An important factor in particle deposition is the pattern of
breathing. During quiet breathing, in which the TV is only two to
three times the volume of the anatomic dead space (i.e, the volume
of the conducting airways where gas exchange does not occur), a
large proportion of the inhaled particles may be exhaled. During
exercise, when larger volumes are inhaled at higher velocities, impaction
in the large airways and sedimentation and diffusion in the
smaller airways and alveoli increase. Breath holding also increases
deposition from sedimentation and diffusion. Factors that modify
the diameter of the conducting airways can alter particle deposition.
In patients with chronic bronchitis, the mucous layer is greatly
thickened and extended peripherally and may partially block the
airways in some areas. Jets formed by air flowing through such partially
occluded airways have the potential to increase the deposition
of particles by impaction and diffusion in the small airways. Irritant
materials that produce bronchoconstriction tend to increase the tracheobronchial
deposition of particles. Cigarette smoking has been
shown experimentally to produce such an effect.