Page 266 - Industrial Ventilation Design Guidebook

S.2 HUMAN RESPIRATORY TRACT PHYSIOLOGY 227

would appear that the molecular form of the inhaled acid may play a significant
role, perhaps through differences in hygroscopic growth and neutralization rate
between H 2SO 4 and NH 4HSO 4 particles.
For a given ambient concentration of acid aerosol, the dose of acid deliv-
ered to the respiratory tract is in large measure determined by the pH and parti-
cle size of the aerosol. Due to the efficiency of the upper airways (particularly
the nasal passages) in filtering coarse (> 3 (xm) particles, subrnicrometric acid
aerosol particles pose the greatest risk to the lower airways. Ambient acid aero-
sols are overwhelmingly subrnicrometric in size distribution at most relative hu-
108
midities. Submicron acid particles therefore merit special attention in the
attempt to understand the action of acid aerosols on airway health, particularly
as they comprise a large proportion of acidic environments. 108
Airstream neutralization of acid aerosols by NH 3 present in the airway
lumen reduces the health risk associated with acid particles by reducing the
78 109
acid concentration prior to particle deposition. ' In addition, the liquid
lining of the respiratory tract probably acts as a chemical buffer, 110 further
reducing the health hazard posed by inspired acid particles. Principal factors
controlling airstream neutralization of acid aerosols, which is considered to
be a diffusion-limited process, are particle surface area, [NH 3] 4, and particle
residence time in the airstream.
Since NH 3 is highly water-soluble and neutralization within the droplet
111
occurs rapidly, the rate-limiting step in acid neutralization is normally NH 3
transport to the air/droplet interface, which is dependent on [NH 3] A and parti-
cle surface area. At high [NH^, the rate of NH 3 uptake across the air/droplet
interface is given by

where C s is the NH 3 concentration in the acid droplet, D g, is the airstream NH 3
diffusion coefficient, r is the droplet radius, H is the Henry's law coefficient, and
q NS is the activity coefficient of neutral undissociated species in solution in the
droplet. 112 Particle size of inhaled liquid aerosols does not remain constant
within the airways, however. Water will condense on the surface of particles as
they move distally along the airway because of local increases in relative humid-
ity^,! 0,114 y^ resulting increase in particle radii due to hygroscopic growth
will reduce NH 3 concentration according to Eq. (5.49), while the increase in
'
particle size will increase particle deposition. 102 115 However, increasing the par-
ticle radius results in greater particle surface area, which should increase NH 3
uptake, thereby opposing the reduction in particle [NH 3].
Elevated inspiratory flow rates reduce airway [NH 3] by diluting the endog-
enous NH 3. The effect of flow rate on [NH 3] A should be most apparent in the
upper airway due to the large NH 3 concentration difference between ambient
air (as the diluent) and the upper airway (containing endogenous NH 3). The di-
luting effects of ventilation should be less evident in the large central airways
and diminish steadily as the inspiratory wavefront moves distally along the air-
way because air velocity declines as airway volume increases and NH 3 is a
highly soluble gas and most likely equilibrates rapidly with blood ammonia. As
a result, flow-rate effects on [NH 3] A should be negligible after approximately
the eighth bronchial generation because the rapid increase in airway volume