Hailed as a major technological breakthrough with enormously positive implications for the future design of electronics, even to the point of perpetuating Moore’s Law, carbon nanotubes (CNTs) are now being labeled as potentially carcinogenic.Medical research involving a small sample of people engulfed in the World Trade Center dust cloud has found CNTs in their respiratory systems, casting doubts upon their safety. Approximately 70,000 people were exposed to the dust cloud.
Seven people who spent time in the cloud have had lung biopsies at Mount Sinai Medical Center. Three of those people had absorbed CNTs. This surprising find was further augmented by evidence of CNTs in samples taken from the dust cloud. These results not only heighten health concerns but also raise the question of why CNTs were found in the dust samples.
Indeed, numerous crucial questions about CNTs need answering. First, is the evidence that they are cancerous conclusive? Second, in what situations would they become a serious risk to human health? And third, what should be done to control those risks to both the general public and people in industry who have to work with CNTs?
In 2011, I asked if the electronics industry, in its enthusiasm to capitalize on CNT technology, had fully recognized the potential health risks that nanotubes may present (see “Will Nanotubes Become The New Asbestos?” at electronicdesign.com).
Back then, nanotechnology was already showing how it could radically affect electronic design. Nanotube activated transistors that could toggle on and off with the flow of a single electron were being demonstrated. Conventional transistors require the movement of millions of electrons, and that naturally causes heat, the natural enemy of electronic component efficiency.
However, even then researchers were concerned that CNTs could enter the human body via the lungs and digestive tract, like asbestos particles (Fig. 1). The worry was that if a CNT enters the lungs, the macrophages in our immune system would be unable to remove the unhealthy tissue once it had passed beyond the lung surfactant.
Where Is The Evidence?
But what evidence exists today that incontrovertibly condemns CNTs as cancerous? Obtaining conclusive results is not easy. Available information shows that, under some conditions, nanotubes can cross membrane barriers. Also, a research study led by Alexandra Porter from the University of Cambridge shows that CNTs can enter human cells and cause cell death.
Results of rodent studies demonstrate that regardless of the process by which CNTs were synthesized and the types and amounts of metals they contained, CNTs could produce inflammation, epithelioid granulomas, fibrosis, and biochemical and toxicological changes in the lungs. The hair-like shape of CNTs is similar to asbestos fibres, raising the concern that widespread use of CNTs may lead to pleural mesothelioma, a cancer of the lining of the lungs, or peritoneal mesothelioma, a cancer of the lining of the abdomen.
A recent pilot study supports this prediction. Scientists exposed the mesothelial lining of the body cavity of mice to CNTs and observed asbestos-like pathogenic behaviour that included inflammation and formation of lesions known as granulomas.
“This study is exactly the kind of strategic, highly focused research needed to ensure the safe and responsible development of nanotechnology. It looks at a specific nanoscale material expected to have widespread commercial applications and asks specific questions about a specific health hazard,” said the study’s coauthor, Dr. Andrew Maynard, an expert in nanotechnology who served as the chief scientific advisor for the Project on Emerging Nanotechnologies at the Woodrow Wilson International Center.
“Even though scientists have been raising concerns about the safety of long, thin carbon nanotubes for over a decade, none of the research needs in the current U.S. federal nanotechnology environment, health, and safety risk research strategy address this question,” he continued.
Exhaustive and meticulous research must continue. But available data suggests that under certain conditions, especially for those people experiencing chronic exposure, CNTs can pose a serious risk to human health. Fortunately, nanotechnology experts are calling for urgent and cohesive government action to ensure that the use of CNTs is precisely regulated.
Research by Dr. David Brown and Professor Vicki Stone at Edinburgh Napier University shows that people 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. However, it’s not yet known if inhaling nanotubes at typical exposure levels could harm workers.
Professor Ken Donaldson of the University of Edinburgh and the Safety of Nano-materials Interdisciplinary Research Centre and his team injected multi-walled CNTs 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.
Donaldson maintains that the problem is that macrophages, cells that usually swallow up invading objects, can’t stretch to engulf fibres that reach beyond about 20 µm (1 micrometre equals one-thousandth of a millimetre, 0.001 mm, or about 0.000039 of an inch). Consequently, researchers are urging caution when it comes to CNTs.
“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,” recommends chest physician Anthony Seaton in a statement that predictably infuriated nanoparticle manufacturers.
Nanotube manufacturers remain unconvinced that stricter health and safety precautions are needed. Del Stark, CEO of the European Nanotechnology Trade Alliance, says companies making CNTs 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 Nanotechnology Industries Association in the U.K., 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 minimize exposure.
Nonetheless, an important point here is to clearly define the risks to the general public using products that include nanotubes and those industrial workers who come into daily contact with CNTs.
A hypothetical parallel can be drawn with asbestos when it comes to CNT safety. If, like asbestos, CNTs are included within a structure or product and are not free to enter the atmosphere, then the only possible danger to users of those products is when the structure is damaged.
Even if damage occurs, the CNTs may remain embedded within a compound that makes up the body of the product and therefore theoretically will still be contained. However, it is a completely different matter for people who are consistently exposed to loose CNTs over long periods.
Voluntary Reporting Schemes
The Department for Environment, Food and Rural Affairs in the U.K. has a voluntary reporting scheme for manufacturers that use nanoscale materials in their products. The United States has a similar system. But until this kind of reporting is legally mandatory and enforceable, it will be difficult to tell just what kinds of nanotubes that manufacturers are using and where they are being used.
This is important because aside from the need for more health and safety research, some studies clearly indicate that little is known about exactly what types of CNTs are used in commercial products.
“There are voluntary agreements for reporting in the U.K. and the U.S. that not too many companies have signed up to,” said Andrew Maynard, chief science adviser with the U.S.-based Project on Emerging Nanotechnologies. Maynard also warns that the nanotube industry and products using CNTs might suffer serious commercial loss if the public loses trust in the technology because of the stigma of asbestos and a lack of transparency.
More work is needed to determine which nanotubes constitute a lethal health risk. Size and how the nanotubes are used in industry both matter. Research has shown that nanotubes under 20 µm and long nanotubes that were tangled up into balls did not cause asbestos-like problems. It would seem that the higher risk is from single nanotubes that are straight and 20 µm or longer.
Japanese research has partially confirmed CNT similarity to asbestos by showing that long, straight CNTs 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 performed by researchers at Japan’s National Institute of Health Sciences, also showed cancerous tumors.
Potential RoHS Contravention
With this in mind it is potentially conceivable that CNTs could at some future point contravene Restrictions on Hazardous Substances (RoHS) Directive 2002/95/EC, which is legally enforceable in the member countries of the European Union. RoHS places stringent restrictions on the use of materials and substances such as lead, mercury, cadmium, and hexavalent chromium, which is a carcinogenic.
However, what still remains inconclusive in a manufacturing environment that uses and disposes of CNTs is just how airborne can they be and what quantities need to be inhaled to represent a health risk. Indeed, some research suggests that getting CNTs airborne is difficult, since they tend to clump together, although more comprehensive research is needed to confirm it.
So electronics companies beware. Pioneering technologies like CNTs could lead to quantum leaps forward in semiconductor design, tempting companies to rush into their development and stake a lucrative claim.
But these companies would be very well advised to make sure stringent health and safety procedures are in place and are proven capable of protecting any employees working with nanotubes or products that contain them. Otherwise, the gold rush may well go into reverse as corporations find themselves having to pay huge compensation claims to employees suffering life-threatening disease.