Exposure
Pathways
Challenge
Exposure
Pathways
Benzene (C6H6)
is the first member of a series of aromatic hydrocarbons recovered
from refinery streams during catalytic reformation and other petroleum
processes. It is a clear, colorless, highly flammable liquid at
room temperature. Its vapor is heavier than air and can travel to
a source of ignition and flash back. It has a pleasant, aromatic
odor detectable at concentrations of 1.5 to 4.7 parts per million
(ppm). (The workplace permissible exposure level [PEL] is 1 ppm).
Common synonyms for benzene include benzol, cyclohexatriene, phenyl
hydride, and coal tar naphtha.
Benzene is one of the world's major commodity chemicals. Its primary
use (85% of production) is as an intermediate in the production
of other chemicals, predominantly styrene (for styrofoam and other
plastics), cumene (for various resins), and cyclohexane (for nylon
and other synthetic fibers). Benzene is an important raw material
for the manufacture of synthetic rubbers, gums, lubricants, dyes,
pharmaceuticals, and agricultural chemicals. Benzene
is a natural component of crude and refined petroleum. The mandatory
decrease of lead alkyls in gasoline has led to an increase in the
aromatic hydrocarbon content of gasoline to maintain high octane
levels and antiknock properties. In the United States, gasoline
typically contains less than 2% benzene by volume, but in other
countries the benzene concentration
may be as high as 5%.
Because of its lipophilic nature, benzene is an excellent solvent.
Its use in paints, thinners, inks, adhesives, and rubbers, however,
is decreasing and now accounts for less than 2% of current benzene
production. Benzene was also an important component of many industrial
cleaning and degreasing formulations, but now has been replaced
mostly by toluene, chlorinated solvents, or mineral spirits. Although
benzene is no longer added in significant quantities to most commercial
products, traces of it may still be present as a contaminant.
Benzene is widespread in the environment. Airborne benzene is usually
produced by processes associated with chemical manufacturing or
the gasoline industry, including gasoline bulk-loading and discharging
facilities and combustion engines (e.g., automobiles, lawn mowers,
and snow blowers). Benzene is a component of both indoor and outdoor
air pollution. Benzene levels measured in ambient outdoor air have
a global average of 6 micrograms per cubic meter (µg/m3)
(range 2-9 µg/m3). In almost all
cases, benzene levels inside residences or offices are higher than
levels outside and still higher in homes with attached garages and
those occupied by smokers. Seasonal variations also affect benzene
levels, with higher levels found in the fall and winter when buildings
are less well ventilated. People living around hazardous waste sites,
petroleum-refining operations, petrochemical manufacturing sites,
or gas stations may be exposed to higher levels of benzene in air.
In addition to being inhaled, airborne benzene is absorbed across
intact skin in experimental animals. For most people, the level
of exposure to benzene through food, beverages, or drinking water
is not as high as their exposure through air.
Leakage from underground storage tanks and seepage from landfills
or improper disposal of hazardous wastes has resulted in benzene
contamination of groundwater used for drinking. Effluent from industries
is also a source of groundwater contamination. In addition to being
ingested, benzene in water can also be absorbed through wet skin
and inhaled as it volatilizes during showering, laundering, or cooking.
Typical drinking water contains less than 0.1 parts per billion
(ppb) benzene. Benzene has been detected in bottled water, liquor,
and food.
Cigarette smoke is another common source of personal and environmental
benzene exposure, representing about half of the benzene to which
the general population is exposed. Persons who smoke one pack of
cigarettes a day inhale a daily dose of approximately 1 milligram
(mg) of benzene, about 3 to 4% of the amount inhaled daily by a
worker exposed at the current occupational PEL. Nonsmokers who live
with smokers and who are passively exposed to environmental tobacco
smoke typically experience 50% greater exposure to benzene than
do nonsmokers who live in a smoke-free environment.
Challenge
(2) Later, the
patient in the case study tells you that his well water has always
tasted "funny" and smells like "solvent." You learn that a chemical
plant was near his property until 9 years ago, when the company
moved. You are concerned about your patient's description of his
drinking water, and you request that the state health department
investigate the problem. The investigator contacts the chemical
company that owns the abandoned site and learns that benzene is
stored at the site in tanks that are above and below ground. Laboratory
analyses of the patient's well water reveal an average concentration
of 20 ppm benzene and traces of 1,1,1-trichloroethane and
toluene.
What questions will you ask to gauge the extent of the patient's
exposure to benzene?
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