Respirable rigid fibres are considered more hazardous than non-rigid fibres. However, there is currently no standardised method to measure fibre rigidity. A new approach uses techniques to identify changes at the molecular level in cells. This approach can distinguish between different types of carbonaceous materials. The researchers could tell the difference between rigid fibres, non-rigid fibres, and non-fibrous carbonaceous materials. For this they looked at which biological pathways were affected. This method will help group different types of fibres into hazard categories based on their rigidity.

Hazards of fibres

There is a relationship between a fibre's structure and its hazard. Specific fibres are considered harmful when inhaled. These fibres are longer than 5 micrometres, thinner than 3 micrometres, and have a length-to-diameter ratio greater than 3:1. When it comes to nanofibres, their diameter limits their rigidity, which makes them less hazardous. However, the threshold for diameter as a measure of rigidity varies depending on the material. Currently there is no method to measure rigidity directly.

A new approach to identifying hazardous fibres

A study suggests a new approach to assessing the risk posed by morphology-induced hazards. This study was performed by the German Federal Institute for Risk Assessment (BfR). It used ‘omics data from different studies, including transcriptomics and proteomics data. Carbonaceous materials were classified into three types based on their structure and rigidity. Fibres with a diameter greater than 30 nanometre were classified as rigid fibres. Thinner fibres with a diameter less than diameter 30 nanometre were classified as tangled fibres. The third category are non-fibrous carbonaceous materials. The 30 nanometre threshold was based on the harmonised classification and labelling report for multi-walled carbon tubes. The fibre lengths for all categories were below that of the hazardous benchmark fibre Mitsui-7. Each category led to distinct cellular changes, with rigid fibres showing a clear difference from other carbonaceous materials.

 ‘Omics data can identify fibre morphology

The statistical random forest model was used to predict morphology based on ‘omics data. It is worth noting that one fibre (MWCNT NM-400) was classified as rigid despite its diameter of 11 nm. This diameter would suggest it to be a tangled structure. Upon electron microscopy analysis, it was revealed that its rigidity was due to secondary structures. Individual fibres formed rigid bundles. The study connects toxicological effects to fibre morphology. This allows for the identification of fibres that possess critical morphologies that should be prioritised for testing. Consequently, regulatory frameworks should consider morphology as a factor to be taken into account.

Reflections by RIVM

One characteristic that determines a fibre's hazard is its rigidity. Nevertheless, no standardised methods exist to measure this characteristic. Evaluating the cellular response to the fibre could then be a feasible alternative. To achieve this, a simplified cell method must be developed to differentiate between rigid and non-rigid fibres. Preferably, such a method should rely on a relatively simple readout, such as a reporter cell line. A reporter cell line is a stable cell line that shows specific gene expression when activated. The ultimate goal should be to have an OECD-approved test guideline to ensure harmonisation of the method. This would facilitate the method to be used in a regulatory context.