Mechanisms of Particle Deposition - Aerosols: Measurement, Dosimetry, and Health Effects - page 60

Flows in physical models of more complex branching networks (e.g., branching models derived
from casts of human or animal lungs) have been studied experimentally. 84,85,90 However, the charac-
terization of features such as secondary currents, particularly in downstream daughter branches, is
dificult in such studies. Recently, CFD has been used to model airlow characteristics in a variety of
branching models. 92-97 Such models, and their use in predicting particle deposition, will be reviewed
in detail in Chapter V, Modeling Inhaled Aerosols.

3.7 PARTICLE MOTION
The quantitative assessment of factors affecting inhaled particles was pioneered by Findeisen. 98 The
subsequent work of Beeckmans 99 and Landahl 100 reined the analysis of particle deposition. The
importance of these early efforts should be recognized.
Particle deposition within airways is governed by three primary mechanisms, (inertial impaction,
sedimentation, and diffusion), and several secondary mechanisms (interception, electric charge,
and cloud motion). The respective deposition eficiencies of these mechanisms are dependent upon
interactions among aerosol characteristics, ventilatory parameters, and lung morphologies.

3.7.1 P riMary d ePosition M ecHanisMs
Particle deposition eficiencies for inertial impaction, sedimentation, and diffusion are dependent
upon luid dynamics, airway geometries, and particle characteristics. It is, therefore, necessary to
formulate expressions for deposition eficiencies that speciically consider these factors. Deposition
eficiency is deined as the ratio of the number of particles deposited within a given respiratory sys-
tem region or airway to the total number of entering (or inhaled) particles.
Many individual equations have been developed to determine particle deposition eficiencies
within different regions of the respiratory system. The equations most commonly used include
those formulated by Martonen, 24,101 Beeckmans, 99 Landahl, 100 and Landahl and Herrman, 102,103
and Ingham. 104-106 The respective equations offered by the authors were derived using differing
assumptions (e.g., turbulent versus laminar conditions) and velocity ields (e.g., uniform versus
parabolic velocity proiles). Some investigators have selected equations for use from the aforemen-
tioned authors without recognizing their incompatibilities. For instance, authors have used a sedi-
mentation equation for a uniform velocity ield while simultaneously using an impaction equation
for a parabolic velocity ield. This has led to confusion in the literature. To address this problem,
Martonen 24,101 presented a consistent, compatible set of formulae, being derived from well-deined
conditions applicable to airways. These expressions are discussed in the following.

3.7.1.1 Inertial Impaction
Inertial impaction occurs when particles have suficient momentum to deviate from luid stream-
lines and strike boundary airway surfaces (Figure 3.5A). Because momentum is the product of mass
and velocity ( mV ), inertial impaction is an important deposition mechanism for large particles,
usually greater than 1 μm in size. Inertial impaction is increasingly effective at higher velocities;
thus, it occurs primarily in the upper airways of the tracheobronchial tree. 75 Inertial impaction as a
mechanism of particle deposition has been widely studied. 107-111

3.7.1.1.1 Laminar Conditions
Martonen 24,101 expressed particle deposition eficiency from inertial impaction under laminar plug
low conditions as

2

⎣

2 1 2 / − 1

⎦

P ( ) I = β (1 − β ) + sin ( ) β (3.22)

π