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.