High Aspect Ratio Nanomaterials - Aerosols: Measurement, Dosimetry, and Health Effects - page 235

1 µm 20 µm

(a) (b)

FIGURE 10.2 Morphological structure of MWCNT as shown by TEM (a) and SEM (b).

Aspect ratio is deined as the relationship between length and diameter of a particle or speciically
the ratio of the longest dimension to the shortest dimension. An aspect ratio of 1:1 would equate to
a spherical particle. The increased disparity between length and diameter results in an elongated or
ibrous shape. Aspect ratio, therefore, deines a iber by describing this relationship. As mentioned,
the WHO deinition of a iber requires it to have an aspect ratio of greater than 3:1 and a length
greater than 5 μm. However, nanoparticles with a high aspect ratio are not necessarily ibers . For
example, a nanoparticle with a diameter of 40 nm and length of 3 μm has a very large aspect ratio
(75:1) but would not be classiied as a iber according to the WHO deinition. To identify a nanopar-
ticle as having a high aspect ratio is, therefore, not solely suficient to classify it as a iber and if it
does not meet all the WHO criteria (minimum diameter, length, and aspect ratio) then it would not
be counted as a respirable iber under the WHO method.
Another point of comparison that should be borne in mind is the level of contaminating metals
found both with asbestos and HARN. The mineralogical nature of asbestos and the nature of the
environment in which they form mean that asbestos ibers are contaminated with various met-
als; the speciic elemental constituents of asbestos ibers can act as a ingerprint of their source.
Amphiboles such as crocidolite are known to be contaminated by iron in the form of ferric oxide
(Fe 2 O 3 ) (Virta and Geological Survey (U.S.), 2002), which has been suggested as a contributing
factor to its toxicity and carcinogenicity (Broaddus et al., 1997; Unfried et al., 2002). Depending
on the synthesis method, substrates, and catalysts used, CNTs can also be contaminated with
metals such as iron (Fe), nickel (Ni), or cobalt (Co). These impurities are often removed dur-
ing processing, but amounts can still remain in the inished product and may contribute to the
observed toxicity, as has been shown in vitro with single-walled CNTs (Kagan et al., 2006).
Other forms of HARN may also be contaminated with reactive metals or may indeed be com-
posed of such metals (e.g., nickel nanowires), which can also present “conventional” elemental/
molecular toxicity.

10.3.2 F iber P atHogenicity P aradigM
The basis of iber toxicity has been the subject of scrutiny over the past 30 years and while asbestos
exposure leading to disease has provided the driving force toward understanding the pathogenicity
of ibers, it is this difference in pathogenicity of different types of ibrous materials that has facili-
tated the understanding and development of structure-activity relationships. Some ibers, such as
asbestos, have been shown to be carcinogenic (IARC group 1 [IARC, 1987]); others such as RCFs
are considered only possible carcinogens (IARC group 2b [WHO IARC, 2002]), while many others
such as soluble glass ibers are considered nonpathogenic.