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of several materials including 'fi ne' carbon black and silica fl our. Results were
assessed in terms of mass in the respirable and inhalable fractions. Both of these
were found to be similar in dustiness to other common materials such as animal
feeds and plaster. Information on dustiness as a function of particle number would
be a more useful basis for comparing nanoparticle powders.
Work to extend this approach into the nanoparticle size range has begun but is
at an early stage. Maynard
et al.
(2004) investigated the relative ease with which
carbon nanotubes could be re-suspended. They used a fl uidised bed to try to gener-
ate aerosols from bulk material. They concluded that it was very diffi cult to gener-
ate an aerosol from these materials. This study is discussed in more detail in Section
8.3.4 .
Schneider and Jensen (2008) investigated the dustiness of nine powders in which
there was thought to be a nanoparticle component. These included ' pigment grade '
TiO
2
, ' ultrafi ne' TiO
2
and fused silica. They used techniques based on the ' rotating
drum' and ' single drop ' methods in EN 15051 (CEN, 2006), but adapted to use
smaller quantities of test material and measured the particle size of the generated
aerosol. For all of the powders they tested they found, for the rotating drum
method, a bimodal distribution with one peak in the range 101- 219 nm and
the other ranging from 1360 to 2564 nm. An exception was pigment grade TiO
2
,
where no particles were detected in the smaller particle mode. The results from the
single drum method were very similar. Interestingly, the total number of particles
in each mode was similar for all cases, at least within one order of magnitude.
Dustiness as quantifi ed by particle number and by the mass-based dustiness index
had a large range. The lowest dustiness was found to be for pigment grade TiO
2
and the largest for ultrafi ne TiO
2
. In the large particle mode, the difference was
about a factor 350 based on particle number mode and a factor 280 based on mass.
In the small particle mode, ultrafi ne TiO
2
had the highest dustiness while none could
be detected for the pigment grade. These fi ndings suggest a correspondingly large
difference in exposure potential. In particular, they suggest that preventive mea-
sures would have to be much stricter if pigment grade TiO
2
were to be replaced by
the ultrafi ne version.
Taken together, these two studies suggest that high aspect ratio nanomaterials
such as CNT may be relatively diffi cult to re-suspend, whereas materials such as
TiO
2
might be relatively easy and be more ' dusty ' in ultrafi ne form. However, given
the complexity of the area it is too early to make such generalisations with
confi dence.
8.3
Nanoparticle Exposure
8.3.1
Exposure Scenarios
There are multiple scenarios through which humans could become exposed
to engineered nanoparticles. Figure 8.1 is taken from the Royal Society/Royal
Academy of Engineering report (RS/RAEng 2004) and illustrates a range of
possible occupational, environmental and consumer exposure scenarios.
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