Mechanisms of Particle Deposition - Aerosols: Measurement, Dosimetry, and Health Effects - page 55

important to note that all of this work deals with the assumption of spherical or semi-spherical par-
ticles (aerodynamic diameters), with no work being performed on iber inhalability. Fiber research
is needed to better understand inhalability, particularly in the wake of the World Trade Center
collapse that released large quantities of ibers into lower Manhattan (man-made vitreous ibers
[MMVFs], asbestos, carbon nanotubules, etc.), 50,51 also because of concerns about asbestos expo-
sure and inhalation during remediations, 52,53 and because of the emerging research and development
area of nanoparticles/nanotubules. 54,55
3.5 DEPOSITION IN THE EXTRATHORACIC REGION
Particle deposition in the extrathoracic region of the human respiratory system has been studied
far less than deposition in the human lung due to the extreme complexities and irregular geom-
etries of the extrathoracic region (nose, mouth, throat, and larynx). The area of the extrathoracic
region that has been studied the most is the nose. Experimental work in nasal casts, nasal repli-
cas, and even resin models has been performed, with the most recent work on the nasal replicas
of infants and children. 56-58 In their human subject study, Bennett et al. 58 explored the contribu-
tion of the nose to ine particle deposition in children. They found that the child's nose was less
eficient at iltering larger (2 μm) particles and under higher low conditions than the adult nose.
Cheng, in his 2003 paper, 59 reviews the literature on the deposition of particles in the extratho-
racic region of the human respiratory system and performs an analysis combining data from
casts as well as human subjects. This analysis resulted in regional deposition eficiency equations
for the oral and nasal cavities based on the diffusion and impaction mechanisms of deposition
discussed in Section 3.7. Recent work by Smith et al. 59 has also explored the clearance of inhaled
particles from the human nose.
Modeling work, both mathematical and CFD in nature, has been performed to predict par-
ticle deposition in the extrathoracic airways. 60-69 One computational particle luid dynamics
(CPFD) model in particular, 70 combines both the extrathoracic and the airways for a complete
three-dimensional model of the human respiratory system. This model was developed using a nose
and mouth constructed from imaging data from the U.S. National Library of Medicine Visible
Human Project 71 and a ive-lobe lung airway model constructed using the morphological data of Yeh
and Schum. 72 For additional information on particle deposition in the extrathoracic region, we refer
the reader to Chapter 5, as well as the emerging literature on this subject.
3.6 FLUID DYNAMICS IN AIRWAYS
Both motion and deposition of aerosols in the respiratory system are dependent upon airlow
conditions within the airways. Particles will be affected by the nature of the velocity and pressure
ields in which they are carried. Particles are entrained in the airlow, and are transported with
both the bulk convective motion of the low and any other secondary low patterns initiated by
airway geometries. Therefore, the luid dynamic conditions (e.g., laminar versus turbulent motion)
of air in the bronchial tree are important considerations in both inhalation toxicology and aerosol
therapy.
The dynamic behavior of air in the respiratory passages is governed by both morphological and
respiratory parameters. Morphological considerations include airway dimensions (airway diam-
eters and lengths), bifurcation angles, spatial arrangement of the branching network, and airway
surface characteristics. Respiratory parameters describe the mechanics of ventilation, and include
respiratory rates and tidal volumes. In this section, different lung airway morphologies and their
corresponding airlow characteristics will be discussed. Both idealized and anatomically realis-
tic morphologies will be introduced. A more advanced discussion of luid low modeling and its
incorporation into aerosol deposition modeling will be provided in Chapter V, Modeling Inhaled
Aerosols.