Carbon Nanotubes - Uptake
Uptake through the lung - inhalation
Since presently, carbon nanotubes are not added to foods or to cosmetic products, it is their uptake via the lung into the body that must be investigated closer. Hence, it is not surprising that several biological studies have been carried out on rats and mice to find out more about that path of uptake. Studies on mice have shown that carbon nanotubes can penetrate through to the deepest regions of the lung.more...
According to present knowledge, carbon nanotubes are most probably taken up via the respiratory tract and the lung. Due to their small size, nanoparticles can advance through to the area of the pulmonary alveoli where, among other things, they can disturb vital gas exchange.
In most studies, carbon nanotubes are administered in an uncommon way. This raises the question whether statements applicable to everyday life can be derived at all: In these studies, carbon nanoparticles are administered either by infusing a small quantity of a highly dosed liquid nanomaterials suspension through the trachea directly into the pulmonary alveoli or by suction of a nanoparticles-containing liquid through the pharynx. None of these two methods corresponds to, e.g. the inhalation of airborne particles which, like the inhalation of particulate matter, may occur in workplaces.
In more recent inhalation studies[3,4,5], carbon nanoparticles are administered via the breathable air as aerosols, which would correspond to a real situation. Such studies on mice have shown that also carbon nanotubes can penetrate down to the deepest regions of the lung[4,7].
The results of the different studies are partly contradictory and are still discussed controversially. Some research groups describe stress reactions in the lung tissues, e.g. inflammation reactions in the lung, granulom formation (i.e. formation of new, lump-type tissues caused by inflammation) in the epithelial tissues of the lung, cell damage, fibroses (abnormal growth of the connective tissue), and changes in the lung function, while others detect only small or no changes at all in the lung and, in contrast to the above, describe modified systemic immune reactions (for example in the spleen and in the lymph nodes).
The studies mentioned differ in several essential aspects: use of different or differently functionalized carbon nanotubes, administration of carbon nanotubes in different ways and for different lengths of time, use of different analysis methods, a direct comparison or final comprehensive toxicity assessment have not been possible as yet.
Although only very few data are available so far about CNT concentrations in industrial workplaces, CNT were detected in the air (approximately 0.7-53 µg/m3) during first laboratory tests under corresponding experimental conditions (Maynard et al., 2004). The uptake of such nanoparticles can be reduced strongly through proper application of precautionary measures such as the use of fume hoods and protective clothing (gloves, face masks). The uptake of the more critical larger and longer fractions of CNT into the lung remains to be investigated.
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Uptake through the skin
Mainly cosmetic or pharmaceutical products, for example suncreams, are concerned when discussing the uptake of nanoparticles via the skin. Since no comparable applications are known or projected for carbon nanotubes, their direct contact with the skin is quite improbable at present or may be rather due to accidents or improper application.more...
In accordance with the findings of the European project Nanoderm, the skin, as a rule, is a very good barrier against nanoparticles. So far, there has been no proof of a transfer of particles via the skin into the blood vessels. However, it has not been investigated yet whether carbon nanotubes can be taken up via the skin. Nevertheless, a conclusion by analogy can be helpful in that case, too: Tattooing ink, especially black ink, contains carbon particles and is deposited in the deeper layers of the skin. From there, only very few of these particles are transported into the nearby lymphatic vessels while most of them remain just where they are to stay permanently in the skin for a lifetime. Since the surface characteristics of carbon nanotubes are very similar to those of these carbon particles, it can be inferred that transport of the former is only weak.
Uptake through the gastrointestinal tract (GIT)
For the time being, carbon nanotubes are not applied to foods. Only in theory, they can be transported into the gastrointestinal tract through unwanted hand-to-mouth transfer or when swallowing particles which are transported through cleaning processes from the respiratory tract into the esophagus. As a rule, particles that have been taken up via the food are excreted via the stool. No proof has been furnished as yet that carbon nanotubes get into the intestines and are taken up from there into the body.more...
Carbon nanotubes are not permitted for use in foods, and there are no indications of their being allowed to be applied in the near future. It is conceivable, however, that, e.g. inhaled carbon nanotubes get into the gastrointestinal tract through coughing up or swallowing and that they are distributed from there further in the body. So far, only few analyses exist on that topic. The direct administration of water-soluble carbon nanotubes into the stomachs of mice showed that for a very short time only, the nanotubes used stayed in the gastrointestinal tract. They were excreted via the stool within 12 hours and did not pass into the bloodstream. Biological effects were not found either. By contrast, only very small parts of unmodified carbon nanotubes that had been injected directly into the bloodstream could be detected in the stool and urine, which means they were hardly excreted but distributed in different inner organs such as the liver, the spleen, or the lung. Just as for the lung, one cannot draw any final conclusion for the gastrointestinal tract. Also in the case of the latter, the behavior of carbon nanoparticles strongly depends on their modification and on the type of their administration.
Uptake and risk for environmental organisms
The large diversity of CNT does not allow general or comparable conclusions on their toxicity. However, several studies find no effects on environmental organisms, even if applied in very high concentrations.more...
Additionally, actual toxic effects may not be caused by the CNT but rather by solvents or catalysts, which are contained in the tubes as residues from the production process. The latter may cause toxic effects in studied organisms, whereas purified CNT are not toxic at all 
Rainbow trouts exposed via the water ingested CNT by drinking the water , without any impact on the metabolism of the animals. But CNT were influencing the respiration, as seen by an increase in respiratory rate and changes on the gills. This was rather due to a physical blockade of the gills because of the fibre form of CNT, bound by the mucous layer on the gills.
New and surprising was the observation of an increasingly aggressive behaviour after CNT exposure. This was observed in fish as well as in another species studied, the African clawed frog [2,3] . But it is not yet clarified whether these changes in behaviour represent a new mode of action for CNT. Furthermore, it is unclear whether the effect is due to brain disturbances or irritation because of the presence of the tubes in the water.
For worms living in the soil as well as for water flea an uptake of CNT from soil or water into the intestine was demonstrated, yet no enrichment into the bodie tissues was found. The CNT were rather completely excreted [4-7] . A similar observation was made for the lugworm, where neither a toxicity nor an accumulation of CNT into body tissues occured  .
The seeds of several agricultural plants (tomato, cabbage, carrot, onion, radish, rape, lettuce, rye grass, corn and cucumber) were used to study the influence of CNT on root growth and germination. Multi walled tubes did not show any effect on the seeds, whereas single walled tubes reduced the root elongation of tomato, but only when the surface was not modified. Tubes carrying a modification for better solubility in water, no effects on seeds were observed. CNT were not taken up into the plant roots [8,9].
From the absorption of environmental pollutants by CNT, a new aspect for environmental organisms arises, namely a nanomaterial-mediated change in the action of these pollutants. The effects on environmental organisms may either be diminished, because the free concentrations of the pollutant decrease, or on the contrary enhanced, when e.g. the contaminant-loaded CNT are taken up by an organism[10,11] .
In conclusion, the toxicity of CNT is currently hard to assess because of their large diversity. However, very low doses as to be expected in the environment at present, pose no risk for environmental organisms. Many additional factors such as residues from catalysts and impurities, as well as mechanical characteristics due to the fibre form, which may lead to skin or gill irritations, have to be considered.
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