Unattached Fraction of Radon Progeny as an Experimental Tool in the Assessment of the Risk of Nanoparticles* - Aerosols: Measurement, Dosimetry, and Health Effects - page 422

16.1.6 n anoParticle d ePosition in tHe l ung (r igHt
b rancH oF tHe d osiMetric r oad M aP )
There is a lack of systematic studies on nanoparticles lung deposition.
One of the most important dificulties in the assessment of the dose as a main cause of the effect
of aerosols, particularly in the nanometer range, is the lack of information on local deposition in
human lung.
Our approach to this fundamental problem we presented in Ruzer and Apte (2010a,b).
Operationally, the concept uses these 218 Po radon progeny as a radiolabel. These particles have
a very high diffusion coeficient and readily attach to other particles in air. When attached, or
aggregated, with the environmental aerosol, these particles are called “attached activity.” Given
their high diffusion coeficient, their attachment eficiency can approach 100%. Thus, almost every
particle in the environmental aerosol becomes labeled with a radioactive radon progeny particle,
destined to decay and emit Gamma particles ( 214 Pb and 214 Bi). Particle inhalation experiments may
be designed in which relatively low concentrations of radon gas is mixed with a non-active study
aerosol that will subsequently be inhaled by subjects. As the radon atoms decay, their progenies
attach to the study aerosol particles and thus radiolabel them.
In this case every measured gamma-quantum corresponds to a nonradioactive aerosol particle
in the nanometer range locally deposited in the lung. So, the measured gamma-activity will repre-
sent dose of nonradioactive nanoaerosols at the target .
According to dosimetric road map for nanoparticle dose assessment we have to have data on
translocation of nanoparticles across the air-blood barrier and cell penetration.
In SCENIHR (2007), Geiser et al. (2008), and Muhlfeld et al. (2007), a review of such studies
is presented.
In animal studies for titanium dioxide as well as for diesel exhaust particles (Geiser et al.,
2005; Muhlfeld et al., 2007; Nemmar et al., 2002), it was demonstrated that a small fraction of the
nanoparticles can be translocated to the circulation and can reach extrapulmonary organs via the
bloodstream.
In human lung one study (Neemar et al., 2002) reported a rapid and signiicant translocation
of inhaled carbonaceous nanoparticles to the systemic circulation and extrapulmonary organs. In
contrast, other subsequent studies failed to conirm this inding and detected only a low degree of
translocation for iridium (Kreiling et al., 2002) or carbon nanoparticles (Mills et al., 2006; Moller
et al., 2008; Wiebert et al., 2006).
The studies of Nemmar et al. and Mills et al. had a very similar design and Mills et al. have
argued supporting the fact that the strong translocation of 99m Tc labeled particles observed by
Nemmar et al. was mainly related to the translocation of soluble 99m Tc.
A very speciic property that has been assigned to inhaled nanoparticles refers to translocation
from the nasal epithelium to the brain. Elder et al. showed that ultraine manganese oxide particles
translocate to the olfactory bulb and other regions of the central nervous system.
Additional support for translocation of ultraine particles via olfactory axons comes from a study
(Oberdörster et al., 2004) that showed that inhaled radioactive carbon particles were signiicantly
enhanced in the olfactory bulb after exposure, in contrast to other brain regions that showed only
inconsistent increases in carbon particles.
In conclusion we could say that nanoparticles can enter the blood circulation from the respira-
tory tract and be deposited mainly in the liver, spleen, and other organs. Even if the amount of these
nanoparticles are small in terms of mass in comparison with the total amount that entered the body,
we cannot say that the locally deposited number at the target, that is, dose and effect will be small
and unsubstantial.
As the knowledge of long-term behavior of nanoparticles is very limited, a conservative estimate
should be assumed that insoluble nanoparticles may accumulate in secondary target organs during
chronic exposure with unknown consequences.