If I rightly recollect:
Determining the toxicity of carbon nanotubes has been one of the most pressing questions in nanotechnology.

Results from various scientific tests on carbon nanotubes have so far proven confusing, with some results indicating it to be highly toxic and others showing no signs of toxicity. This is primarily because of difficulties arising in spotting the nanotubes entering the cells from other carbon-based cell structures such as membranes.
*never heard of Wikipedia face*