, moderate and high. The two-dimensional risk matrix builds on a hazard and an exposure banding. Parameters for hazard banding are the biopersistence and the fibre rigidity. The latter has been derived from an extension of the classic fibre principle with the intrinsic material property flexural rigidity, hypothesized to play a significant role in limiting the toxicity of inhaled fibres. Current research efforts of BAuA focus on further investigating the influence of flexural rigidity on fibre toxicity aiming at threshold values e. g. for fibre diameter, which can be used as band limits. In addition, we are developing methods for measurement of fibre rigidity.
For exposure banding, relevant parameters are the material dustiness, the propensity of release of fibres with a critical morphology and the degree of fibre agglomeration. Fibres with a critical morphology are defined according to the WHO-convention for asbestos (thinner than 3 µm, longer than 5 µm, aspect ratio greater than 3:1). To characterize the dustiness of fibrous materials, we have developed and evaluated the ‘fluidizer’ as an aerosol generator to adequately perform dustiness testing with powders of HARM. In combining aerosol characterization and sampling with subsequent particle-morphological analysis, we enable the identification and characterisation of HARM with relevant potential for release of fibres with a critical morphology.
We improved our risk banding scheme with 15 different types of multi-walled CNT (MWCNTs) by determining their parameters for hazard, exposure and risk banding. MWCNTs show high variance resulting in allocations to all three risk levels. The use of risk banding scheme also enables identification of low risk MWCNTs to support a safe design of nanoproducts.