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 risk 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 fiber rigidity. The latter has been derived from an extension of the classic fiber principle with the intrinsic material property flexural rigidity, hypothesized to play a significant role in limiting the toxicity of inhaled fibers. Current research efforts of BAuA focus on further investigating the influence of flexural rigidity on fiber toxicity aiming at threshold values e.g. for fiber diameter, which can be used as band limits. In addition, we are developing methods for measurement of fiber rigidity.

For exposure banding, relevant parameters are the material dustiness, the propensity of release of fibers with a critical morphology and the degree of fiber agglomeration. fibers of critical morphology are defined according to the WHO fiber counting convention to be thinner than 3 μm, longer than 5 μm and exhibiting an 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 particlemorphological analysis, we enable the identification and characterisation of HARM with relevant potential for release of fibers with critical morphology.

We applied the new risk banding scheme to 15 different types of multi-walled CNTs (MWCNTs) by determining their hazard and exposure parameters and adapting risk bands in accordance to the current knowledge on the risk potential of MWCNTs. A central result of our work is that the tested MWCNT variants cover large parameter ranges and can be allocated to all three risk levels. Our risk banding scheme also enables identifying those properties that are relevant for low risk materials. This way, our risk banding scheme can support the development of safe by design strategies for HARM and other advanced materials.

The complete article "A human risk banding scheme for high aspect-ratio materials" is a chapter of the book "Synthetic Nano- and Microfibers" (2020).