Page 335 - Industrial Ventilation Design Guidebook

5.3 TOXICiTY AND RISKS INDUCED BY OCCUPATIONAL EXPOSURE TO CHEMICAL COMPOUNDS 29 I

soluble compounds are also easily transported in the blood, mainly hound to
lipoproteins. They move freely from the circulation to the organs depending
on the lipid content of various organs. Thus, at equilibrium, organs such as
the brain and other lipid-containing organs have the highest concentration of
the agent at equilibrium. Typical examples of very lipid-soluble components
are aromatic solvents such as benzene, xylenes, toluene, styrene, and ethyl-
benzene. Also, chlorinated hydrocarbons such as tri- and tetrachloroethylene
2 68
belong to this category. '
The reactivity of a compound greatly affects its distribution and, there-
fore, the potential target organs. For example, formaldehyde is a very reactive
and irritating gas. Because of its reactivity, inhaled formaldehyde binds with
mucus and proteins in the nasal and oral cavities and perhaps in the upper res-
piratory tract, but it does not reach the alveolar region or the systemic circula-
tion through inhalational exposure. For this reason, the most serious health
effects of formaldehyde, notably cancer, are only seen in the upper respiratory
tract. In fact, a considerable amount of formaldehyde is being formed endoge-
nously in normal metabolism. However, it does not cause any harm under
these conditions because it is tightly bound to serum proteins. Thus, harmful
reactions of formaldehyde with macromolecules such as DNA only occur in
very limited areas in the body. Due to its reactivity, formaldehyde also readily
forms protein adducts which in some cases can be used for biomonitoring ot
formaldehyde exposure. 129

Tox/c/ty to the Central and Peripheral Nervous Systems
The nervous system consists of two main categories of cells: neurons and
glial cells. Neurons are the actual nerve cells, which are responsible for trans-
mitting information. There are fewer nerve cells than glial cells present in the
brain. Glial cells play a variety of supportive functions. The brain and spinal
cord form the central nervous system (CNS). Most parts of the CNS are iso-
lated from other parts of the body by the blood-brain barrier, which is a func-
tional rather than a morphological entity that consists of tightly connected cell
membranes. Some substances, however, pass through the blood-brain barrier
due to their lipophilicity. In addition, there are active transport mechanisms
for hydrophilic nutrients and minerals which are vital for CNS function. Some
toxic compounds can use these mechanisms to cross the barrier. The remain-
ing parts of the nervous system are called the peripheral nervous system
l
1 >
(PNS). The PNS can be considered, in fact, as an extension of the CNS. • -^~ ^'
Neurons have three parts: the cell body and dendrites, the axon, and axon
terminals. The cell body contains the nucleus and the organelles needed for
metabolism, growth, and repair. The dendrites are branched extensions of the
ceil body membrane. The axon is a long, thin structure which transfers electri-
cal impulses down to the terminals. The axon divides into numerous axon ter-
minals and it is in this specialized region that neurotransmitters are released to
transmit information from one neuron to its neighbors. The synapse has been
defined as the space between two subsequent interrelated neurons. 1117
The glial cells support the neurons physically. Certain glial cells (oligoden-
droglial cells) synthesize myelin, a fatty insulation layer wrapped around the
axons, Myelin is necessary for the so-called saltatory conduction of electrical