On Monday, the National Institute for Occupational Safety released new data on the potential role multi-walled carbon nanotubes play as a cancer-promoter – a substance that promotes the development of cancer in the presence of a carcinogen. In the study, mice were injected with methylcholanthrene – a cancer initiating agent – and subsequently exposed to airborne multi-walled carbon nanotubes. Compared to a control group, the methylcholanthrene and carbon nanotube-exposed mice were significantly more likely to develop tumors than a control group, developed more tumors, and developed larger tumors. The study provides a strong indication that this particular form of carbon nanotube material can synergistically increase the likelihood and severity of cancer in the presence of a carcinogen.
The NIOSH research was one of a number of presentations on carbon nanotube safety at this year’s Society of Toxicology meeting. The results won’t appear in the peer review literature for some time yet, and so it is premature to draw too many conclusions. However, there is a growing body of research indicating that some forms of carbon nanotubes can present a significant health hazard if used inappropriately.
Unfortunately, there is still considerable uncertainty over which of the many attributes of different carbon nanotubes and nanotube materials are associated with their toxicity, release routes leading to significant exposure, and what constitutes a significant exposure. These are all areas of extremely high research priority if carbon nanotube material is to be produced, used and disposed of/recycled responsibly.
The carbon nanotubes in this study were inhaled multi-walled carbon nanotubes with a predominantly long, straight fiber-like morphology. Mice were exposed at a level of 5 mg/m3 for 5 hours per day, over a 15 day period. From the NIOSH post:
Mice receiving both the initiator chemical plus exposure to MWCNT were significantly more likely to develop tumors (90% incidence) and have more tumors (an average of 3.3 tumors/mouse lung) than mice receiving the initiator chemical alone (50% of mice developing tumors with an average of 1.4 tumors/lung). Additionally, mice exposed to MWCNT and to MWCNT plus the initiator chemical had larger tumors than the respective control groups. The number of tumors per animal exposed to MWCNT alone was not significantly elevated compared with the number per animal in the controls. These results indicate that MWCNT can increase the risk of cancer in mice exposed to a known carcinogen. The study does not suggest that MWCNTs alone cause cancer in mice.
For further background insight into the complexities of interpreting carbon nanotube toxicity with respect to nanotube exposure and physicochemical characteristics, check out this week’s Risk Bites video:
Wikipedia: What are carbon nanotubes used for?
Donaldson et al. (2006) Carbon Nanotubes: A Review of Their Properties in Relation to
Pulmonary Toxicology and Workplace Safety. Toxicol. Sci. 92 (1): 5-22. [Free download]
Poland et al. (2008) “Carbon nanotubes introduced into the abdominal cavity of mice show asbestos-like pathogenicity in a pilot study” Nature Nanotechnology 3, 423 – 428