Environmental Engineering Reference
In-Depth Information
to established criteria for the measurement of occupational exposure to airborne
particles, based on measurement of particle size.
International standards (CEN, 1993; ISO, 1992; ACGIH, 1993) have been devel-
oped which defi ne the size of particles capable of penetrating to the various regions
of the respiratory tract. The three most commonly used standards relating to occu-
pational exposure are the inhalable fraction (representing that fraction of aerosol
which can enter the respiratory tract), the thoracic fraction (particles capable of
penetrating to the airways below the larynx defi ned according to a selection curve
with a 50% cut-off at 10
m (10 000 nm)) and the respirable fraction (particles,
defi ned according to a selection curve with a 50% cut-off at 4
µ
m (4000 nm), which
can penetrate beyond the ciliated airways to the gas exchange region of the lung).
These conventions represent size fractions which are much larger than that nor-
mally considered relevant to nanometre-diameter particles.
These standards defi ne sampling conventions for particle size fractions that
are to be used in assessing the possible health effects resulting from inhalation
of airborne particles in the workplace. In principle, they are derived from
experimental inhalation data for healthy adults. These specifi cations are stated
in terms of mass fractions, but they may also be used when the intention is to
evaluate the total specifi c surface area or the number of particles in the collected
material.
Although in common use, the terms ultrafi ne particle, ultrafi ne aerosol, nanopar-
ticle, nanoparticle aerosols and nanostructured particles, have not been so rigor-
ously defi ned leading to potential confusion.
In relation to environmental pollution studies it has been common to use the
term ' ultrafi ne particles', although often the term is not defi ned. Where defi nitions
are provided there has been broad agreement that ultrafi ne particles were those
with a diameter ' less than ' 100 nm (Preining, 1998). Generally this was taken to
imply the physical diameter of the particles, although it could imply a diffusion
diameter, as instruments to measure particles in this size range often use diffusion
as a classifying mechanism. Particles are seldom present as a single size (monodis-
perse) but rather can be represented by a distribution of sizes, which is commonly
characterised by median (in terms of mass or number) and a geometric standard
deviation. This simplistic defi nition (less than 100 nm) fails to take account of size
distribution. It is not clear, for example, whether the defi nition implies all particles
less than 100 nm, 95% of particles less than 100 nm, a mean (or median) of less than
100 nm or any particles less than 100 nm. Sometimes, the term ' nominal diameter '
has been used to provide an effective dimension, where information about particle
shape was not available or the particles were known not to be spherical. The ter-
minology used by industry can be even less consistent. For example, the website of
a company involved in powder handling and processing defi nes micronising as the
'production of an ultrafi ne powder with a top size of 20 microns' (i.e. equivalent to
20 000 nm).
A recent attempt to develop a more structured approach has been published by
the British Standards Institution (BSI) (BSI, 2007a). In its 'Terminology for nano-
materials' (BSI, 2007a) it defi nes nanoscale as 'size range from approximately 1 nm
to 100 nm ', a nano-object as a 'discrete piece of material with one or more external
µ
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