Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 332

Another microscopy technique, atomic force microscopy (AFM), also provides
sub-nanometre resolution and, unlike EMs, can image nanoparticles in air or water
without loss of resolution. AFMs offer signifi cant advantages and some limitations
and are a useful additional technique, but are little used in this area. Chapter 6
contains more information about FM and EM methods.

8.3.3.3 Sampling Strategy

All of the measurement methods that may be used clearly fall short of what would
be an ideal sampling and measurement system for nanoparticles. However, all have
some potential to provide useful information about particular aspects of nanometre
size particles, particularly when they are used in combination. The key issue is to
develop an appropriate exposure assessment strategy, particular to the process and
materials under consideration, which optimises the information available from the
various sources. By using suitable combinations of instruments and methods, iden-
tifi cation of appropriate surrogate measures and using appropriate models, taking
account of determinants of exposure in a structured way and recognising the limita-
tions of all of these, good information concerning the nature of the aerosol and, in
particular, how it may change can be obtained. However, there is a pressing need
for more research into the development of new and improved measurement
methods, combination approaches and the development of generic strategies to
provide reliable assessments of exposure to nanoparticles.
The US National Institute for Occupational Safety and Health (NIOSH)
provides advice on a possible sampling strategy (NIOSH, 2008). Since, at the
current time, there is no single sampling method that can be used universally to
characterise exposure to nano-sized aerosols, it recommends a multifaceted
approach incorporating many of the sampling techniques mentioned above. This
follows the approach suggested by Brouwer et al. (2004) , which recommends that
all relevant characteristics of nanoparticle exposure be measured. The NIOSH
approach recommends a step-by-step process, using different instruments at each
step so as to maximise the information gained. NIOSH recommends that the fi rst
step would involve identifying the source(s) of nanoparticle emissions in a work-
place. A CPC would provide acceptable capability for this purpose, being portable
and simple to use. It is necessary to determine ambient or background particle
counts before measuring particle counts during the process of interest. This may
be possible by taking measurements before and after a process is turned on and
comparing these with measurements during operation of the process. For identifi -
cation of specifi c nanoparticles of interest, area sampling with a fi lter suitable for
analysis by electron microscopy can also be employed. Transmission electron
microscopy (TEM) can then be used to identify specifi c particles and to estimate
the size distribution of the particles.
NIOSH recommends that once the source of emissions is identifi ed, more exten-
sive investigations may be carried out. This could include aerosol specifi c surface
area measurements conducted with a portable diffusion charger and aerosol size
distributions determined with an SMPS or ELPI using static (area) monitoring. It
suggests that a small portable specifi c surface area instrument could be adapted to