| dc.description.abstract | Carbon nanotubes (CNTs) can be seen as graphene sheets rolled to form cylinders.
CNTs may be categorised as single- (SWCNT) or multi-walled (MWCNT). Due
to the small size, the number of particles as well as the surface area per mass unit
is extremely high.
CNTs are highly diverse, differing with respect to e.g., diameter, length, chiral
angles, chemical functionalisation, purity, stiffness and bulk density. Today, CNTs
are utilised primarily for the reinforcement of composite polymers, but there is
considerable potential for other applications. The rapidly growing production and
use of CNTs increases the risk for occupational exposure. Since CNTs in bulk
form are of very low density and much dust is produced during their handling,
exposure by inhalation appears to represent the greatest potential risk in the work
place. However, most work place measurements involved sampling periods that
are too short, varying sampling techniques and non-specific analytical methods.
CNTs may be absorbed via inhalation and ingestion. Systemic uptake via the
skin has not been demonstrated.
Human toxicity data on CNTs are lacking and interpretation of animal studies
is often problematic since the physical properties and chemical composition are
diverse, impurities may be present and data are sometimes omitted. Because of the
physical similarities between asbestos and CNTs, it can be suspected that the latter
may also cause lung fibrosis, mesothelioma and lung cancer following inhalation.
Intraperitoneal and intrascrotal administration of CNTs causes mesothelioma in
animals, but no inhalation carcinogenicity studies have been conducted. Thus, it is
too early conclude whether CNTs cause mesothelioma and lung cancer in humans.
Both SWCNTs and MWCNTs cause inflammation and fibrosis in the lungs of
relevant animal types and for MWCNTs these effects are also seen in the pleura.
For instance, minimal histiocytosis and mild granulomatous inflammation in the
lungs and lung-draining lymph nodes have been observed in rats exposed for 13
weeks to 0.1 mg/m3 MWCNTs (lowest observed adverse effect level, LOAEL),
with more pronounced inflammation in both mice and rats at higher doses. Thus,
inflammatory responses in the lungs may be considered as the critical effect. However,
the LOAEL of CNTs should be interpreted cautiously, since their toxicity is
likely to vary widely, depending on the structure and physicochemical properties,
as well as the contribution from non-carbon components. It is also uncertain which
dose metric (e.g., mass, number or surface area per air volume unit) is most appropriate.
Some studies indicate that longer straight CNTs evoke more pronounced
biological effects than shorter or tangled fibres. | sv |
