Much Ado About Carbon Nanotubes...Or Not
Researchers at the University of Paris-Saclay in France have discovered that fluid samples taken from the airways of 64 asthmatic children contained carbon nanotubes (CNTs). In addition, the France-based researchers determined that five other children studied also had CNTs in macrophages found in their lungs.
While this will no doubt add fuel to the fury of NGOs bent on shutting down research into nanotechnology immediately, there is little in this research that breaks new ground—at least qualitatively.
“From past studies, the conditions in combustion engines seem to favor the production of at least some CNTs (especially where there are trace metals in lubricants that can act as catalysts for CNT growth),” explained Andrew Maynard Director, Risk Innovation Lab and Professor, School for the Future of Innovation in Society at Arizona State University, in an e-mail interview. Says Maynard:
What, to my knowledge, is still not known, is the relative concentrations of CNT in ambient air that may be inhaled, the precise nature of these CNT in terms of physical and chemical structure, and the range of sources that may lead to ambient CNT. This is important, as the potential for fibrous particles to cause lung damage depends on characteristics such as their length—and many of the fibers shown in the paper appear too short to raise substantial concerns.
It’s not even clear from the research whether the nanoparticles in question are in fact carbon nanotubes. At this point, they are best described as carbon nanotube-like fibers.
Nonetheless, Maynard praises the research for establishing that these carbon nanotube-like fibers are part of the urban aerosol and therefore end up in the lungs of anyone who breathes it in. However, he cautions that the findings don’t provide information on the potential health risks associated with these exposures.
“Because of this,” Maynard told IEEE Spectrum, “it would be highly premature to draw any conclusions on health risk from the study.” He added that, “It would be appropriate to conduct further study into whether there is an association between these unusual carbon-based fibers and ill health.”
At least some of the coverage of the research has made the misleading point that “nanotubes have shown great potential in areas such as computing, clothing and healthcare technology” with the obvious implication being that the CNTs used in these applications are the ones found in the lungs of children.
While the research doesn’t draw a distinction between manufactured CNTs and the natural and incidental varieties produced by, say, car exhausts, there is little to suggest they are anything other than particles that have been around with us since the introduction of the internal combustion engine. Meanwhile, there has been little evidence showing that a manufactured CNT, once embedded in the matrix of a material, can ever be separated from that matrix so that it’s free to float around in the air.
“Some studies have indicated that occasionally single nanotubes might be released from abraded materials,” Maynard admits. “But it looks like the release rates are extremely low. This is what would be expected given how tightly carbon nanotubes bind to polymers used in composite materials, and the amount of energy that would be required to release them.”
Maynard points that it may be possible in principle to create fingerprints for different types of CNTs based on their source, allowing scientists to determine definitively whether a sample of CNTs is from car exhaust, or tennis racquets and bicycles. He adds:
For instance, CNTs that are lab generated will often be associated with trace amounts of catalyst materials such as nickel or iron. However, I suspect that such fingerprinting will require a level of characterization rarely used on such materials.
Such characterization may also be a moot point from a health perspective. Maynard notes that while ambient carbon nanotubes may be analytically difficult to distinguish from engineered carbon nanotubes, it’s reasonable to assume that our lungs will also find it hard to make the distinction.