Dust and Human Health - Mineral Dust: A Key Player in the Earth System - page 382

Saliva
slightly acidic to slightly
alkaline pH; high K,
HCO 3 - , P; high enzymes;
glycoproteins

Mucus
mucoproteins, water

Nasal Cavity

Oral Cavity
Pharynx
Larynx
Trachea

Sweat
acidic pH, high
urea, lactic acid,
etc.

Cilia

Aqueous
fluid
near neutral
pH, high P O 2

Primary bronchi

Secondary bronchi

Interstial fluids,
plasma
exposed at
breaks in skin:
pH 7.4; plasma has
high proteins

Terminal bronchioles

Interstitial fluids
pH 7.4; high Na, low K;
low proteins
Intracellular fluids
pH 7.4; high K, Mg, P

Alveolar duct, sac,
and alveoli

Large
intestine
near neutral pH,
reducing

Alveolar
macrophage
(acidic lysosomal fluids
with acid protease
enzymes)

Stomach
pH 1.5, electro-
lytes, enzymes

Lung fluid
pH 7.4; high P O 2 ; high Na,
low K; mucoproteins,
phospholipids

Small
intestine
near-neutral pH,
reducing

Capillary—plasma
(pH 7.4, moderately high
P O 2 , high proteins)

Fig. 15.1 A schematic diagram illustrating exposure pathways and the variability in the composi-
tion and pH of the body fluids encountered during exposure (After Plumlee et al. 2006 )

The respiratory tract (Fig. 15.1 ) serves as a portal of entry, a deposition site, and
target organ (McClellan 2000 ). The structure of the respiratory tract provides the
first line of defense against the deposition of inhaled particles (Lehnert 1993 ). This is
accomplished by its structural design creating a filtration system that supports
deposition in the nasopharyngeal region (nose, mouth, and larynx) and consequently
limits deposition in the lower pulmonary region. The efficiency of this system is
dependent on many factors, including individuals' breathing patterns, the geometric
structural configuration of the airways (airway diameter, branching, and bifurcation
angles), and the physiochemical characteristics of the inhaled particle (Lehnert
1993 ). During exercise, heavy labor, or very dusty conditions, mouth breathing may
occur that allows larger particles to enter the respiratory system. There is evidence
that increases in the ventilation rate during exercise or exertion may increase the
total deposition for all respirable particles (Schultz et al. 2000 ). Inhaled particles
may then be exhaled or deposited by coming in contact with wet airspace surfaces.
The respiratory system may be divided into three components or regions and
particle size deposition roughly correlates with these regions (Fig. 15.1 ). The
nasopharyngeal region operates as an initial barrier to larger particles, generally
PM10 or greater. The tracheobronchial compartment consists of conducting airways