Environmental Engineering Reference
In-Depth Information
inhaled particle deposition. Modeling can be a powerful research tool, as it can be used to predict
behaviors, phenomena, or physiological parameters that cannot be measured. In addition, modeling
has the potential to help maximize both inancial and animal resources, by aiding in the design of
appropriate experiments for a given scientiic hypothesis. As noted by Martonen,
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modeling studies
should be integrated in a complementary manner with human inhalation exposure studies.
One of the main challenges in simulation is that its valid and rigorous use may require uniquely
trained scientiic personnel. For example, performing CFPD requires appropriately trained interdis-
ciplinary scientists who are capable of understanding the computational, mathematical, and physi-
ological nuances of modeling complex biological systems. Therefore, rigorous modeling studies
may require collaboration among physicians, toxicologists, and engineers.
5.4 FACTORS INFLUENCING AEROSOL DEPOSITION PATTERNS
In Section 5.3, a brief overview of the different classes of lung deposition models was presented.
Now we will discuss some of the morphological, ventilatory, and situational factors that affect depo-
sition patterns in the human respiratory system. These are factors that may be considered in both
deterministic and stochastic models of particle deposition and distribution.
5.4.1 a
erosol
P
roPerties
As described in detail in Chapter 6, the primary mechanisms by which particles deposit in the
respiratory tract are inertial impaction, sedimentation, and diffusion. The inluence of each of these
mechanisms is dependent on various particle characteristics. Therefore, the vast majority of depo-
sition models will include input parameters describing the size and density of the particles being
studied. Some models only consider a single size of particles, while some may allow the user to
describe a polydisperse aerosol having multiple sizes. Recently, the study of deposition of particles
of nanometer or ultraine scale (nanoparticles) has increased, both in inhalation toxicology
82,83
and
aerosol medicine.
84,85
In addition to size, models also may consider the inluence of particle shape
(e.g., spherical, tubular, ibroid) on deposition.
5.4.2 i
nHalability
Inhalability is the ability of particles to be inhaled from the ambient environment into the mouth or nose.
For small particles (those less than 5-10 μm in diameter) inhalability is essentially equal to 1. For larger
particles, inhalability becomes a concern and, thus, should be considered when modeling respiratory
tract deposition. Particle aerodynamic diameter, breathing conditions, and ambient conditions (such
as wind speed) may all have an inluence on the fraction of particles that can be inhaled. Typically,
inhalability curves are empirical models that have been developed using experiments.
86,87
Millage
et al.
88
provide a review of several algebraic models of inhalability. In addition, CFPD has recently
been used to examine particle inhalability,
89
including investigation of the effect of facial morphology.
90
5.4.3 r
esPiratory
s
ysteM
M
orPHology
Any modeling of the deposition of aerosols in the human respiratory system requires a description
of the morphology of the airway(s) being studied. Both the overall branching structure of the airway
tree and dimensions (e.g., diameters and lengths) of individual airways must be considered. Both
idealized morphology models and models based on speciic experimental observations have been
used in particle deposition modeling.
5.4.3.1 Idealized Models
Many morphology models of the respiratory system have been derived from experimentally
obtained morphometric data. Early morphometric models were simpliied to provide idealized
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