TOXICITY OF CARBON NANOTUBES - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 234

In a very recent article, Ryman-Rasmussen and collaborators 48 showed
that MWCNTs reached the subpleura in mice after a single inhalation exposure
of 30 mg/m 3 for 6 hours and that subpleural ibrosis increased after 2 weeks
following inhalation (Fig. 8.4).
As emerges also by comparison with the data reported in all the articles
mentioned in the present chapter, it is evident that the exposure of 30 mg/
m 3 is a huge dose which is unluckily to occur if nanometerial is handled
cautiously. Even in the case of aerosols, typical drug concentrations by weight
may range up to 1% so that luid metering, of say 25-100 μL of suspension,
commonly releases the drug in around 100 μg quantities. 49 Moreover, the
authors admitted that none of these effects was seen in mice that inhaled
lower doses of nanotubes (1 mg/m 3 ), thus implying an overestimation of
the risks and a general scepticism until further long-term assessments are
conducted.
Additional studies by Huczko et al. have contributed to a further
understanding of the impact of nanomaterials on health. The researchers
tested the effect of fullerenes and CNTs on skin irritation and allergen risks. 50,51
Before these studies, there were only a few evidences of contact dermatitis
caused by carbon ibre exposure. 52,53 The authors applied two protocols
usually adopted for testing skin irritation, namely a patch and a Draize rabbit
eye test, respectively. In the irst case, 40 people showing predisposition to
irritation and allergy were treated with a patch of ilter paper that Whatman
saturated with a water suspension of CNTs, and then they were examined
for 96 hours. In the second method, four albino rabbits were instilled with
0.2 mL of CNT aqueous suspension, and observed after 24, 48 and 72 hours.
The absence of adverse effects seemed to be reliant neither on the time of
exposure nor on the type of material used, since there were no differences in
comparison with the reference material (which did not contain CNTs).
In a different study, aimed to evaluate the effects of ingested CNTs, SWCNTs
or MWCNTs were delivered through the food to the larval stage. They both had
no detectable consequence on egg to adult survivorship, despite evidence that
the nanomaterials are taken up and become sequestered in tissue. 54 However,
when these same nanocarbons were exposed in dry form to adults, some
materials (such as SWCNTs) adhered extensively to ly surfaces, overwhelmed
natural grooming mechanisms and led to impaired locomotor function
and mortality. Conversely, MWCNTs adhered weakly, could be removed by
grooming and did not reduce locomotor function or survivorship. Therefore, it
seems more likely that these discrepancies are primarily due to differences in
nanomaterial superstructure, or aggregation state, and that the combination