Page 302 - Industrial Ventilation Design Guidebook

258 CHAPTER 5 PHYSIOLOGICAL AND TOXiCOLOGICAL CONSIDERATIONS

reaches the alveolar region only at high concentrations because it reacts
with proteins in the mucus and in the cells of the epithelial lining of the up-
per respiratory tract. On the other hand, as a consequence of its reactivity,
high concentrations of formaldehyde cause serious lung injury and lung
58 9
edema upon reaching the alveolar region. ''*
Aerosols reach the alveolar space depending on their particle size and
physico-chemical characteristics. Small particles that reach the alveolar region
(see Sections 2.3.7 and 3.1.1) may reach the circulation through the lymphatic
drainage of the alveolar region.

Dermal Exposure
Skin is also important as an occupational exposure route. Lipid-soluble
solvents often penetrate the skin, especially as a liquid. Not only solvents, but
also many pesticides are, in fact, preferentially absorbed into the body
through the skin. The ease of penetration depends on the molecular size of the
compound, and the characteristics of the skin, in addition to the lipid solubil-
ity and polarity of the compounds. Absorption of chemicals is especially effec-
tive in such areas of the skin as the face and scrotum. Even though solid
materials do not usually readily penetrate the skin, there are exceptions (e.g.,
37 60 61
benzo(#)pyrene and chlorophenols) to this rule. ' '
Oral Exposure
In the occupational setting, oral exposure is of minor significance,
being mainly due to poor personal hygiene. In addition, gases that dis-
solve or are otherwise trapped in the upper respiratory tract, usually are
swallowed and enter the gastrointestinal tract. Particles that are removed
as such or are captured by macrophages by the mucociliary escalator
from the respiratory tract are also ultimately swallowed and enter the
gastrointestinal tract.

5.3.2.3 Physico-Chemical Determinants of Exposure
Physico-chemical characteristics greatly determine the entry of chemicals
into the body, and also their behavior in the body (distribution, biotransfor-
mation, and excretion). Therefore, the physico-chemical characteristics of a
compound affect its dose and its subsequent effects by determining how
quickly and extensively a chemical reaches the target organs. In the following
section, some of these important physical-chemical characteristics of chemi-
cals will be discussed.

Water Solubility
The site and the severity of the effect of respiratory irritant gases depend
largely on their water solubility. Very water-soluble gases and vapors, such as
ammonia, hydrogen chloride, sulfur dioxide, and formaldehyde dissolve in the
mucus of the nose and upper airway and cause inflammation. Poorly water-
soluble gases, such as nitrogen dioxide and ozone, are able to reach the deep
lung area. Inflammation results from damage to cellular membranes of bron-
chiolar and alveolar cells, and subsequent accumulation of liquid in the lungs
(edema). Because the alveoli have no receptors for irritation, the effects are
generally noticed only several hours after the exposure when the amount of