Page 263 - Industrial Ventilation Design Guidebook

224 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

The nasal passages form an efficient filtration mechanism for inspired air,
removing larger particulates ( >3 ^m MMAD) before they can enter the tho-
racic airways. The very largest inspired particles (roughly 10 u,m MMAD and
larger) impinge on nasal hairs (vibrissae) and are mechanically removed from
the nasal cavity (e.g., by blowing one's nose). Particle inertia generally causes
the remaining larger particles to deposit along the nasal cavity surfaces by im-
paction because of convoluted nasal geometry. A particle impacts an airway
wall when the path length to the wall equals the lateral displacement, L, oc-
curring while the particle moves at a velocity u along a streamline altering di-
rection by an angle 0, which is given by

where terminal velocity, u t, is the particle velocity at which particle inertia is
balanced by drag forces. For 1.0 fjum < J < 40 u-m,

where g is the gravitational constant, fji a is the air viscosity, and p p and p a are
the density of the particle and air, respectively. Larger particles that success-
fully traverse the nasal passages typically impact the nasopharyngeal wall at
the 90° turn beyond the distal edge of the nasal cavity.
Finer particles ( < 3 |xm), termed respirable particles, pass beyond the ex-
trathoracic airways and enter the tracheobronchial tree. Impaction plays a sig-
nificant role near the tracheal jet, but sedimentation predominates as the
effects of rapid conduit expansion dampen in the distal trachea and beyond.
Sedimentation occurs when gravitational forces exerted on a particle equal
drag forces, i.e., when particle velocity falls to u t,. As mean inspiratory air-
stream velocity gradually declines along the tracheobronchial tree, particle
momentum diminishes and 0.5-3 u,m MMAD particles settle out of the air-
flow and onto mucosal surfaces.
Mean airflow velocities approach zero as the inspired airstream enters
the lung parenchyma, so particle momentum also approaches zero. Most
of the particles reaching the parenchyma, however, are extremely fine
(< 0.5 jam MMAD), and particle buoyancy counteracts gravitational
forces. Temperature gradients do not exist between the airstream and
airway wall because the inspired airstream has been warmed to body
50 51
temperature and fully saturated before reaching the parenchyma. '
Consequently, diffusion driven by Brownian motion is the only deposition
mechanism remaining for airborne particles. Diffusivity, D c, can be
described under these conditions by

where k is the Boltzmann constant, T is the absolute temperature, jx is the air
viscosity, and d is the particle diameter. Particle displacement, <5, is a function
of residence time, £, and D c such that