Wednesday May 21 2008
Coloured scanning tunnelling micrograph of carbon nanotubes
Scientists have found that “carbon nanotubes could pose a cancer risk similar to that of asbestos”, The Guardian reports. The researchers have suggested “the government should restrict the use of the materials to protect human health”, the newspaper says. Carbon nanotubes are strong, light cylindrical molecules of carbon that are used industrially to add strength but not weight to products. They are reported to be a similar in size and shape to certain asbestos particles.
The study in mice showed that long carbon nanotubes could cause inflammation of the membrane which surrounds organs (the mesothelium), and this is similar to what is seen with certain types of asbestos. With blue and brown asbestos, inflammation of the mesothelium of the lungs can lead to the development of a rare lung cancer (mesothelioma); however, the mice in this study were not studied for long enough to see if they developed cancer. Carbon nanotubes that are embedded in other materials, like those in tennis rackets, car body panels and bike frames, are thought to be relatively harmless, but the researchers suggest that further studies are needed to confirm this.
Where did the story come from?
Dr Craig Poland and colleagues from the University of Edinburgh, other Universities and research institutes in the UK and US carried out this research. The study was funded by the Colt Foundation, the Engineering and Physical Sciences Research Council (EPSRC) and the Royal Academy of Engineering. The carbon nanotubes were donated by Mitsui & Co. It was published in the peer-reviewed scientific journal: Nature Nanotechnology.
What kind of scientific study was this?
This was a laboratory study looking at the effects of carbon nanotubes injected into the abdominal cavity of mice. It is known that exposure to brown and blue asbestos in the air can lead to inflammation, scarring and, in some cases, a rare form of cancer of the mesothelium of the lungs (mesothelioma). In this study, the researchers used the mesothelium of the abdominal cavity in mice as a model for what might happen in the lungs. Long fibres of asbestos are more hazardous than short fibres and the researchers wanted to assess whether the length of the carbon nanotubes, and whether they were straight or tangled, determined how hazardous they were. The carbon nanotubes used in this study were “multi-walled” meaning that they consisted of between two and 50 cylinders of carbon, one inside the other.
The researchers injected the abdominal body cavity (the area below the diaphragm, which contains organs like the stomach and intestines, liver and kidneys) of different groups of mice with solutions containing: long straight carbon nanotubes, short tangled nanotubes, long or short brown asbestos fibres or a sample of carbon not formed into nanotubes. These solutions were washed out of the body cavity after either 24 hours or seven days. The researchers looked to see whether there were signs of inflammation in the body cavity after 24 hours exposure (indicated by the presence of certain types of white blood cells and proteins). They also looked at the lining of the body cavity after seven days in the different groups of mice, to see if it was inflamed or had developed any lesions, called granulomas.
What were the results of the study?
The researchers found that long carbon nanotubes caused inflammation in the abdominal cavity after 24 hours and caused lesions to form in the mesothelium of the mice after seven days. Long fibres of brown asbestos had the same effect. Short carbon nanotubes, short brown asbestos fibres and carbon that was not formed into a nanotube did not have these effects.
What interpretations did the researchers draw from these results?
The researchers concluded that their results were very important because carbon nanotubes are widely used in research and commercially, and people had assumed that they were as safe as other forms of carbon such as graphite. They suggest that more research is needed, and that in the interim there should be caution about introducing products containing carbon nanotubes into the market.
The Guardian reports that Professor Ken Donaldson, who carried out the study, “stressed that the team had not demonstrated that carbon nanotubes actually caused cancer but they thought the government should take the threat seriously and prevent people from being exposed”.
What does the NHS Knowledge Service make of this study?
This was a well-conducted study that raises concerns about the safety of long carbon nanotubes. As the authors point out, they did not look at whether the mice exposed to long carbon nanotubes went on to develop mesothelioma, and therefore have not shown that long carbon nanotubes cause cancer. However, the inflammatory response they cause is similar to that seen with long fibres of asbestos and these can be a precursor to cancer in some cases of asbestosis. The authors also make it clear that they have not looked at whether inhaled long carbon nanotubes would cause inflammation or cancer of the mesothelium of the lungs, and if so, whether the levels in workplaces dealing with long carbon nanotubes would be high enough to cause these effects. Further research will clarify these issues.