Page 259 - Industrial Ventilation Design Guidebook

220 CHAPTER S PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

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where X = particle composition at time t. Particle growth continues until equilib-
rium is reached between particle surface and bulk airstream water vapor pressure.
Given sufficient growth, extremely fine particles that might otherwise pass entirely
through the airway during the breathing cycle deposit along the airway because of
the increase in mass. Compromised extrathoracic subniucosal blood flow due to in-
jury or disease will change water vapor exchange between the mucosal surface and
inspired airstream. This in turn will alter the growth patterns of inspired hygro-
scopic particles. This process may play a role in lower airway injury caused by in-
spired toxins (e.g., acid aerosols) or succumbing to diseases normally present in the
ambient environment (e.g., pneumonia) that seem to affect weakened individuals,

5.2.6 Endogenous Ammonia Production

Evidence suggests that the highest airway NH 3 concentrations occur in the oral
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cavity, the only segment of the respiratory system that is normally colonized by
bacteria, and that the remainder of the airway, including the nasal passages, have
significantly lower levels. Diffusion of NH 3 from the bloodstream into the airway
lumen is probably the primary source of NH 3 for the entire airway except the oral
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cavity. Blood ammonium concentration, [NH 4+] B, is normally the consequence
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of protein deamination during dietary protein digestion, though deamination of
AMP in muscle tissue during strenuous exercise can significantly increase
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[NH 4+] B. ~ Ureolysis by gastrointestinal bacteria can also contribute to
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[NH 4+] B. It is theorized that airstream NH 3 concentration, [NH 3]4 ( is in equi-
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librium with [NH 4+1 B throughout most of the respiratory tract, though this
has not been demonstrated. Airway mucus may impede diffusion of blood ammo-
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nia into the airway lumen because of its net negative charge. The effect this
Donnan exclusion phenomenon may exert on airway NH 3 diffusion has not been
demonstrated, since [NH 4 +] B has not yet been correlated with [NH 3] 4 in hu-
mans.
Bacterial catabolism of oral food residue is probably responsible for a
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higher [NH 3] A in the oral cavity than in the rest of the respiratory tract. Am-
monia, the by-product of oral bacterial protein catabolism and subsequent ure-
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olysis, desorbs from the fluid lining the oral cavity to the airstream. ' Saliva,
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gingival crevicular fluids, and dental plaque supply urea to oral bacteria and
may themselves be sites of bacterial NH 3 production, based on the presence of
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urease in each of these materials. ' Consequently, oral cavity [NH 3] A is con-
trolled by factors that influence bacterial protein catabolism and ureolysis. Such
factors may include the pH of the surface lining fluid, bacterial nutrient sources
(food residue on teeth or on buccal surfaces), saliva production, saliva pH, and
the effects of oral surface temperature on bacterial metabolism and wall blood
flow. The role of teeth, as structures that facilitate bacterial colonization and
food entrapment, in augmenting [NH 3]^ is unknown.
The significance of pH is particularly interesting since pH may either aug-
ment or diminish NH 3 production. The possible mechanisms by which pH af-
fects NH 3 production are: (a) inhibition of bacterial metabolism, (b)
pH-dependent changes in urea metabolic pathways, (c) pH-dependent bacterial
utilization of glucose and urea as energy sources, and (d) increased bacterial uti-