Measurement and Monitoring Strategy for Assessing Workplace Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 232

morphological analyses. The measurements often focused on particle number concen-
trations and their size distributions, because they are considered to be the most sensi-
tive particle metrics when it comes to assessing exposure to nanomaterials. Although
nanoscale particles usually occur in high number concentrations, they commonly only
contribute negligibly to the total particle mass concentration, because the particle mass
scales with the third power of the particle size. Some studies also suggested that health
effects of inhaled particles may correlate better with their number rather than their
mass (Peters et al. 1997) while other studies have reported that the particle surface area
may play a more critical role (Oberdörster 2000; Tran et al. 2000).
Measurement instruments that are commonly used in exposure assessment are
condensation particle counters (CPCs, McMurry 2000) for measuring number con-
centrations and scanning mobility particle sizers (SMPSs, Wang, and Flagan 1990)
or similar electrical mobility spectrometers to determine the submicron particle
number size distributions (see Chapter 2). Since nanoscale particles can occur as
micron-sized agglomerates, concentrations of larger particles are often assessed by
means of optical spectrometers or aerodynamic particle sizing. These instruments
are rather bulky and mains operated. Simplified devices for measuring concentra-
tions of airborne particles in workplaces are provided by different handheld, bat-
tery driven instruments (see Chapter 2), such as handheld CPC or diffusion charger
based instruments (Fierz et al. 2011; Marra, Voetz, and Kiesling 2010; Asbach et al.
2012a; Kaminski et al. 2013). These instruments all measure the particle number
and/or lung deposited surface area (LDSA) concentration and are hence usable for
an easy screening of the workplace aerosol. Most diffusion chargers additionally
measure the mean particle size. All the above-mentioned measurement techniques
have in common that they only measure physical quantities, that is, particle size
and/or concentration, but inherently they do not allow for the differentiation of the
engineered nanomaterials from ubiquitous background particles, typically stemming
from outdoor sources, combustion processes, or other human activity in the near and
far field of the workplace (for a perspective on background sources and effects, com-
pare Chapter 5). Workplace exposure measurements therefore always require knowl-
edge about the particle background in the workplace, for example, by measuring the
background concentration prior to and after the work process under investigation
or simultaneously with a second set of equipment at a representative background
site. Additionally, all these measurements can be accompanied by particle sampling
for subsequent analysis of the particle morphology or the chemical composition.
Sampling is typically done on filters or electrostatically onto flat surfaces, using
an electrostatic precipitator (Dixkens and Fissan 1999). These analyses can then
provide a more definitive proof for the presence or absence of the engineered nano-
material in the workplace air. Still, the choice of instruments has to be carefully
considered depending on the measurement task. Their use and data evaluation has to
follow harmonized protocols concerning, for example, the placement of instruments,
particle size and concentration ranges to be covered, averaging time intervals, and
so on. It is obvious that such exhaustive measurement campaigns are very labor and
cost intensive. Since there is no legal framework that outlines the measurements and
their data evaluation, the results of such measurements are not comparable with sim-
ilar measurements carried out elsewhere and also not suitable for storage in exposure