Mechanisms of Particle Deposition - Aerosols: Measurement, Dosimetry, and Health Effects - page 66

than charge-neutralized aerosols. Other experimental work has been carried out in human subjects.
Scheuch et al. 150 found that negatively charged particles deposit more effectively than uncharged
particles, with the greatest effects of charge occurring for submicron particles. Melandri et al. 153
found that unipolar charge on monodisperse aerosols resulted in an increase in deposition eficiency
in the human lung. Several models of the effect of charge on particle deposition in the respiratory
system have been developed. 151,152,154
The published data from experimental and modeling studies, albeit limited, indicate that electric
forces may have highly relevant effects on the deposition of inhaled particles. Therefore, the role
of electric forces should not be ignored. However, their signiicance may be more of a concern for
aerosol therapy (in which larger mass concentrations may be inhaled) than for inhalation toxicology.

3.7.2.3 Cloud Motion
Under certain circumstances, an array of particles may behave as an entity rather than as individual
constituent units. This type of behavior is called cloud motion (Figure 3.6). The motion of such an
entity in a gas differs from that predicted by theory for individual particles. Cloud motion may play
a role in the deposition of cigarette smoke in the human lung. 155
Cloud motion occurs when the terminal settling velocity of a cloud of particles is much greater
than the settling velocity for a single particle. The terminal settling velocity of a particulate cloud
having a characteristic diameter, d c , and drag coeficient, C D , is given by 156

1/2

= ⎛

⎞
⎟

4ρ d g

c c

V (3.42)

⎜

c

3 C ρ

D g

The cloud drag coeficient C D is a function of cloud Reynolds number:

= ρ

V d
c a c

Re c (3.43)

μ

Using the relationship between C D and Re recommended by Klyachko, 2 the following expression
was derived for V c (Equation 3.41) for Re c between 3 and 400 156 :

2

ρ
μ +

gd c

c

V = (3.44)

c

2 3 /

3 6 ( Re )

c

Particle cloud

Boundary

Constituent
particles

Internal
circulation
currents

V c cloud velocity

FIGURE 3.6 Cloud motion of concentrated aerosols. The settling velocity of the cloud is superimposed upon
the motion of the constituent particles.