Mechanisms of Particle Deposition - Aerosols: Measurement, Dosimetry, and Health Effects - page 48

3.1 INTRODUCTION
Particle deposition is an important topic of concern to diverse areas of aerosol science and technol-
ogy, ranging from the design and manufacture of equipment for aerosol generation and charac-
terization to the study of human health effects in aerosol therapy and inhalation toxicology. This
chapter shall focus on the application of aerosol science to particle deposition in the human respi-
ratory system. Particle deposition in human airways is governed by multiple mechanisms. Inertial
impaction, sedimentation, and diffusion are often considered the primary mechanisms of deposi-
tion, while interception, charging, and cloud motion may be important in some situations. These
respective mechanisms are described and formulated in this chapter. The deposition eficiency of
each mechanism is dependent upon interactions among aerosol characteristics, ventilatory param-
eters, and respiratory system morphologies.
In this chapter, we discuss factors governing both the motion and deposition of particles
in the respiratory system. We review the kinematic behavior of particles immersed in a luid
medium. We then consider the motion of particles and luids within tubular structures (e.g.,
airways), and we discuss speciic mechanisms of particle deposition in branching networks.
We discuss particle inhalability and the current state of knowledge. We also briely discuss
deposition in the extrathoracic airways, but refer the reader to the peer-reviewed literature for
more information on this evolving ield.

3.2 FUNDAMENTALS OF INHALED AEROSOLS
An aerosol is a suspension of particulate matter in a gaseous carrying medium. Several textbooks
have been written that address the general ield of aerosol science and technology. We refer the
interested reader to the works of Fuchs, 1 Mercer, 2 Reist, 3 Hinds, 4 and Friedlander. 5
Particle size, density, and shape are important factors in the prediction of particle kinetics and
aerosol deposition in human airways. In this section, we discuss particle characteristics, their effects
on Stokes's Law and terminal settling velocity, and their role in particle deposition.

3.2.1 s tokes ' s l aw
The interaction between an aerosol particle and the carrying gas is quantiied by Stokes's Law.
Stokes's Law provides a basis for the study of aerosol particle motion. The drag force on a particle
moving through a luid may be expressed as

F = πμ 3 d V (3.1)

D p p

It is important to recognize that the application of Equation 3.1 requires the following assumptions:
(1) the particle is a rigid sphere, (2) the carrying gas is incompressible, (3) the inertia of the particle
is negligible compared to drag force, (4) there are no hydrodynamic interactions or boundary effects
affecting the particle, (5) particle motion is constant, and (6) the luid velocity at the particle surface
is zero. In practice, these conditions must be checked for validity.

3.2.2 t erMinal s ettling v elocity and r elaxation t iMe
An aerosol particle will reach its terminal settling velocity when the drag force, F D , on it is equal
and opposite to the force of gravity, F g :

F

=

F (3.2)

D