Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 311

per hour. Production of the fi nest grades of material was previously referred to as
micronising. Production of particles in the nanometre size range is referred to as
ultrafi ne grinding (Mende et al. , 2003 ) or nanosizing (Merisko - Liversidge et al. ,
2003 ).
The process involves wet milling in high shear media mills. Mende et al. (2003)
used a stirred media mill to produce suspensions of fused carborundum with a
median per hole size of 50 nm. The milling chamber comprised rotating perforated
plates. Alumina suspensions were also produced. Due to increasing particle- parti-
cle interactions in this process, it was necessary to stabilise the suspension by adjust-
ment of the pH to prevent particle recombination.
Merisko - Liversidge et al. (2003) used a similar process (described as Nanocrystal
Technology) to produce nanometre size drug particles of poorly-water-soluble
compounds. Again stabilisation was required to prevent recombination of the par-
ticles. Milligramme quantities of the drug were produced although it is understood
that higher production rates can be achieved in commercial systems (www.elan.
com). Particles with a diameter of 147 nm were produced by this method.

8.2.4 Nanoparticle Behaviour

In general, occupational hygiene has largely focussed on exposure from the
inhalable route based on the general belief that this was generally the highest in
terms of risk. Hence an understanding of aerosol behaviour is necessary. Aerosol
science is a well understood and described fi eld of science that has been investi-
gated and described over the last 100 years or so. Many excellent textbooks are
available, including Davies (1966) , Fuchs (1964) and Hinds (1999) , which describe
in great detail the fundamental properties of aerosols, their behaviour, their mea-
surement and their applications; the reader is also referred to Chapter 5 in this
volume.
Particle size is the principal parameter governing the behaviour of aerosols.
Aerosol behaviour is governed primarily by inertial, gravitational and diffusional
forces. For particles in the micrometre scale, inertial and gravitational forces domi-
nate. As particle size decreases into the nanometre scale, diffusional forces domi-
nate and particle behaviour is more like a gas or a vapour. In considering how
nanoparticle aerosols may differ in behavioural terms from larger aerosols, impor-
tant aspects include the rates at which particles diffuse, agglomerate, deposit and
re-suspend. It is useful to consider the differences between large (inertial) and small
(diffusional) particles in relation to aspects of exposure and control.

8.2.4.1 Diffusion

As particle size decreases towards the molecular level, their behaviour is more like
that of a gas or vapour (ICRP, 1994). The kinetic behaviour of nanoparticles follows
basic laws of gaseous diffusion.
Particle diffusion (Brownian motion) occurs because particles suspended in a gas
are bombarded by gas molecules causing the particle to move in a random fashion.
The rate at which particles diffuse is determined by the diffusion coeffi cient, which
is inversely proportional to the particle diameter. Particles with high diffusion coef-