Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 346

and subsequent processing are powder handling activities which, if suffi cient energy
is used, may result in the generation of respirable or inhalable concentrations of
agglomerated nanoparticles. It is, however, very probable that these powder han-
dling activities will not generate separated nanoparticles, due to the relatively high
energies which would be necessary to break the (Van der Waals') forces which keep
particles agglomerated. Even generation of larger agglomerations of nanoparticles
as aerosols may be quite diffi cult.
All of the production processes described could potentially result in dermal
exposure, particularly at the powder handling, packaging and bagging stages.
Dermal exposure is likely to result in ingestion exposure from hand-to-mouth
contact. It has been postulated that nanoparticle exposure to the skin can result in
direct penetration through the skin. At least one pharmaceutical company is devel-
oping drug delivery systems based on topical application of lipid nanoparticles. It
is, therefore, reasonable to conclude that nanoparticles depositing on the skin could
potentially penetrate into the epidermis and possibly beyond. As far as the authors
are aware toxicological effects arising from dermal exposure to nanoparticles, or
from ingestion, have not as yet been investigated or identifi ed.
There is evidence to suggest that specifi c surface area is the most appropriate
metric to use to assess exposure by inhalation. This appears to fi t best with current
toxicological evidence and would deal directly with the issue of agglomeration.
Ideally, a personal sampler would be available which could assess this metric.
Although there are now methods by which exposure in terms of specifi c surface
area may be measured, these are largely untried. While a strong case may be made
for using surface area as a metric, this may not be universally so. For high aspect
ratio nanoparticles (HARN), particle number may be more appropriate, as is the
case currently for other high aspect ratio particles, such as asbestos fi bres.
In any case, it is also necessary to consider characterising exposures against
aerosol mass and number concentration until further information and improved
methods are available. For each of these exposure metrics, but particularly in the
case of mass concentration, size selective sampling will need to be employed to
ensure only particles within the relevant size range are sampled. That in itself pres-
ents a problem, however, since at this point it is not clear at what size the selection
should be made. The use of 100 nm is overly simplistic, since there is no reason to
expect that a particle with a size of 105nm would have a signifi cantly different
potential to cause harm compared with a particle with a size of 90 nm. A 100 nm
size selection could also exclude aggregates of sub-100 nm particles. Again, from a
hazard perspective, there is no reason to do this.
For dermal exposure and for ingestion exposure, measurements should also be
biologically relevant. At this stage there is insuffi cient evidence to indicate whether
mass, number or specifi c surface area is the most appropriate metric. For dermal
exposure, measurement approaches should ideally also consider the skin area
exposed and the duration of exposure.
At the current time, information about the exposure of workers engaged in the
manufacture and use of new nanoparticles is very limited. Only a handful of studies
have been published thus far and it is too early to draw any solid conclusions from
these. For high aspect ratio nanomaterials, Maynard et al (2004) demonstrated that