Unlike asbestos and silica fibers, nanofibers are created by human designers. All three substances, however, are proven or suspected carcinogens.
Why carcinogenic? To borrow a phrase, size does matter, at least for some kinds of fibers. Science to date tells us that longer asbestos fibers are in general more lethal than are shorter fibers, especially for the needle-like amphibole fibers. In addition, the RJ Lee Group and Dr. Ilgren published new 2012 work indicating that width also matters, at least for crocidolite mined in Bolivia, and perhaps for other fibers. Now, the BBC reports on new research in the UK which indicates that carbon nanofibers become more carcinogenic once the length exceeds 5 micro-meters:
"Ken Donaldson, professor of respiratory toxicology at the University of Edinburgh, said: "Concern has been expressed that new kinds of nanofibres being made by nanotechnology industries might pose a risk because they have a similar shape to asbestos."
Silver nanofibres of varying lengths were injected into the lungs of mice.
Those larger than five micrometres, or five-thousandths of a millimetre, tended to become lodged in the lungs and cause inflammation. The smaller ones were cleared from the lungs.
Prof Donaldson said: "We knew that long fibres, compared with shorter fibres, could cause tumours, but until now we did not know the cut-off length at which this happened.
"Knowing the length beyond which the tiny fibres can cause disease is important in ensuring that safe fibres are made in the future as well as helping to understand the current risk from asbestos and other fibres."
Prof Stephen Spiro, from the British Lung Foundation, said cases of mesothelioma had almost quadrupled in the past 30 years because of asbestos.
He added: "This research is particularly interesting as it gives us an indication of the size of fibre that might lead to mesothelioma if inhaled.
"If confirmed by subsequent studies, this minimum fibre length can be cited in industry guidelines to help ensure people are not exposed to the sorts of fibres that may lead to such deadly diseases."
The abstract for the research paper
says much the same, and then closes by referring to designing fibers to fall below the apparently dangerous size. The result? Engineered fibers could be "benign-by-design." That could become a popular watchword in product liability litigation, and design defect claims could evolve in new ways. Thus, the abstract states:
"Suspicion has been raised that high aspect ratio nanoparticles or nanofibres might possess asbestos-like pathogenicity. The pleural space is a specific target for disease in individuals exposed to asbestos and, by implication nanofibres. Pleural effects of fibres depends on fibre length, but the key threshold length beyond which adverse effects occur has never been identified up to now since all asbestos and vitreous fibre samples are heterogeneously distributed in their length. Nanotechnology advantageously allows for highly defined length distribution of synthetically engineered fibres which enable for in depth investigation of this threshold length. We utilised the ability to prepare silver-nanofibres of five defined length classes to demonstrate a threshold fibre length for acute pleural inflammation. Nickel-nanofibres and carbon nanotubes were then used to strengthen the relationship between fibre length and pleural inflammation. A method of intrapleural injection of nanofibres in female C57Bl/6 strain mice was used to deliver the fibre dose and we then assessed the acute pleural inflammatory response. Chestwall sections were examined by light and by scanning electron microscopy to identify areas of lesion; furthermore cell-nanowires interaction on the mesothelial surface of the parietal pleura in vivo, were investigated. Our results showed a clear threshold effect demonstrating that fibres beyond 4 µm in length are pathogenic to the pleura. The identification of the threshold length for nanofibre induced pathogenicity in the pleura has important implications for understanding the structure-toxicity relationship for asbestos-induced mesothelioma and consequent risk assessment with the aim to contribute to the engineering of synthetic nanofibres by the adoption of a benign-by-design approach." (emphasis added)