Health Effects of Aerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 571

23.3 DEPOSITION AND CLEARANCE OF AEROSOLS

FROM THE HUMAN RESPIRATORY TRACT*

23.3.1 d ePosition and r etention
For most people during quiet breathing, air enters the respiratory tract primarily through
the nose at high velocity (Figure 23.6). Particles 0.5 μm are removed eficiently at this level.
Particles <100 nm are also removed at this level, largely by impaction. Inertial impaction is
proportional to velocity, the square of the particle diameter, and the sharpness of the angle of
the airway [21,22]. Therefore, impaction occurs at places where there is a sudden change in the
direction of the airstream, such as airway branch points, and involves a similar size range as that
for gravitational impaction [23]. As air moves through the trachea and bronchi, impaction and
sedimentation both play a role in the deposition of particles. Sedimentation due to gravity occurs
in airways, except those that are vertical and for particles >0.5 μm aerodynamic diameter (Vol.
1 of Ref. [19]). As airway diameters narrow, particles whose distance to the airway wall surface
is less than the particles' size are removed by interception [21]. As the small particles (<1 μm
down to the nanometer range) move down the airways in an ever-slowing airstream, diffusion
plays a greater role in deposition. Diffusion is an important mechanism for particles <0.5 μm
[22]. Small particles that carry a surface charge (e.g., freshly generated combustion particles)
may also be deposited as a result of electrostatic forces. Electrostatic forces are not important
for particles >4 μm [22].

Keratinized
epithelium
(basal cells)

Air velocity

ET 1

Posterior
nasal passage
Nasal part
Oral part

Extrathoracic
region

Impaction

Pharynx

Stratified
squamous
epithelium
(basal cells)

ET 2

Larynx

BB

Trachea

Sedimentation
interception

Ciliated bronchial
epithelium
(secretory and
basal cells)

Main bronchi

Tracheobronchial
region

Bronchi

Bronchioles

bb

Bronchiolar
Alveolar interstitial

AI

bb

Ciliated
bronchiolar
epithelium
(clara cells)

Bronchioles

Terminal bronchioles

Diffusion

Alveolar
region

Respiratory bronchioles

AI

Electrostatic
precipitation

Alveolar endothelium,
epithelium and interstitium
(endothelial cells, Type II
epithelial cells and clara cells)

Alveolar duct +
alveoli

FIGURE 23.6 Schematic of human respiratory tract. (Adapted from U.S. Environmental Protection Agency
(EPA), Air quality criteria for particulate matter—Third external review draft, EPA 600/P-99/002aC, Report
No.: EPA 600/P-99/002aC, Research Triangle Park, NC, 2001; Casarett, L.J., Toxicololgy of the respiratory
system, in Casarett, L.J. and Doull, J., Eds., Toxicology: The Basic Science of Poisons ., MacMillan Publishing
Co., New York, pp. 201-224, 1975; Lippmann, M. and Schlesinger, R.B., J. Toxicol. Environ. Health , 13, 441,
1984.)

* An exhaustive discussion of issues related to deposition of particles in the human respiratory tract can be found in
Ref. [19].