The size of a particle in an aerosol is one of the main determinants where in the respiratory tract a particle will deposit. Local conditions may also be important factors to consider.


Processes affecting deposition and disposition of the particles:

(1) impaction: A larger particle impinges against the wall of the upper airway, and it may stick if the surface is moist. The particle will then be moved by the cilia of the respiratory epithelial cells and swallowed or expectorated.

(2) sedimentation: A particle of intermediate size may drop out onto a bronchial wall because of greater weight and less buoyancy. The particle may be moved by the cilia or ingested by macrophages.

(3) diffusion: The smallest particles are well ssuspended and reach the alveolar air spaces during the inhalation where they can cross the walls and enter the circulation. On exhalation these particles may be expired out of the body.

Deposition Site

Diameter of the Particle

difficult to suspend in air

> 100 microns

upper respiratory tract (mouth, nose, oropharynx)

30 - 100 microns

trachea and large bronchus

9 - 30

bronchus to small bronchiole

2.1 - 9 microns


0.5 - 2.0 microns

remains suspended (fails to deposit, and lost during exhalation)

< 0.5 microns



• 1 micron = 1 µm = 0.001 mm or 0.000001 meters


Diameter in microns

Amount Reaching Alveoli

Amount on Bronchi

Amount in Upper Airways

0.01 to 0.1

about half


essentially none

0.1 to 1.0

about a third

a little

trace to a little

1.0 to 5.0

about a quarter

a little

some to most

5.0 to 10

about a sixth

a little


10 to 50

about a tenth

a little


> 50

essentially none

essentially none

essentially all


Other factors affecting the site of deposition:

(1) density of particles in the air (parts per million)

(2) moisture along airways

(3) depth of inhalation

(4) airway obstruction (pulmonary edema, pneumonia, bronchial plug, tumor)

(5) aggregation of particles


In addition, particle size is important for:

(1) smoke inhalation

(2) other toxic and occupational inhalations

(3) viral infections


surface area of the particle in square microns =

= PI() * ((diameter in microns)^2) =

= 4 * PI() * (((diameter in microns) / 2)^2)


volume of the particle in cubic microns =

= 4 / 3 * PI() * (((diameter in microns) / 2)^3)


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