Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 306

dimensions in the nanoscale' and a nanoparticle as a ' nano - object with all three
external dimensions in the nanoscale'. A nanomaterial is a 'material having one or
more external dimensions in the nanoscale or which is nanostructured', with nano-
structured being defi ned as 'possessing a structure comprising contiguous elements
with one or more dimensions in the nanoscale' . Defi nitions are also provided for
nanorods ('nano-object with two similar external dimensions in the nanoscale and
the third dimension signifi cantly larger than the other two external dimensions' ),
nanofi bre ('fl exible nanorods') and nanotubes (' hollow nanorods ' ).
A further key issue to consider is the role of agglomerates. It is useful here to
distinguish between aggregates and agglomerates. These terms are not used rigor-
ously in the literature but in ISO (2007) the following defi nitions are proposed:

• Agglomerate: A group of particles held together by relatively weak forces, includ-
ing Van der Waals' forces, electrostatic forces and surface tension.
• Aggregate: A heterogeneous particle in which the various components are not
easily broken apart.

Hence, while an aggregate may be considered to be permanent, agglomerates
may break up under certain circumstances. The issue is whether an aerosol of
primary particles in the nanometre scale that exists as loosely bound agglomerates
in the micrometre scale can be considered a nanoparticle aerosol? A strong argu-
ment for including these agglomerated aerosols as nanoparticle aerosols is that in
the lung they may break up, resulting in primary nanometre scale particles becom-
ing distributed in the respiratory tract. Oberdorster et al. (1994) found that particles
in the form of agglomerated TiO 2 aerosol had increased toxicity when compared
with an aerosol of larger TiO 2 particles, even though both aerosols were similar in
aerodynamic size distribution. This has important implications for how nanoparticle
aerosols are measured or even detected.
All of these types of objects are potentially of interest in relation to occupational
and other types of exposure.

8.2.2 Nanoparticle Types

The development of new nanomaterials is a rapidly progressing science and it is
beyond the scope of this chapter to track all of these developments. However,
several excellent summaries are available. A special edition of the Journal of
Materials Chemistry (Rao, 2004; Rao et al. , 2004) published 47 papers concerning
the development of new nanomaterials, including metallic nanoparticles, germa-
nium, ceramic and aluminium oxide nanowires, carbon, silicon and germanium
nanotubes, zinc oxide nanocrystals, gold nanowafers and copper oxide nanocubes.
Table 8.1, adapted from Jortner and Rao (2002), summarises the main categories
of nanoparticle according to their morphologies, material from which they are
composed and the type of application in which they may be used.

8.2.2.1 Fullerenes

Fullerenes are a family of carbon allotropes, molecules composed entirely of carbon,
in the form of a hollow sphere, ellipsoid, tube or plane. Spherical fullerenes are also