Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 327

One important issue to consider is the upper size limit of the particles to be col-
lected or measured. The actual size cut that should be used for assessing exposure
which relates to potential human health impact will depend on particle behaviour
and subsequent biological interactions. A simplistic view would be to collect only
nanoparticles, which would imply that exclusion of all particles larger than 100
nanometres be appropriate. Two factors, however, mitigate against this simplistic
approach. Firstly, it seems highly unlikely that particles which are just larger than
the 100 nanometre limit are likely to have any signifi cant differences in terms of
potential health impact from particles which are just smaller than 100 nanometres.
Selection of 100 nanometres is therefore somewhat arbitrary. Secondly, in many of
these scenarios, which are described in Section 8.3.2, it is clear that nanoparticles
are unlikely to be present in the air as single discrete particles. More likely is that
these particles will be formed in agglomerates or attached to other materials or
matrices which are likely to result in individual entities which are greater than 100
nanometres. At the current time it is unclear whether the biological impact of dis-
crete particles depositing within the respiratory tract is similar to or different from
the impact of larger conglomerate of the same particles. In terms of specifi c specifi c
surface area, three or four nanoparticles clumped together to form an aggregate
are likely to have a similar specifi c specifi c surface area to three or four individual
nanoparticles. In addition, it is conceivable that aggregates of nanomaterials will
disaggregate following deposition in the lung resulting to an identical dose to the
lung as it would result from the inhalation of four discreet nanoparticles. Taking
these issues into account, it seems that any decision at this time to exclude particles
greater than 100 nanometres is premature.
Methods of measurement of nanoparticles in air have been reviewed by Maynard
and Aitken (2007) amongst others. Table 8.4, which is adapted from Maynard and
Aitken (2007), provides a summary of the various types of devices and approaches
that can used to provide measurement information on nanometre sized aerosols
for the estimation of exposure. Additional information on methods is available in
in PD ISO/TR 27628 (ISO, 2007) and is discussed in more depth in Chapter 5.
Measurement of number concentration. Simple optical particle counters such as
the Grimm 1.104 Work-Check or SKC 3886 Handheld Laser Particle Counter (SKC
Inc.) have a lower detection size limit that is governed by the wavelength of their
light source. In most cases this means that they cannot ' see ' or count particles less
than 300 nm. As a result, these devices have little value in the measurement of
nanoparticles.
There are, however, several more sophisticated devices which can provide esti-
mates of particle number concentration, some of which have been used in studies
investigating nanoparticle exposure levels (Wake, 2001; Maynard et al. , 2004 ). None
of these are personal sampling devices.
The most widely used type of instrument for detecting and counting nanoparticle
aerosols is the Condensation Particle Counter (CPC). These devices operate by
condensing a vapour (typically isopropanol, butanol or water) onto particles which
have been drawn into the instrument, to grow them to a size range that can be
detected optically by a standard optical counter. These devices are produced