Summary
Particles greater than about 6 ,u (microns) radius
are retained in the nose. With mouth breathing
they would not be deposited below the secondary
bronchi. Deposition of these and larger particles
is mainly due to impingement. At 2 -radius
penetration of the nose is considerable and most
particles are deposited by sedimentation in the
bronchioles; a small proportion reach the alveoli.
Above 2 IL scarcely any particles are exhaled.
Maximal deposition in the alveoli and fine
bronchioles occurs between 0-4 and 0-8 'L radius
and is also due to sedimentation. Some 80 per
cent. of particles of radius 0 1 to 0-15 IL are breathed
ott again, this being the range over which retention
is a minimum. Deposition in this range takes
place entirely in the alveoli. Still smaller particles
deposit, on account of their Brownian motion, in
the alveoli and bronchioles.
Retention is enhanced by slow breathing because
it allows more time for sedimentation of particles
on to the surfaces of the bronchioles and alveoli.
Alveolar retention is enhanced by deep breathing
because exchange between tidal and alveolar air
is then more effective.
The figures quoted relate to spherical particles
of unit density. For other densities the radii given
above should be divided by the square root of the
density of the particle which may be several times
smaller than the density of the material dispersed.
Thus particles of low density penetrate deeper.
This does not apply to very small particles, below
0.1 ,u, where Brownian motion is important, since
this is independent of density. Particles which are
not spherical will penetrate to greater depths
because the surface to mass ratio is greater; this
is not the case for extreme shapes, for example
fibres, when collision with the walls of the respiratory
passages is facilitated.
Particles deposited in the trachea, bronchi, or
bronchioles are normally removed by ci