Page 336 - Industrial Ventilation Design Guidebook

292 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

impulses. The myelin layer is not continuous but has breaks called the nodes
of Ranvier. Action potentials occur only at those nonprotected nodes where
they "jump" (the Latin verb saltare means "jump") from one node to the
next. The glial cells also have various maintenance functions (e.g., maintaining
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ionic equilibrium). 136 137
The nervous system is vulnerable to attack from several directions. Neu-
rons do not divide, and, therefore, death of a neuron always causes a perma-
nent loss of a cell. The brain has a high demand for oxygen. Lack of oxygen
(hypoxia) rapidly causes brain damage. This manifests itself both on neurons
and oligodendroglial cells. Anoxic brain damage may result from acute carbon
monoxide, cyanide, and hydrogen sulfide poisonings. Carbon monoxide may
also be formed in situ in the metabolism of dichloromethylene. 107 138
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Organic solvents have acute narcotic effects. Aromatic and chlorinated
hydrocarbons seem to be especially effective. As stated, the combined effect of
several organic solvents is usually considered to be additive. However, there is
some evidence that the combined effect may in fact be synergistic. The symp-
toms caused by organic solvents, often called prenarcotic symptoms, resemble
those caused by the use of alcohol. A decrease in reaction time and impair-
ment in various psychological performances can be observed. Acute neurotox-
icity can also be detected as abnormalities in the electroencephalogram (EEG),
which records the electrical activity of the brain. 107
Chronic neurotoxic effects can be divided into four groups. Neuronopa-
thy, where the whole neuron is destroyed, is the most dramatic of these four.
In axonopathy, the axon partly degenerates. The damage usually begins from
a certain site and progresses towards the terminal. One can imagine it as a
break in the axon. Carbon disulfide and n-hexane are examples of chemicals
causing this kind of damage. The toxic effect results in reactions with the
arnino groups in proteins. In the case of n-hexane, the toxic compound (2,5-
hexadione) is formed via oxidative metabolism. The reactions cause precipita-
tions of neurofilaments in the axons which hinder its transport capabilities.
The sensomotoric neuropathy caused by n-hexane exposure appears in ex-
tremities as numbness, weakness, and muscle pain. Myelinopathy slows the
velocity of nerve conduction. The damage cannot be easily rectified in the
CNS. Demyelination may also occur in the PNS. Lead is the most common
agent causing myelinopathy. 107 139
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Eye Toxldty
Vision is vital for human activities, and eyes are very sensitive to a number
of toxic insults induced by chemical compounds. The most serious outcome is
permanent eye damage which may be so severe as to cause loss of vision. The
eye consists of the cornea and conjunctiva, the choroid, the iris, and the ciliary
body. It also contains the retina, which is of neural origin, and the optic nerve.
The retina contains photoreceptors, a highly specific light-sensitive type of neu-
ral tissue. The eye also contains the lens and a small cerebrospinal fluid system,
the aqueous humor system, that is important for the maintenance of the steady
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state of hydration of the lens and thus the transparency of the eye. °
The cornea must be transparent to allow normal function of the eye.
Therefore even a tiny degree of scar formation, commonly induced by exposure