Environmental Engineering Reference
In-Depth Information
can be used to trace aerosol lung or surface deposition behavior. To do this, a size-characterized
monodisperse aerosol is inserted into an exposure chamber containing particle-iltered air. Rn
progenies are also inserted into the chamber at a controlled or known rate. By measuring the
unattached fraction in the air we can assess the particle surface area of aerosols, see Ruzer
(2008) and Ruzer and Apte (2005), and for monodisperse aerosols consequently we assess the
particle concentration. The ratio between particles attached to the nanoparticle activity and the
nanoaerosol particle concentration itself will be known. After the nanometer aerosol (labeled
with radon progeny) is inhaled into the lungs, the nanometer particles will be locally deposited
according to their size depending on some breathing parameters (volume breathing rate, humidity,
and temperature).
Until now, experimental data on nanoaerosol deposition in human lungs have not been avail-
able. For larger size aerosols mostly bulk deposition data are available, based on the difference in
concentration in exhaled and inhaled air. However, it is well known that biological effects depend
on local deposition.
After exposure, the local gamma emission distribution in the lung can be measured using a
gamma-spectrometer, with the local activity being proportional to the aerosol deposition (dose). In
addition, it may be possible to use SPECT scanning to provide a more precise spatial resolution of
particle deposition and local dosing (Kao et al., 1997; Piai et al., 2004).
As with all such radiotracer studies, the protocol must meet the approval of an IRB and radiological
screening review. In these experiments, as in other studies, when radiation is used as a tool, for exam-
ple, in using radiation in the study of Alzheimer disease, we have to compare the risk with beneit. The
use of such experiments will enable us to close the gaps in our knowledge. Quantitative assessment of
the local deposition of aerosol is at the core of aerosol, and particularly nanoaerosol exposure and risk
assessment. So, our goal will be to ind the safest possible and most appropriate marker.
Radon progenies are attractive as a marker for several reasons:
1. Radon and its progeny belong to the natural background of radioactivity to which the
general population is exposed during their lifetime. Therefore, it is easy to assess the
additional risks due to its use by the methods proposed.
2. Part of radon progenies, called “unattached activity,” are 1 nm sized particles with dif-
fusion coeficient close to 0.06 cm
2
/s (a size that attaches readily to nanoaerosols), which
make it very attractive as a marker for nanoaerosols with a built-in signal.
3. Radon decay products are easy to generate.
4. Radon decay products are short-lived nuclei.
Direct measurement on humans is needed in order to validate the hollow cast, animal studies, and
modeling. From our point of view, this kind of study will be strategically important in nanoaerosol
dosimetry and risk assessment. And it will partially close one of the many gaps in our understand-
ing of nanoaerosol exposure.
The irst step of this study should be human experiments with monodisperse spherical nanopar-
ticles. In the case of nonspherical particles, typically found in aerosol studies, we should use the term
“equivalent diameter,” that is, the diameter of a monodisperse aerosol with the same local deposition
as the aerosol of interest. Study of polydisperse aerosols adds complexity that can be resolved after
the monodisperse aerosol lung deposition characterization across a broad nanometer size range is
completed.
16.2.4 r
adon
P
rogeny
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ool
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ssessMent
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article
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urFace
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rea
As discussed earlier, one very important property of radon decay is that after radon decay, the
newly formed atom of Po forms clusters that are useful as markers in studies of properties of
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