Certain types of carbon nanotubes could cause the same health problems as asbestos, according to the results of two recent studies. In one, published yesterday, tests in mice showed that long and straight multiwalled carbon nanotubes cause the same kind of inflammation and lesions in the type of tissues that surround the lungs that is caused by asbestos. The other study, also done in mice, showed that similar carbon nanotubes eventually led to cancerous tumors.
Carbon nanotubes, tube-shaped carbon molecules just tens of nanometers in diameter, have excellent electronic and mechanical properties that make them attractive for a number of applications. They have already been incorporated into some products, such as tennis rackets and bicycles, and eventually they could be used in a wide variety of applications, including medical therapies, water purification, and ultrafast and compact computer chips. “It’s a material that’s got many unique characteristics,” says Andrew Maynard, a coauthor of one of the studies, which appears in the current issue of Nature Nanotechnology. “But of course nothing comes along like this that is completely free from risk.”
Carbon nanotubes that are straight and 20 micrometers or longer in length–qualities that are well suited for composite materials used in sports equipment–resemble asbestos fibers. This has long led many experts to suggest that these carbon nanotubes might pose the same health risks as asbestos, a fire-resistant material that can cause mesothelioma, a cancer of a type of tissue surrounding the lungs. But until now, strong scientific evidence for this theory was lacking.
The new studies partially confirm the carbon nanotubes’ similarity to asbestos by showing that long, straight carbon nanotubes injected into mesothelial tissues in mice cause the sort of lesions and inflammation that also develop as a result of asbestos. Such reactions are a strong indicator that cancer will develop with chronic exposure. One of the studies, which appeared in the Journal of Toxicological Study and was done by researchers at Japan’s National Institute of Health Sciences, also showed actual cancerous tumors. The Nature Nanotechnology study was done primarily by researchers in the United Kingdom at the University of Edinburgh and elsewhere.
What isn’t known is whether, during nanotubes’ manufacture, use, and disposal, they can become airborne and be inhaled in sufficient quantities to cause problems. Indeed, earlier work has shown that it is actually difficult to get carbon nanotubes airborne, since they tend to clump together, says Maynard, the chief science advisor for the Project on Emerging Nanotechnologies, at the Woodrow Wilson Center for Scholars, in Washington, DC. He says that this could decrease the chance that they will be inhaled. He adds that further research is needed to confirm this.
Not all types of carbon nanotubes behave like cancer-causing asbestos. The Nature Nanotechnology article showed that short nanotubes (those less than 15 micrometers long) and long nanotubes that have become very tangled do not cause inflammation and lesions. Also, while the study did not look explicitly at single-walled nanotubes, these tend to be shorter and more tangled than multiwalled nanotubes, so they probably won’t act like asbestos, the researchers say. The authors suggest that this could be because such nanotubes can easily be taken up by immune cells called macrophages, and long, straighter ones can’t. (Macrophages can only stretch to 20 micrometers, which makes it difficult for them to engulf nanotubes longer than that.) This finding is consistent with results published in January that suggest that certain types of short carbon nanotubes are nontoxic to mice, says Hongjie Dai, the professor of chemistry at Stanford University who published the earlier work. Short nanotubes are likely to be useful in electronics and medical applications, while long, multiwalled nanotubes are more attractive for composite materials because of their mechanical strength. Dai says that it’s important not to lump all carbon nanotubes together, since they can have very different characteristics depending on how they are manufactured.
The Nature Nanotechnology study is a strong one because it establishes the link between a particular type of nanotube and asbestos-like symptoms, while controlling for chemical impurities that are a by-product of manufacturing carbon nanotubes, says Vicki Colvin, a professor of chemistry and chemical and biological engineering at Rice University in Houston, TX. Such chemical impurities have led to contradictory results in earlier toxicity studies on nanoparticles. The Journal of Toxicological Study paper, which showed not only that long carbon nanotubes could cause lesions, but also that these can actually lead to cancerous tumors, had the drawback that the researchers used genetically modified mice that are particularly sensitive to asbestos, Colvin says. But that study still shows a relationship between these particular kinds of carbon nanotubes and mesothelioma.
As is the case with asbestos, carbon nanotubes are not likely to cause problems while they’re embedded inside products. It’s most important to protect workers involved in the manufacturing and disposal of these products, at which point the nanotubes could be released into the air, the authors of the Nature Nanotechnology study say. This could be done with established methods for handling fibrous particles, Colvin says, and by starting to keep track of what products have the potentially dangerous nanotubes–something that’s not done systematically now. Armed with the results, engineers could possibly use types of carbon nanotubes that are safer, Maynard says.
Anthony Seaton, one of the authors of the Nature Nanotechnology paper, a researcher at the University of Aberdeen, and a medical doctor who has treated people exposed to asbestos, draws a connection between the promise of carbon nanotubes and the hope people once had for asbestos. Asbestos, like carbon nanotubes, was seemingly ideal for many applications. At one point, Seaton says, asbestos was “almost ubiquitous.” But whereas the dangers of asbestos weren’t recognized and dealt with until people got sick, the new findings present a chance to keep people from being hurt, he says, by taking preventative measures. “We’ve learned a serious lesson from asbestos,” Seaton says.