Health Effects of Aerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 572

The nasal passages are effective in removing particles larger than 5 μm and remove virtually
all particles larger than 10 μm [22]. In addition, condensation nuclei are effectively removed in
the nasal passages by diffusion. Figure 23.7 and Table 23.2 summarize the estimated deposition
fractions for particles of various mass median aerodynamic diameters (MMAD). Particles <10 μm
MMAD are deposited at all levels of the respiratory tract, with particles <1 μm being deposited
primarily in the tracheobronchial tree and the alveoli (“Pulmonary” in Figure 23.7). A number of
factors affect deposition (Table 23.3). Patterns of ventilation and underlying lung disease are of
particular importance for health considerations. Exercise is associated with a switch from primarily
nasal breath to oral breathing with a resultant increase in the alveolar deposition of particles >1 μm
(see Section 10.5.1.4 in Ref. [19]).

1.0

Nasopharyngeal

0.8

0.6 Pulmonary

0.4

Tracheobronchial

0.2

0.04 0.05 0.1 0.5 1.0 5.0 10.0

Mass median aerodynamic diameter (µm)

FIGURE 23.7 Fractional deposition of particles in the respiratory tract of humans. The x-axis is plotted on a
logarithmic scale. (From Morrow, P.E., Health Phys ., 12, 173, 1966; as cited by Brain, J.D. and Valberg, P.A.,
Am. Rev. Respir. Dis ., 120, 1325, 1979.)

TABLE 23.2
Respiratory Tract Penetration of Particles of Various Sizes

Particle Size Range ( μ m) Level of Penetration (Generation Number) a

≥11 Do not penetrate

7-11 Nasal passages

4.7-7 Pharynx

3.3-4.7 Trachea and primary bronchi (1st)

2.1-3.3 Secondary bronchi (2nd-7th)

1.1-2.1 Terminal bronchi (8th)

0.65-1.1 Bronchioles (9th-23rd)

<0.65 Alveolar ducts (24th-27th) and alveoli

Source: Adapted from Wilson, S.R. and Spengler, J.D., Emissions, dispersion,
and concentration of particles, in Wilson, S.R. and Spengler,
J.D., Eds., Particles in Our Air: Concentrations and Health Effects ,
Harvard University Press, Cambridge, MA, pp. 41-62, 1996.
a Generation numbers are from [271].