20 May 2008Long straight carbon nanotubes may be as dangerous as asbestos fibres, potentially causing cancer in cells lining the lung, a pilot study in mice has shown.
Toxicologists say that those manufacturing and disposing of nanotubes - produced in thousands of tonnes per year worldwide - are most likely to be at risk of an asbestos-like illness, though it's not yet known if workers could be harmed just by inhaling nanotubes at typical exposures. 'We need more research on the toxicology of these materials, and the exposure to them in workplaces,' says Ken Donaldson, who led the research at the University of Edinburgh, UK.
Donaldson's team injected multiwalled carbon nanotubes and asbestos fibres between the membranes lining the lungs and abdominal organs in mice. They found that long straight nanotubes caused inflammation and lesions in membrane cells of the sort that have been shown to lead to cancer - just like asbestos fibres.
The problem, Donaldson explains, is that macrophages, cells which usually swallow up invading objects, can't stretch to engulf fibres that reach beyond about 20 micrometres. Such 'frustrated phagocytosis' leads to inflammation and eventually tumours. 'Anything that's thin, long, and doesn't easily dissolve in body fluids has got to come under suspicion as behaving like asbestos,' Donaldson says.
Nanotubes under twenty micrometres, and long nanotubes which were tangled up into balls, did not cause asbestos-like problems, the researchers found - although the study was not set up to investigate any other potential toxic effects of nanotubes.
'Much more work will be required to provide definitive proof [of whether particular types of nanotubes behave like asbestos fibres], and to show if the same effects are seen if particles are inhaled, and whether exposure levels reach the threshold for the development of cancer,' comments Mike Horton, director of life sciences at the London Centre for Nanotechnology, UK.
But given the terrible effects of asbestos that emerged in the 1960s, researchers are urging caution. 'Those tubes that resemble asbestos should be treated as though they were asbestos and regulated accordingly. In this way, workers involved in their manufacture, use and ultimate disposal will be protected,' says Anthony Seaton, a chest physician who annoyed nanoparticle manufacturers by linking carbon nanotubes and asbestos fibres two years ago.
But nanotube manufacturers are unconvinced that the study means that stricter health and safety precautions are needed.
Del Stark, the CEO of the European nanotechnology trade alliance (Enta), says companies making nanotubes already take the strictest possible safety precautions, so it's hard to see how the research will change manufacturing practice.
Steffi Friedrichs, director of the UK's Nanotechnology Industries Association says that it is not surprising that long insoluble fibres of any material should behave in this way - glass wool has similar effects. Nanotube makers already take measures to minimise exposure, Friedrichs points out.
'We welcome the study - it gives us a very good insight into the potential problems of some types of carbon nanotubes,' she adds. 'But the study needs to be verified, and the researchers have noted important caveats - for example whether nanotubes can actually get to the place in the body that's going to cause damage.'
It's unlikely that those using nanotube products right now (such as lightweight composites in sports equipment) will be in danger of breathing in dangerous doses of free nanotubes, but researchers agreed they would have to demonstrate, rather than assume, low public exposure. 'Even if you took a mallet and hammered a tennis racket, there's probably no danger because the nanotubes are held in a polymer matrix. So if it turns out there's no long fibres for the public to be exposed to - that's great,' says Donaldson
Aside from the need for more health and safety research, the study flags up that not much is known about exactly what types of carbon nanotubes are used in commercial products, says co-author Andrew Maynard, chief science adviser with the US-based Project on Emerging Nanotechnologies. 'There are voluntary agreements for reporting in the UK and the US that not too many companies have signed up to,' he says, warning that the nanotube market might suffer if the public lost trust in the technology because of the stigma of asbestos and because of a lack of transparency. 'It is up to governments to give industry as much guidance as possible,' he adds.
Richard Van Noorden