John Balbus, M.D., M.P.H., is Chief Health Scientist.

A new study published today in Nature Nanotechnology finds that multi-walled carbon nanotubes (MWCNTs) cause inflammatory changes in mice that closely resemble those caused by asbestos.  This is the second study in a few months to make this finding.  (I posted on the first, Takagi et al., a few weeks ago.)  So is the case closed on multi-walled carbon nanotubes?  Or is too early to draw conclusions?

These two studies used different approaches to compare the effects of MWCNTs and asbestos.  Although both directly injected the nanotubes into the peritoneal cavities of mice (the peritoneum is the lining of the abdominal organs and walls, and is the part of the body – along with the pleura lining the lungs – where asbestos causes mesotheliomas to arise), the new study used a 60-fold lower dose and measured only inflammatory changes, not the actual development of mesotheliomas.  Takagi et al., on the other hand, used special mice that were genetically highly susceptible to cancer formation, and observed the mice until they developed tumors. 

Despite these differences, these studies together make a strong case that multi-walled carbon nanotubes cause inflammation and cancers in a similar fashion to asbestos, at least if they have been deposited directly within the peritoneum. 

But the studies only tell a part of the story.  In order to have a better handle on the risks from inhaling MWCNTs, studies need to be done to determine whether there could be substantial concentrations in the air that workers (or other exposed persons) breathe.  Then we need to find out whether inhaled MWCNTs will make their way from the airspaces of the lungs, through the lung tissue, to the lung and abdominal cavity linings – something asbestos does with relative ease. 

If these first two steps in causing disease are demonstrated, it is likely that MWCNTs will persist and continue to cause inflammation as long as asbestos does, but this needs further investigation as well.  So the case isn’t closed, but the evidence is stacking up.

Until these questions are answered, companies using MWCNTs should carefully characterize their materials and refrain from exposing workers to nanotubes longer than 5 microns with fibrous shapes.  And researchers developing applications for MWCNTs should look to develop forms that don’t have those size and shape characteristics.  These studies don’t show that other forms are harmless, but it can’t hurt to sidestep the trouble you can see.