Copyright © 2008 Published by Elsevier Ltd.
Carbon nanotubes could be as harmful as asbestos Nanotechnology
Available online 20 June 2008.
Toxicological studies of carbon nanotubes indicate that long needle-like fibers could have similar effects on lung tissue as asbestos [Poland et al., Nat. Nanotechnol. (2008) doi: 10.1038/nnano.2008.111].
US and UK researchers compared the effects of four samples of multiwalled nanotubes (MWNTs), three of which came from commercial sources, with short- and long-fiber amosite (brown asbestos) as controls. Of the MWNT samples, two contained a high proportion of long, straight fibers >20 μm and two consisted of low-aspect-ratio tangled aggregates. Samples of each material were injected into the peritoneal (abdominal) cavity of mice and washed out 24 hrs or seven days later.
The normal response of the cell layer that covers the chest (pleural) peritoneal cavities – the mesothelium – to pathogenic particles is inflammation. This is typically characterized by raised levels of white blood cells (morphonuclear leukocytes) and proteins, and the formation of scar-like structures (or lesions) called granulomas. In the tests, only the samples containing long fibers showed these effects. Although one of the short-fiber MWNT samples caused the formation of granulomas, the samples without long fibers did not show the hallmarks of an inflammatory response.
“The results are clear,” says Ken Donaldson at the University of Edinburgh. “Long thin carbon nanotubes show the same effects as long thin asbestos fibers.”
This is the first time it has been clearly shown that carbon nanotubes can behave like asbestos in the mesothelium. With asbestos, it is exposure to this lining of the lungs that can lead to cancer (or mesothelioma). However, this short-term study does not show whether the inflammatory and granulomatous response of mice leads to mesothelioma.
However, it is not just exposure that is key but whether a sufficient number of fibers can reach the mesothelium to cause chronic inflammation. This study, as the researchers themselves emphasize, does not address that issue. Nor does it address whether it is even possible to be exposed to sufficient quantities of MWNTs in the workplace or environment.
“We still don't know whether carbon nanotubes will become airborne and be inhaled,” says Donaldson, “or whether, if they do reach the lungs whether they can work their way to the sensitive outer lining.”
On the plus side, the results indicate that short, tangled aggregates of carbon nanotubes, which make up the majority of commercial samples tested, do not behave like asbestos.
“Researchers should be cautious, but not necessarily concerned,” says Andrew Maynard of the Woodrow Wilson International Center for Scholars. “By taking precautions, there is no reason why exposures cannot be kept acceptably low.”
There have been concerns about the safety of carbon nanotubes for many years, but governments have been slow to react, say the researchers. “Targeted research is urgently needed to answer the many outstanding questions about using carbon nanotubes safely,” says Maynard. “At the end of the day, this is an opportunity to get carbon nanotube-based technologies right.”