High Aspect Ratio Nanomaterials - Aerosols: Measurement, Dosimetry, and Health Effects - page 234

light intensity scattered by particles as they diffuse through a luid. However, the size information
obtained from PCS will be the average diffusion coeficient of the particles and the size is correlated
to the equivalent sphere diameter. Hence, PCS is more likely to be successful in measuring size
if the nanoparticle under investigation is close to being spherical. In the case of HARN, size
information will be limited and the information obtained will be neither the length nor the width
of the particle. Nonetheless, it is possible to probe the state of dispersion, that is, evidence of
agglomeration, taking place in the suspension (relative to the nanoparticles if in the dispersed state,
that is, as close to the “primary” particle size distribution as possible). For example, it has been
used by Lee et al. (2005) for measuring polydispersity and the stability of single-walled CNTs.
10.2.3.2.4 Scanning Mobility Particle Sizer
If the aerosolized form is to be analyzed, then a different technique is needed. For example,
the scanning or fast mobility particle sizers are able to present a particle size distribution of the
aerosolized material, typically for particles between 5 and 500 nm. In the case of the scanning
mobility particle sizer, particles are classiied with an electrostatic classiier, whose voltage is
“scanned” and used to deliver singly charged, monodisperse aerosols of known size and composition
to a condensation particle counter or electrometer, yielding particle size distribution information
(Berne and Pecora, 2000). Again, the interpretation of the results for nonspherical/agglomerated
nanoparticles is not straightforward (Van Gulijk et al., 2004). Limited research has been done in
relation to the characterization of HARN using this technique and again its suitability is questionable
as the nanoparticles under investigation move away from the idealized spherical model.
10.3 TOXICOLOGY OF HARN
There has been considerable concern about the potential health effects that could be caused by
occupational or environmental exposure to HARN. This concern and speciically the link between
carbon nanotubes (CNTs) and asbestos has been raised as early as 1998 in the journal Science under
the title “Nanotubes: The next asbestos?” (Service, 1998). This article was a mere 7 years after the
seminal papers by Iijima (1991; Iijima and Ichihashi, 1993), which reignited the interest in CNT
and paved the way for other HARN. The basis for this comparison of CNT and asbestos is the
morphological similarity that exists between the two forms of material and the general suspicion of
all new industrially used ibers. This is due to the pandemic of disease caused by asbestos exposure
and the ensuing considerable legal and inancial fallout from the use of asbestos that has left both
the public and governments suspicious of new industrial ibers. It is worth considering that this
suspicion has not just been leveled at HARN, but has also been considered for other industrially
relevant ibers. For example, the organic iber para-aramid, which is more commonly known by its
trade name, Kevlar (Donaldson, 2009), has also been under considerable scrutiny for toxicological
effects due to its morphological similarity to asbestos.
10.3.1 P rinciPles
In drawing a comparison between HARN and asbestos (or indeed any other iber such as refractory
ceramic ibers [RCF] or man-made vitreous iber [MMVF]), the focus is on iber toxicity, but a poten-
tial particle effect for ibrous materials is still possible as many types of HARN, such as CNTs, can
be found in ibrous (e.g., as singlet ibers or agglomerated “nanoropes”) and non-ibrous forms (e.g.,
very short rodlike particles, tightly curled spherical bundles, or agglomerates) as shown in Figure 10.2.
Currently, information on the toxicity of HARN exists mainly for CNTs and reviews of the
paradigm which forms the basis of iber toxicity as it relates to CNT has been conducted recently
(Donaldson et al., 2006, 2010, 2011).
Using the WHO guidelines for counting asbestos ibers (WHO, 1997), a particle should be con-
sidered a iber if it has a length of >5 μm, a diameter of <3 μm, and an aspect ratio greater than 3:1.