For over two decades, carbon nanotubes have been attracting attention. First, they were seen as a super-strong, super-conductive new form of carbon that could potentially revolutionize everything from space travel to drug delivery. Later, concerns were raised that these long, thin, fiber-like materials might cause or exacerbate lung diseases if inhaled.
Now, a new study in the journal EBioMedicine has suggested that these microscopic carbon fibers are ubiquitous in the air many of us breathe every day. And the obvious question that results is: should you be worried?
The new paper – a collaboration between scientists in Paris in France, and Texas in the US – analyzed carbon particles found in lung fluid samples from 64 asthmatic children living in Paris. Using high resolution Transmission Electron Microscopy (TEM), they found carbon nanotube-like fibers in each sample. Similar fibers were found in lung cells from five patients, and dust samples taken from deposits around vehicle tailpipes, and inside buildings close to minor roads.
The authors concluded that carbon nanotube are the main component of inhaled particulate matter.
At first blush, the paper seems alarming – carbon nanotubes that could be harmful were found in the lungs of children with a lung condition. However – as the authors acknowledge in the paper – the results, while interesting, don’t provide evidence that these exposures are a health risk.
To start with, it wasn’t too surprising that some fibrous carbon-based particle were found in the samples. Research over the past ten years has indicated that carbon nanotube-like particles are incidentally formed as a by-product in a number of high temperature processes. In 2006 for instance, Murr and Guerrero found multiwalled carbon nanotubes in soot collected from burning pine wood. And in 2013, Jung and colleagues found carbon nanotubes amongst diesel exhaust particles under controlled engine tests.
These and other studies have indicated that it’s not uncommon to find carbon nanotubes where carbon-based materials are burnt at high temperatures – especially where there are small quantities of metal present that can act as seeds or catalysts to promote their creation and growth.
Internal combustion engines in vehicles are a particularly interesting case. Here, trace quantities of metal in lubricants within the engine, and possibly particles in catalytic convertors, potentially promote carbon nanotube formation and growth. Because of this, it would be surprising not to find at least some carbon nanotube-like materials coming out of tailpipes.
From a health perspective, the possible emission of carbon fibers from vehicles and their presence in lung samples on their own don’t indicate whether they present a health problem. While the Paris study provides interesting information that will no doubt prompt further research, a lot more information is needed before any connection between the nanotubes observed and health impacts can be drawn, including more detailed information on the physical and chemical nature of the fibers, how many of them are produced, how many eventually end up in the lungs of people, and whether, once there, they retain an ability to cause harm.
The length of the fibers is particularly important. Research over the past few years has indicated that the toxicity of inhaled carbon nanotubes depends strongly on how long they are. Nanotubes that are longer than several micrometers are not readily cleared from the lungs, and as a result present a long term risk to health. On the other hand, shorter nanotubes like those observed in the Paris study behave very much like non-fibrous particles in the lungs – in other words, you wouldn’t expect inhaled fiber-like toxicity from the materials seen here..
Of course, even non-fibrous carbon-based particles could present a health risk if the concentrations are high enough. Research from the past twenty years or so has established that long-term exposures to elevated levels of fine airborne particles does on average increase the chances of respiratory and cardiovascular diseases developing within populations.
These exposures still follow the usual rules of risk though, where the probability of harm occurring depends on how much substance a person is exposed to. And this is a vital piece of information that’s missing from the Paris study. The researchers have indicated that carbon nanotube-like materials are breathed in by children, and that some of these particles remain in the lungs. But they did not measure how much material the children were exposed to.
Despite this, the study is interesting in that it does show the presence of these fiber-like particles. What is surprising though is how many they found compared to the non-fibrous carbon particles. The authors actually state
“our results show that PM [Particulate Matter] is mostly composed of anthropogenic MWCNTs in all analyzed samples”
There’s been a considerable amount of research on what particles produced by internal combustion engines look like, and to my knowledge, none of this research has indicated that most particles produced are carbon nanotube-like. To the contrary, studies typically show that the particles are spherical, and usually heavily agglomerated (see for instance this paper by Zhang and Cook).
This apparent discrepancy between what is known about vehicle emissions and what the Paris study indicates is worth further checking – it’s one of those niggling things that suggests something isn’t quite adding up somewhere, or there’s something happening that doesn’t align with what we’d expect from the current state of the science.
Putting this discrepancy aside, there is still the issue that these fiber-like particles were found in every sample from asthmatic children. Does this suggest that there’s a connection here?
Unfortunately not. Asthmatic children were used simply because it was easier to obtain samples from them. Because no non-asthmatic children were included in the study, and because there was no way to correlate detected particle types with incidence and severity of respiratory illness, there is no way of even drawing tentative associations from the research.
This is not to say that the study isn’t relevant. It’s useful to have tentative confirmation that fibrous carbon-like particles seem to be present in the air we breathe, and can deposit in our lungs, as it indicates that potential exposures to engineered carbon nanotubes may not be as unique as we once thought. It also potentially paves the way to establishing benchmarks for short carbon nanotube exposures that can be used in understanding the relevance of workplace exposures – with the proviso that health risk with these materials is strongly dependent on the precise nature of the nanotubes.
In other words, the Paris study indicates you might be inhaling carbon nanotubes – albeit very short ones. But it doesn’t say anything to indicate you should be worried at this point.
Feature image: Ruben de Rijcke, Wikimedia Commons