Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 312

Table 8.2 Coagulation half-life (from Preining, 1998)

Particle diameter (nm) Half life

1 g/m 3 1 mg/m 3 1 mg/m 3 1 ng/m 3

1 2.2 ms 2.20 ms 2.2 s 36.67 min

2 12 ms 12.00 ms 12 s 3.34 h

5 0.12 ms 0.12 s 2 min 33.34 h

10 0.7 ms 0.7 s 11.67 min 8.1 d

20 3.8 ms 3.8 s 63.34 min 43.98 d

fi cient have high mobility and mix rapidly in aerosol systems. Nanometre size
particles will have much higher mobility than particles in the micrometer scale. This
has implications for the ease in which they can be enclosed in systems and the ease
in which they can be controlled. In an enclosed system which has a leak, nanometre
sized particles would be much more likely to escape than larger particles because
of their higher mobility. Therefore, for nanometre particle systems, an enclosure
system design needs to provide a higher level of integrity than for micrometer sized
aerosol systems. Systems normally used to contain gaseous emissions would be
appropriate.
Where particles are released into the workplace, atmosphere diffusion will cause
migration from a higher concentration to a lower one. In this case nanometre size
particles escaping will mix rapidly through the workplace air and will be quickly
dispersed. This has both positive and negative aspects. Nanoparticle aerosols will
not remain localised so the concentration at the site of the leak will fall rapidly.
However, leaking nanoparticles could end up at great distance from the source,
potentially leading to larger numbers of individuals being exposed.

8.2.4.2 Agglomeration

As a result of diffusion, particles will undergo multiple collisions leading to coagula-
tion, agglomeration/aggregation and growth in size. The rate at which agglomera-
tion occurs depends primarily on the particle number concentration and their
mobility; both of these factors increase as particle size decreases. The agglomeration
half-life of different concentrations of nanoparticles of various sizes is shown in
Table 8.2 .
It can be seen from this that very small nanoparticles (e.g. 1 nm) coagulate rapidly
even at low mass concentrations. However, the outcome of this coagulation is still
a nanoparticle, albeit a slightly large one. In turn this will coagulate and grow
further. For larger nanoparticles the coagulation half-life is much longer and there-
fore the growth is slower. While this mechanism will lead to rapid coagulation and,
therefore, very short lifetimes of very small nanoparticles, larger nanoparticles will
persist for longer times.