Prized by materials scientists for their great strength and conductivity, carbon nanotubes are, however, worrying health scientists as two preliminary studies show that the biological activity of the tiny tubes looks strikingly similar to that of asbestos fibres.

“Our results suggest the need for further research and great caution before introducing such products into the market if long-term harm is to be avoided,” warn Craig Poland, of Queens Medical Research Institute in Edinburgh, and colleagues in Nature Nanotechnology this week¹.

Thousands of tonnes of carbon nanotubes are already produced each year, in a business that is predicted to be worth US$1–2 billion in five years' time. The high demand stems from their incredible strength, and high thermal and electrical conductivity. Yet, despite their cherished physical properties, their biological effects remain largely unknown. But concern is growing that they might be a risk to health, particularly to people involved in their manufacture and disposal.

“Research and business communities continue to invest heavily in carbon nanotubes for a wide range of products under the assumption that they are no more hazardous than graphite,” note Poland and colleagues.

The toxicity of asbestos fibres is in part due to their thin, needle-like shape, which is difficult for the body to break down, they explain. Carbon nanotubes have a shape reminiscent of asbestos fibres, with a nanoscale diameter and length up to millimetres, and they persist in biological fluids. Moreover, they can make their way into the outer surface layer of animals' lungs. They may even be able to reach the mesothelial layer where asbestos fibres characteristically cause the most damage, which can range from fluid accumulation and scarring to cancer (mesothelioma).

In a pilot study conducted to see whether nanotubes are indeed biologically active in the mesothelial layer, Poland and colleagues injected multiwalled nanotubes into the mesothelia of mice, and compared the responses with those provoked using asbestos. They found that the nanotubes caused an “asbestos-like… pathogenic behaviour”, with inflammation and the formation of abnormalities called granulomas, which can lead to cancer.

In a separate study², Maricica Pacurari of the National Institute for Occupational Safety and Health in Morgantown, West Virginia, and colleagues examined the effects of single-walled nanotubes on mesothelial cells in culture.

“Since [carbon nanotubes] behave like asbestos with bio-persistence and ability to generate ROS [damaging reactive oxygen species], the potential human health impacts and risks compel us to understand the toxic and molecular interactions,” they explain in Environmental Health Perspectives.

The team analysed the responses of a number of cell signalling pathways known to be involved in the development of mesothelioma, and compared the effects of the nanotubes and asbestos fibres. Exposure to the nanotubes induced generation of ROS, increased cell death, enhanced DNA damage, and activated several cellular pathways in a dose-dependant manner.

“These events recapitulate some of the key molecular events involved in mesothelioma development associated with asbestos exposure,” Pacurari and colleagues conclude.

Both sets of authors note that their studies are only preliminary and that much more research is required to ascertain whether exposure to carbon nanotubes is a health threat. In particular, there is no evidence that inhaled nanotubes do in fact reach the mesotheolia of human lungs in sufficient quantities to cause any problems. Yet the questions raised need to be addressed with some urgency, Poland and colleagues suggest, before the commercial use of carbon nanotubes becomes widespread.