Biomedical Engineering Reference
In-Depth Information
100
H region
TB region
A region
80
60
40
20
0
1
10
100
1000
Diameter (nm)
FIGURE 2.1
Calculated deposition curves of nanoparticles for the head airway (H), tracheobronchial (TB),
and alveolar (A) regions of the respiratory tract using the LUDEP model software.
mechanism for this region is the diffusion of nanoparticles. In particular, when the nanoparticles are
smaller in size, an increased effect would result with particle diffusions. The head airway does not
have cilia. Therefore, the pharyngeal lymphatic ring located in the pharynx might provide the most
defense against the deposition of foreign matter in this region.
The entire tracheobroncial region (except for a part of the larynx) is coated with cilia. These
flickers move in a coordinated manner with a frequency of 10-20 times per second to transport
mucus and deposited particles upwards to the larynx. It takes ~0.5, 2, and 5 h for deposited particles
to be transported from the broad, medium broad, and fine airways to the larynx, respectively.
In the lowermost airways (alveolar region), there is very limited mixture between just inhaled
air and residual air (i.e., air that remains from previous breaths). The deposition of particles with
diameters between 100 nm and 1 μm in this region is governed by the transfer of particles from
newly inhaled air to residual air. Almost all the particles will deposit at this region through diffusion
and sedimentation for small and large particles, respectively. The mechanisms for clearing particles
from the alveolar region are not completely known yet. The main mechanisms might be associated
with the devouring of the particles by phagocytes.
The International Commission on Radiological Protection (ICRP) developed a model to predict
particle depositions at different regions of the human respiratory tract (ICRP, 1994). The model covers
various breathing characteristics governed by the involved workloads, and a wide range of particle
sizes from 1 nm to 100 μm. It is interesting to note that most micrometer-sized particles deposit at the
head airways. Therefore, the nose is considered as an effective prefilter for protecting against particles
larger than a couple of micrometers, which makes the human lungs fairly well protected from coarse
particles depositing at the more vulnerable lower respiratory tract. But for nano-sized particles with
diameters <10 nm, they will deposit at the head airways (up to 80%), whereas those in the 10-100 nm
range will mainly deposit at the alveolar region (up to 50% for 20 nm particles). For 1 nm particles,
however, 90% deposit at the head region, with 10% at the tracheobronchial region, and nearly noth-
ing at the alveolar region. Figure 2.2 shows the regional deposition as a function of particle size for
adults engaged in light work estimated according to the ICRP Task Group lung model based on Hinds'
parameterization, gender, and activity-weighted average deposition efficiencies (Hinds, 1999).
