A special category of fibres are nanotubes, which may be of a few nanometres in diameter but with a length that could be several micrometers. Risks should be assessed bearing in mind the well known carcinogenic effects of certain asbestos fibres. In two recently published in vivo studies, single-wall carbon nanotubes (SWCNTs) were demonstrated to induce lung granulomas after intratracheal administration (Lam 2004, Warheit et al 2004), indicating that these nanotubes cannot be classified as a new form of graphite on material safety data sheets. On a dose per mass basis the nanotubes were more toxic than quartz particles, well known for their lung toxicity, although the mass dose was very high and mechanical blockage of some airways was noted. Carbon black, carbonyl iron and graphite produced no significant adverse effects. Multifocal granulomatous lesions were observed without accompanying inflammation, cell proliferation or cytotoxicity, which was suggested to indicate a potentially new mechanism of pulmonary toxicity and injury by the nanotubes, not following the normal paradigm of toxic dusts (Warheit et al 2004). In vitro studies using a human keratinocyte cell line showed that carbon nanotube exposure resulted in accelerated oxidative stress and cellular toxicity, which may be interpreted as potential for dermal toxicity (Shvedova et al 2003).

Interesting reference to asbestos.