Asbestos Fibers and Other Elongate Mineral Particles: State of the ...

sedimentation, and diffusion [Asgharian and
Yu 1988]. In a study to assess EP deposition in
the tracheobronchial region, Zhou et al. [2007]
evaluated the deposition efficiencies of carbon
fibers (3.66 µm diameter), using two human
airway replicas that consisted of the oral cavity,
pharynx, larynx, trachea, and three to four generations
of bronchi. Carbon fiber deposition
was found to increase with the Stokes number,
indicating that inertial impaction is the dominant
mechanism. Also, fiber deposition in the
tracheobronchial region was lower than that of
spherical particles at a given Stokes number,
indicating a greater likelihood for small-width
EPs to move past the upper respiratory tract
and reach the lower airways where diffusional
deposition predominates [Yu et al.1986].
These results were confirmed by results of later
studies evaluating the deposition of asbestos,
using a similar tracheobronchial cast model
[Sussman et al. 1991a, b]. The probability of
deposition of a particle in a specific location in
the airways is not the same as the probability
of penetration to that region, and for particles
in a certain range of AEDs, the difference between
penetration and deposition may be substantial
[ICRP 1994].
2.9.2 Clearance and Retention
A variety of mechanisms are associated with the
removal of deposited particles from the respiratory
tract [Warheit 1989]. Physical clearance
of insoluble particles deposited in the lung is
an important physiological defense mechanism
that usually serves to moderate any risk that
might otherwise be associated with exposure to
particles. Inhaled particles that deposit on respiratory
tract surfaces may be physically cleared
by the tracheobronchial mucociliary escalator
or nasal mucus flow to the throat, and then
they may be either expectorated or swallowed.
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Clearance depends upon the physicochemical
properties of the inhaled particles, the sites of
deposition, and respiratory anatomy and physiology.
For example, inhaled insoluble particles
with larger AEDs tend to deposit on the
nasopharyngeal mucus and are generally cleared
by sneezing or nose blowing or by flow into the
oropharynx, where they are swallowed. Insoluble
particles with smaller AEDs tend to deposit
lower in the respiratory tract, with associated
longer retention times. Those deposited in the
alveolar region are subject to longer retention
times than those deposited on the bronchial region
[Lippmann and Esch 1988].
The most important process for removal of insoluble
particles from the airways is mucociliary
clearance, which involves a layer of mucus propelled
by the action of ciliated airway cells
that line the trachea, bronchi, and terminal
bronchioles [Warheit 1989]. The mucociliary
transport system is sensitive to a variety of
agents, including cigarette smoke and ozone
[Vastag et al. 1985]. These toxicants affect the
speed of mucus flow and consequent particle
clearance by altering ciliary action and/or modifying
the properties and/or amount of mucus.
Chronic exposure to cigarette smoke has been
shown to cause a prolonged impairment of particulate
clearance from the bronchial region.
This impaired clearance is associated with increased
retention of asbestos fibers in the bronchi,
where they stimulate inflammatory processes
in the bronchial epithelium [Churg and
Stevens 1995; Churg et al. 1992].
Because the alveolar region of the lung does
not possess mucociliary clearance capability,
particles (generally