Research scientists from Oak Ridge National Laboratory are publishing the results of their recent in vivo pulmonary toxicity testing of single-walled carbon nanohorns (SWCNHs) in the next print edition of Nanotoxicology.

R. Lynch, et al., “Assessing the pulmonary toxicity of single-walled carbon nanohorns,” Nanotoxicology, Month 2007; 00(0):1-10 (forthcoming).

The scientists explored the toxicity of SWCNHs because prior toxicity testing of multi-walled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT) indicated in certain circumstances SWCNTs can be more toxic than both MWCNTs and/or fullerenes. Accordingly, the authors theorized "the results from previous SWCNT toxicity studies do not immediately imply the toxicity of other carbon based nanoparticles,” and “the potential toxicity of a nanomaterial cannot be inferred from its elemental composition alone.”

Regarding the test material, the scientists described it as self-aggregated SWCNHs formed into 80-100nm structures with the tips of the individual nanohorns projecting outward from the center in all directions. The researchers chose an inhalation exposure route because of concerns that nanoparticle aerosols may deposit in the alveolar lung regions and may not be expelled through normal lung clearing mechanisms. The article explained that this can lead to oxidative stress and pulmonary inflammation, which can in turn trigger fibrotic change and loss of pulmonary function.

Regarding the test itself, lab mice were subjected to 30 mg of surfactant-suspended SWCNHs through pharyngeal aspiration for 24 hour and 7 day intervals. The scientists selected a 30 mg dose because Shredova (2005) determined mouse exposure to a 20 mg dose of SWCNTs is equivalent to human exposure to 20 eight hour workdays of graphite particles at OSHA permissible exposure limits.

The results were an “early but mild inflammatory response which is primarily resolved by 7 days post-exposure.” Lung microarray analysis showed no robust changes in gene expression, and histological analysis showed no evidence of granuloma formation or fibrosis. However, the researchers noted the lungs of exposed mice were slightly darker than those of non-exposed mice 7 days out, “indicating that the nanoparticles were well distributed through the lung and that complete clearance of SWCNHs had not occurred.”

The scientists concluded “[t]here combined results suggest that SWCNH is a relatively innocuous nanomaterial when delivery to mice in vivo using aspiration as a delivery mechanism.”