CNTs: Culprit or witness of air pollution?

Common and widespread

Have you ever wondered what convenient Indian autorickshaws, powerful American heavy-duty engines and popular European (not necessarily German) cars have in common? They have all been reported to generate carbon nanotubes (CNTs) and emit them from their exhausts [1–3].

Carbon nanotubes, if an introduction is required, are cylindrical, sp²-bonded carbon nanostructures that can be single- or multi- walled. As the building blocks of nanotechnologies, CNTs can be synthesized in a laboratory under controlled conditions, where rare structural defects and low impurity content result in unique and exceptional physical properties. Alternatively, CNTs can be made (less perfectly and less purely) outside the labs through different processes of common biomass combustion [4].

Diesel, the sustained source

Particles derived from biomass burning contribute to the major share of airborne carbonaceous particles. Among them, traffic-derived, especially diesel-derived, particulates (as shown, A), have been recognized as major particulate matter (PM) components [5]. Interestingly, in the opinion of the International Agency for Research on Cancer, based on the evidence of increased risk for lung cancer, diesel engine exhaust is carcinogenic to humans (Group 1). Be that as it may, according to the statistics published by the European Automobile Manufacturers Association (http://www.acea.be/statistics/tag/category/share-ofdiesel-in-new-passenger-cars), the proportion of new diesel passenger cars continues to increase and currently represents more than 50% of sold vehicles. The roaming sources of diesel-derived particulates, including airborne anthropogenic CNTs [6], consequently, continue growing.

Commonality between ice core, ancient steel, and ... children

Carbon nanotubes appear as past [7] and present invaders of the atmosphere, and what is alarming is that they can reach our lungs as we have recently confirmed [8]. Inhaled CNTs (similar to those collected in airborne dusts and vehicle exhausts) were present in all examined samples (N = 69) obtained from Parisian asthmatic children [8].

An unhealthy mix

Diesel-related health effects are vastly reported [9] and diesel soot particulates are recognized inducers of inflammation, immune response and oxidative stress [10]. Apart from primary diesel particles’ intrinsic toxicity, atmospheric transformations, including reactions with OH radicals, ozone, NO₃ radicals and light [11], accentuate the toxicity. In addition, carbonaceous particles adsorb polycyclic aromatic hydrocarbons [12], which have an extensively documented carcinogenic effect.

By analogy, CNTs (as proven at least for synthetic ones), also interact with OH radicals [13], and exhibit increased toxicity when applied in combination with ozone [14]. While CNTs are associated with radical-scavenging activities, they also induce reactive oxygen species production within cells [15]. And, whilst they appear to be less toxic than carbon black and diesel particulates (in vitro) [16], CNTs also have a high adsorption capacity for polycyclic aromatic hydrocarbons [17].

Deceptive appearance

The idea that black carbon [5] penetrates the lungs is not new and was already proposed in a study performed on children from Leicestershire [18]. This study reported a strong dose-dependent link between the carbon content in alveolar macrophages and the decline in lung function[18]. But carbon, characterized only by optical microscopy [18], was probably mistaken for black-appearing lamellar bodies (B), the lipid storage and secretory organelles, present within alveolar cells [8]. Obviously, as substantiated in our study, carbon nanoparticles cannot be distinguished from lamellar bodies by optical microscopy only [8]. Thus, previous studies based on optical microscopy, linking carbon content of alveolar macrophages to lung dysfunction [18], must be reconsidered.

Another study reported that CNTs were extracted from the lungs of the victims of theWorld Trade Center attack [19], but the samples were only observed by means of low magnification transmission electron microscopy (TEM) [19], which might provide erroneous evidence, as pulmonary surfactant filaments (C) [8] might strongly resemble filamentous single-walled CNT bundles (D).

A needle in a haystack

The synthesis of novel nanomaterials blooms and blossoms and the identification studies of airborne nanoparticles [20] are numerous, yet, reports of the presence of anthropogenic CNTs in the indoor and outdoor air are still rather scarce [4,21]. In a similar fashion, qualitative identification studies of inhaled PM are extremely rare [22]. While we are intrigued by the reason why environmentalists do not report more findings on airborne CNTs, it is worthy of note that finding carbon particulates within carbon-based biological samples is a real challenge. Particulate matter, including CNTs, can be characterized in biological samples only by a limited number of techniques, which are not common in clinics. And yet, applied clinical research definitely needs appropriate easy-to-use methods for the detection of airborne carbon nanoparticles. Some methods have already been developed for synthetic CNTs [23,24] and could certainly be adapted to suit anthropogenic CNTs detection needs.

In our recent study [8], we used optical microscopy and TEM to prescreen biological samples (lung cells and bronchoalveolar lavage fluids, respectively). Subsequently, we used specific techniques, such as energy dispersive X-ray spectroscopy, high-resolution TEM, near infrared fluorescence microscopy and Raman spectroscopy. These methods allowed us to characterize the PM present in biological samples and discriminate between lamellar bodies, lung surfactant and PM. Consistent with theoretical calculations, which suggested the rareness of inhaled PM even after significant particulate exposure (i.e. a daily deposition of about 50 g of PM would result in particles covering of 0.2–0.5 millionths of the lung surface area) [25], our results showed that inhaled PM were particularly rare [8]. In addition, intracellular PM consisted mainly of anthropogenic CNTs and some amorphous black carbon nanoparticles (E), which cannot be identified by optical microscopy [8], in contrast to previous results [18].

Causal or non-causal bases

Particulate-matter-associated pathological conditions, including respiratory and cardiovascular disease, cancer and diabetes, to mention but a few, have been extensively reported. In contrast, quantitative assessments of the threshold for PM penetrating our bodies and causing deleterious health effects are non-existent. Is that because there are no causal effects [26] between airborne PM levels and health effects or because health effects are a result of multiple (particle-related and particle non-related) variables? While epidemic studies blamed PM derived from open fire biomass burning, a very recent study involving more than 10 000 Malawian children fund no evidence that cleaner burning biomass-fuelled cook stoves reduced the risk of pneumonia in young children [27].

Cleaner is not always dirt-free and “Smaller is not always better”

Considerable effort is now being directed towards the optimization of “cleaner burning” biodiesels and “advanced gasoline” direct injection engines. Paradoxically, both of them result in smaller and more numerous particles. While, in our opinion, the emergence of studies reporting (“ultra short”?) anthropogenic CNTs in “cleaner fuels” and “advanced engines” is just a matter of time, current studies indicate that primary particles derived from biodiesels and direct injection gasoline engines have smaller diameters, typically less than 20 nm [28,29] (whereas conventional diesel particulates measure approximately 50 nm [30]). But smaller particles are more likely to translocate from airways to blood [31,32].

So help me nanotechnologists

Besides being a general societal issue, particulate pollutants should attract more nanotechnologists’ attention, as nanotechnologists could fill the methodological lacunae in current biomedical and environmental approaches, which seek to determine and reduce the health impacts of anthropogenic nanoparticles. In a similar attempt, the purpose of our study [8] was to characterize the particulates that penetrate the lungs, which we found contain CNTs [8]. Further studies are evidently necessary to establish any potential link between CNT-lung content and lung dysfunction.

Additional research is also necessary to determine which constituents of diesel engine exhausts are the most responsible for deleterious health outcomes. A method of detection and quantification of intracellular PM (and especially CNTs) should be found, and should, preferentially, be applicable in clinics. If deleterious health effect are confirmed, appropriate measures should be taken (e.g. effective filters and air purifiers should be developed and applied) to diminish the atmospheric CNT burden. Besides, there might be other (toxicological) components, including volatile ones, involved in pollution’s deleterious effects [33], the effect of which should also be assessed.

Fine-tuning the research

Observational epidemiological studies reporting relationships between PM exposure and morbidity/mortality are prone to confusions and interpretational uncertainties [5]. While the “carbon content” [18] or “CNT content” [19] are probably yet another example of dubious identifications of carbonaceous nanoparticles [5] in biological samples, anthropogenic CNTs might represent a potential risk, and their effects should be assessed in detail. Indeed, this should add to other laboratory studies that found more or less diverging outcomes after CNTs administration [34–41]. In addition, as it has been suggested that alveolar removal might occur [25], and demonstrated that CNTs might degrade [42,43], empirical evidence of PM accumulation over a lifetime exposure should be investigated.

Judgment and action plan

Forthcoming studies should help answer the question, are CNTs the culprit or just a witness of exposure to air pollution? In whatever way, even if CNTs are slightly harmful or non-toxic, due to their large specific-surface and ability to adsorb a large variety of substances and interact with atmospheric components, they probably act as nanovectors for some hazardous pollutants present in the particulate mix. 

Authorities and media continue talking about pollution on daily basis but drastic action is not being taken. If the issue of particulates with a diameter smaller than 20 nm (probably including CNTs) is not addressed immediately, the potential cumulative effects over time are likely to be catastrophic. If anthropogenic CNTs are guilty as charged, appropriate measures to stop their discharge into the air we breathe should be taken S-T-R-A-I-G-H-T-A-W-A-Y.

Acknowledgements

We kindly acknowledge the reviewers for his or/and her worthy comments regarding “cleaner burning” fuels.

The article is dedicated to the Memory of Dr Henri Szwarc, a dear friend and our best scientific advisor, whom we lost while preparing this manuscript. May his kind and witty soul rest in peace.

Author affiliations: LETIAM, Lip(Sys)², IUT d’Orsay, Paris-Saclay University, Plateau de Moulon, 91400, Orsay, France; Department of Biochemistry, Trousseau-La Roche Guyon Hospital Group, Assistance Publique—Hôpitaux de Paris, Pierre et Marie Curie-Paris 6 University, Paris, France.

This article was originally published in Nano Today 15 (2017) 11-14.

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