Unattached Fraction of Radon Progeny as an Experimental Tool in the Assessment of the Risk of Nanoparticles* - Aerosols: Measurement, Dosimetry, and Health Effects - page 434

deposition parameters is still needed in order to validate the results of modeling. We have discussed
the potential use of radon progeny as a radioactive marker, which can be used as an experimental
tool in measurement of local deposition parameters and dosimetry of nanoaerosols.
The irst problem faced in using radioactive markers is that of safety. We discuss three settings
of human exposure experiments using radon decay products. It is clear from the comparison, with
careful controls, that the risk of such experiments is minimal. The beneits from gaining information
on lung deposition and dosimetry will be extremely valuable. Inter-species variability in respiratory
system morphology and physiology is so great that similar data will be impossible to obtain in
animal experiments.
A review of literature on environmental health in the new rapidly developing nanotechnology
industry shows that the problem of exposure has not been adequately assessed (Oberdörster
et al., 2005). Worker health and safety is of initial concern as occupational groups are likely
to be among the irst to be exposed to elevated concentrations of nanomaterials. A gap exists
between existing particle measurement methods and those truly appropriate for nanoaerosol
exposure assessment. Until now, the primary tools available for the measurement of nano-
sized aerosols have been CPCs, and DMA. Results of the particle counting detection eficiency
of the Condensation Particle Counter TSI CPC 3762 for different operating parameters have
been presented (Banse et al., 2001). This study showed a substantial decrease in the eficiency
in the range of particle diameter of 6-10 nm. DMA measurements suffer mainly due to the
low probability with which in this range of sizes nanoparticles are charged (NSF, 2003). Even
with improved aerosol instrumentation for nano-sized particles, the issues of respiratory tract
deposition quantitation cannot be resolved without a direct localized measurement of particle
dose.
The experiments at PSI and the measurements on miners serve as a model for experiments
that can be performed in laboratory conditions. These experiments can provide accurate data on
human breathing characteristics, deposition, lung dosimetry, and the assessment of true eficiency
of respirators.
A new instrument on the market, the Nanoparticle Surface Area Monitor, is used for the assess-
ment of DSA in the lung. Lung deposition estimates from this instrument are based on correlations
developed (Wilson et al., 2004) between the electrical signal and modeled DSA. The instrument is
said to be capable of detecting particles with diameters down to 10 nm. Another attempt to solve
the problem of surface area assessment, previously presented (Maynard, 2003), is based on simul-
taneous number and mass concentration measurements. Of course, with these data, only for air
concentration, it is not possible to calculate the true dose for target cells in the respiratory system in
terms of the density of the number of particles per cm 2 , especially because results from modeling
(Balásházy and Hoffman, 2000) showed that such particles can form “hot spots,” that is, extremely
high local density of particles.
Clearly, all human experiments with nanometer particles labeled with radioactive markers need
careful consideration from the point of view of both radioactive and nonradioactive nanometer
aerosols safety. Still, in the balance, the scientiic need for information from good human lung
dosimetry experiments suggests that the problem should be considered.
From these observations, the following conclusions can be made:
1. Radon decay products (as a radioactive marker) can be used in a controlled manner that
provides minimal risk in human studies under laboratory conditions.
2. Particle deposition, lung dosimetry, measurement of the breathing characteristics, and
respirator eficiency for nanometer-sized particles may be measured by noninvasive
techniques under laboratory conditions, in the range of exposures described in this chapter.
3. The unattached fraction of radon progeny can be used for quantitative assessment of a
very important characteristic of nanometer-sized aerosols, that is, the aerosol particle
surface area.