Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 313

8.2.4.3 Deposition

Particles may be removed from the atmosphere by depositing onto fl oors, walls and
other surfaces. The gravitational settling velocity of the particle is proportional to
its diameter. Therefore, airborne nanoparticles will fall out much more slowly than
larger particles and gravitation settling will not be an effective removal process.
This, potentially, would have the implication of causing higher and longer exposures
for more workers. However, deposition of particles will occur primarily because of
diffusion. The high mobility of nanoparticles means that they will migrate quickly
to and become trapped in the boundary layer on all surfaces including walls, ceilings
and fl oors. Deposition of nanoparticles will therefore be independent of orientation
but will be diffusion controlled and not sedimentation controlled.
Hence, a leakage of nanoparticles would end up widely dispersed and deposited
on all surfaces throughout the workplace. This contrasts with larger particles in
which any leakage would tend to be more localised and produce relatively localised
contamination. Decontamination after a leakage of nanoparticles would therefore
be much more diffi cult than after a leakage of larger particles and so the clean up
processes may not be as effective. This could mean that small, widely dispersed
deposits of nanoparticle material could remain attached to the surfaces for much
longer periods, leading to possible chronic exposure resulting from other routes
such as dermal and ingestion exposure.
These behavioural aspects all relate to an airborne release of nanometre size
particles. In principle, this event could occur within the synthesis process of a gas
phase or vapour phase production system. There would be much less likelihood of
a release of this type in a liquid phase process.

8.2.4.4 Re - suspension

An alternative scenario is the re-suspension of nanoparticle material during powder
handling or powder mixing activities in the recovery stage of the process. In this
case, the issue is whether or not a powdered nanoparticle material, which has been
aggregated into bulk form, is likely to become re-suspended as a nanoparticle
aerosol. Re-suspension of aerosols from bulk powders is extremely complex and
not easily predicted on a theoretical basis. Many factors can infl uence the possibility
of re-suspension, including particle size, particle shape, particle charge, the energy
used in the activity which causes the re-suspension and the moisture content of the
powder.
Aerosol particles that contact one another generally adhere and form aggregates
or agglomerates. These particles tend to stick together because of attractive Van
der Waals' forces, which act over very short distances relative to particle dimen-
sions. Van der Waals' forces would also act to keep a particle attached to a surface.
The implication of this is that once attached (or agglomerated) small particles
would be much more diffi cult to split up or re-suspend than large particles.
In the United Kingdom, the concept of dustiness has been applied to powders.
Dustiness is an index of the relative ease by which powder materials can become
re-suspended (HSE, 1996) and is assessed by agitating bulk materials and measur-
ing the aerosol which is produced. Lyons and Mark (1994) assessed the dustiness