PRINCIPLES OF TOXICOLOGY

394 PROPERTIES AND EFFECTS OF ORGANIC SOLVENTS
Elimination of methyl bromide is predominantly via the lungs as unchanged methyl bromide. A
significant amount of methyl bromide, however, is metabolized in the body and may appear as inorganic
bromide in the urine. “Normal” bromide ion concentration is below 1 mg/100 mL of blood serum, in
the absence of the dietary sources (e.g., grains, beverages, medications). Five milligrams per 100 mL
of blood may be considered evidence that exposure to bromide has occurred. If blood bromide is to
be used in determining the exposure to methyl bromide, consideration must be given to other possible
sources of bromide. The most common sources of inorganic bromide are medications, food, or water.
Methylene chloride (see Figure 16.21) (or dichloromethane) is used as a common “blowing” agent
for foams and as a solvent for many applications, including the coating of photographic films, in aerosol
formulations, and to a large extent in paint processes, where its high degree of volatility is desirable.
Owing to this volatility, high concentrations may be rapidly attained in poorly ventilated areas. It is a
powerful solvent that is effective in dissolving cellulose esters, fats, oils, resins, and rubber. Methylene
chloride is more water-soluble than most other chlorinated solvents.
Methylene chloride is probably the least toxic of the four chlorinated methanes; the predominant
toxic effect is CNS depression (expressed as narcosis). Other systemic effects in humans following
nonlethal exposures to methylene chloride may include headache, giddiness, stupor, irritability,
numbness, psychomotor disturbances, and increased carboxyhemoglobin levels. It is mildly irritating
to the skin and dermal absorption is not considered a significant threat to human health unless massive
concentrations are encountered. Eye contact may be painful but is not likely to cause serious injury.
Adaptation to methylene chloride vapors occurs with repeated exposure, decreasing the ability to detect
exposure.
PHYSIOLOGIC RESPONSE TO METHYLENE CHLORIDE IN HUMANS
Concentration (ppm) Response
200–300 Odor threshold
1000 Unpleasant odor level
2300 Dizziness, fainting
The symptoms of excessive exposure may be dizziness, nausea, tingling or numbness of the
extremities, sense of fullness in the head, sense of heat, stupor, or dullness, lethargy, and drunkenness.
Exposure to sufficiently high concentrations of methylene chloride may lead to rapid unconsciousness
and death. Prompt removal from exposure area typically results in complete recovery.
Historical industrial experience with methylene chloride has been remarkably free of serious
adverse effects. Nephrotoxic and hepatotoxic potential of methylene chloride is considered low under
typical industrial environments. Reports of systemic injury are rare and, although dermatitis has been
reported because of common usage in paint remover formulations, only a few anesthetic deaths have
occurred, all at extreme concentrations. Methylene chloride is known to be metabolized to carbon
monoxide, but symptoms of carbon monoxide poisoning, such as headaches, have not been a common
feature of methylene chloride reports. This indicates that carboxyhemoglobin levels alone are not a
good measure of the toxic effect of methylene chloride. Acute exposures have resulted in liver or kidney
Figure 16.21 Methylene chloride.