Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 329

commercially by several manufacturers. Generally they are small, battery
operated and hand-held with some cases claiming lower detection limits of down
to 3 nm. One example of this type of device is the TSI Model 3007 Condensation
Particle Counter which is a hand-held device, highly portable, with a claimed size
range window of 10-1000 nm and a concentration range of 0- 100 000 particles per
ml (www.tsi.com). Claims of manufacturers for instrument measurement ranges
must be treated with caution and validated prior to use. If this type of validation
and calibration is not performed adequately, collected data is likely to be of poor
quality.
One of the diffi culties in using CPC to measure nanoparticles in occupational
scenarios is that they cannot discriminate between particles produced by a work-
place source and those particles arising from combustion processes or natural
sources which are generally present in the ambient atmosphere. Unless the work-
place is isolated from the external environment and all air entering the working is
fi ltered, for example in a clean room, then particle counts in indoor workplaces are
likely to be similar to those in the outside atmosphere. Other sources on the work-
place, for example heaters, may also contribute towards the overall count. These
diffi culties may be overcome to some extent by the choice on an appropriate sam-
pling strategy (Section 8.3.4).
Measuring particle number concentration in isolation can, however, be mislead-
ing. In all particle number concentration measurements, the integration limits over
which a particular instrument operates are critical in interpreting the reported
results. CPC instruments become increasingly insensitive to particles smaller than
10-20 nm. Concentrations measured with instruments with different sensitivities
might therefore differ substantially, particularly if the particle count median diam-
eter is close to or in this range.
A further limitation of CPC devices is the lack of size information. Instruments
that provide both size and number information are, not surprisingly, larger, more
complex and more expensive. The most commonly used instrument of this type is
the scanning mobility particle sizer (SMPS). Devices of this type (www.tsi.com) are
apparently capable of measuring aerosol size distribution from 3 to 800 nm, although
not simultaneously over the complete range. The size distribution is expressed in
terms of particle mobility diameter. The SMPS operates by charging particles and
separating them based on their mobility passing between electrodes. Separated
particles are then counted to give size range of mobilities. Application of these
devices in occupational hygiene investigations has been limited due to their lack
of mobility, expense and complications in use.
Measurement of particle number concentration for high aspect ratio nanoparticles
( HARN ). In the same way, CPCs do not provide any information about particle
shape and indeed may give biased data if (nano)particles are permeable or non-
spherical. CPCs, therefore, cannot discriminate between HARN and compact
nanoparticle forms and indeed may give misleading data. Off-line assessment of
nanoparticle number concentrations is feasible by collecting particles on to suitable
media and analysis (counting) by electron microscopy in an analogous approach to
that used for counting asbestos fi bres (WHO, 1997), although this approach has to