Description
Polychlorinated biphenyls (PCBs) are one of the most widely
studied environmental contaminants, and many studies in
animals and human populations have been performed to
assess the potential carcinogenicity of PCBs. PCBs are mixtures
of up to 209 individual chlorinated compounds (known as
congeners or chlorinated hydrocarbons), which are no longer
produced in the United States, but are still found in the environment.
Exposure to PCBs includes acne-like skin conditions
in adults and neurobehavioral and immune alterations in
children. PCBs are carcinogenic to animals, and they have been
found in at least 500 of the 1598 National Priorities List sites
identified by the US Environmental Protection Agency (EPA).
PCBs (US trade name Aroclor) have no known natural sources,
are either oily liquids or solids that are colorless to light yellow,
and can exist as a vapor in air. PCBs typically do not exhibit any
smell or taste.
The US Toxic Substances Control Act (TSCA) was enacted by
the US Congress to control the distribution, use, and disposal
of harmful chemicals, including PCBs. PCBs have been
demonstrated to cause a variety of adverse health effects.
Studies in humans provide supportive evidence for potential
carcinogenic and noncarcinogenic effects of PCBs, and based
on the weight of the data, it is suggested that PCBs are probable
human carcinogens.
Chemical Properties
PCBs are a class of biphenyl compounds with one to ten hydrogen atoms replaced by chlorine. At room temperature, they range in physical state from light- to dark-yellow oily liquids to white crystalline solids and hard noncrystalline resins (IPCS 1992, HSDB 2009). PCBs are produced commercially by chlorination of biphenyl, resulting in 209 possible PCB congeners (Silberhorn et al. 1990). However, McFarland and Clarke (1989) reported that about half of these molecules accounted for nearly all environmental contamination by PCBs, and they considered only 36 to be environmentally relevant, because of their potential toxicity, environmental prevalence, and relative abundance in animal tissues. Commercial PCB formulations are complex mixtures of chlorinated biphenyls that vary in the degree of chlorination, and similar mixtures can show significant lotto-lot variation in composition (ATSDR 2000). Of the 209 possible PCB congeners, about 100 are present in commercial PCB mixtures, and about 70 have been detected in human adipose tissue (Mühlebach et al. 1991). At least 20 of the 209 possible congeners have not been identified in commercial mixtures of PCBs (Kimbrough 1987).
Physical and chemical properties of PCBs are affected by the numbers and positions of chlorine atoms (Carpenter 2006). PCBs with fewer chlorine atoms tend to be more soluble in water, more volatile, and more easily metabolized. Larger numbers of chlorine atoms are associated with increased resistance to biodegradation, which can increase bioaccumulation in the environment. PCBs are practically insoluble in water, but soluble in organic solvents and fats (IPCS 1992). They are very stable and persistent in the environment. Physical and chemical properties representative of PCBs are listed in the following table.
PCBs have been categorized as “dioxinlike” or “non-dioxinlike,” based on their ability to exert biochemical and toxic effects similar to those of TCDD through activation of the aryl hydrocarbon receptor (Carpenter 2006, Knerr and Schrenk 2006). Dioxin-like activity is seen for PCB congeners with chlorine atoms occupying meta (carbon atoms 3, 3′, 5, or 5′) and para (carbon atoms 4 or 4′) positions, with no more than one ortho (carbon atoms 2, 2′,6,or 6′) chlorine; these molecules are likely to exist with a planar conformation. Twelve tetra-, penta-, hexa-, or hepta-chlorobiphenyls meet these criteria and have been assigned toxic equivalency factors (TEQs) of 0.0001 to 0.1 by the World Health Organization based on their toxicity relative to that of TCDD, which has the highest toxic potency for activation of the aryl hydrocarbon receptor (TEQ = 1.0). PCB 126 has a TEQ of 0.1, which is the highest value for this class of molecules.
Uses
PCBs were used as coolants, lubricants, and insulators in electrical
capacitors, electrical transformers, vacuum pumps, and
gas transmission turbines. They have also been used as
hydraulic fluids, plasticizers, adhesives, fire retardants in plastics,
wax extenders, lubricants and cutting oils, inks, and
cleaning agents. PCBs were banned from production internationally
in the late 1970s, but are still widespread as contaminants
in the environment. They are also produced
unintentionally in the production of organic chlorinated
herbicides, bleaching of papers, in the incineration of wastes, and in industrial and thermal processes involving chlorine and
chlorinated chemicals.
Uses
Before 1974, PCBs were used in the United States for both enclosed applications, such as transformers, capacitors, and heat transfer and hydraulic fluids, and open applications, such as inks, flame retardants, adhesives, carbonless duplicating paper, paints, plasticizers, wire insulators, metal coatings, and pesticide extenders (IARC 1978, ATSDR 2000). After 1974, all uses of PCBs were limited to enclosed applications (transformers and capacitors), and after January 1979, no PCBs were used in the manufacturing of transformers or capacitors. The only remaining permitted uses of PCBs are as a mounting medium in microscopy, as an immersion oil in low-fluorescence microscopy, as an optical liquid, and for other research and development purposes (ATSDR 2000).
Production Methods
PCBs are synthesized by the chlorination of biphenyl
and the resulting products are designated according
to their percent (by weight) chlorine content (2). For
example, Aroclors 1221, 1242, and 1260 contain 21,
42, and 60 wt% chlorine. The commercial Aroclors
were produced by the Monsanto Chemical Corp. and
similar PCB mixtures were manufactured worldwide
by other chemical companies. Over 600 million kg
of commercial PCBs were produced in the United
States and the estimated worldwide production is
approximately double this quantity (Table 1). Properties
of the commercial PCBs varied from highly fluid liquids
(Aroclor 1221) to viscous liquids or solids. All of these
preparations contained a complex mixture of isomers and
congeners and as the degree of chlorination increased
there was a corresponding increase in the relative
concentrations of the more highly chlorinated congeners.
There are 209 possible PCBs and the properties of
these commercial mixtures and the individual PCBs
have been extensively investigated. More recent studies
indicate that the commercial PCBs contained 132 different
compounds (18).
Definition
ChEBI: 3,4,5,3',4',5'-Hexachlorobiphenyl is a trichlorobenzene and a hexachlorobiphenyl.
Safety Profile
Confirmed carcinogen
with carcinogenic and tumorigenic data.
Moderately toxic by ingestion. Some are
poisons by other routes. Experimental
reproductive effects. Like the chlorinated naphthalenes, the
chlorinated diphenyls have two distinct
actions on the body, namely, a skin effect
and a toxic action on the liver. This hepato-
toxic action of the chlorinated diphenyls
appears to be increased if there is exposure
to carbon tetrachloride at the same time.
The higher the chlorine content of the
diphenyl compound, the more toxic it is
liable to be. Oxides of chlorinated diphenyls
are more toxic than the unoxidmed
materials. In persons who have suffered
systemic intoxication, the usual signs and
symptoms are nausea, vomiting, loss of
weight, jaundice, edema, and abdominal
pain. If the liver damage has been severe the
patient may pass into a coma and die.
Combustible when exposed to heat or
flame, When heated to decomposition they
emit highly toxic fumes of Cl-. See also
specific compounds.
Potential Exposure
Several studies have reported relatively high levels of PCBs in the serum or adipose tissues of occupationallyexposed individuals, e.g.,>3000 ppb in the serum (57 58). Not surprisingly, after these exposures were terminated, the PCB serum concentrations tended to decrease (59 61). Chloracne and related skin problems have been observed in several groups of workers and it was suggested that the air concentrations of commercial PCBs > 0.2 mg/m3 were associated with this effect (62). It was also reported that after occupational exposure to PCBs was terminated there was a gradual decrease in the severity and number of dermatological problems in the exposed workers, and this paralleled a decrease in their serum levels of PCBs (61). The effects of occupational exposure to PCBs on the concentrations of several serum clinical, chemical, and hematological parameters have been reported (58). Mildly elevated SGOT and γ -glutamyl transpeptidase (GGTP) suggest some liver damage and induction of hepatic monooxygenase enzymes; these results are similar to those observed in animal studies. In one study, it was reported that as PCB serum levels decreased over time the GGTP serum levels also decreased to normal values.
Carcinogenicity
Polychlorinated biphenyls (PCBs) are reasonably anticipated to be human carcinogens based on sufficient evidence of carcinogenicity from studies in experimental animals. Not all PCB mixtures caused tumors in experimental animals.
Environmental Fate
Before being banned and before the US Clean Water Act
regulated wastewater discharges, PCBs could be found, often at
high levels, in wastewaters from industries handling PCB
equipment. These wastewaters either were discharged directly
to surface waters or sent to municipal sewage treatment plants.
Urban industrial areas are more likely to have higher PCB
contamination than rural areas. While not highly volatile,
PCBs, especially the less chlorinated ones, will partition into
the air. Atmospheric transport is the most important mechanism
for dispersion of PCBs.
Those PCBs with a high degree of chlorination are much
more persistent in the environment than those with lower
degrees of chlorination because they are more resistant to
metabolism. Microbial metabolism is the most important
mechanism for the removal of persistent organic pollutants as
the PCBs from the environment. Anaerobic dehalogenation of
the highly chlorinated PCBs in aquatic sediments is a major
mechanism for their elimination by generating lower chlorinated
congeners that are more readily metabolized by aerobic
enzymes. As a consequence, the environmental levels of PCBs
are slowly decreasing with time.
Toxicity evaluation
PCBs and related halogenated aromatic hydrocarbons elicit a diverse spectrum of toxic and biochemical responses in laboratory animals dependent on a number of factors including age, sex, species, and strain of the test animal and the dosing regimen (single or multiple) (27 32). The
toxic responses elicited by most PCB preparations are
also observed for other classes of HAHs (27–32) and
include a progressive weight loss not simply related
to decreased food consumption and accompanied by
weakness, debilitation, and ultimately death, i.e., a
wasting syndrome; lymphoid involution, thymic and
splenic atrophy with associated humoral and/or cellmediated
immunosuppression and/or associated bone
marrow and hematologic dyscrasia; a skin disorder
called chloracne accompanied by acneform eruptions,
alopecia, edema, hyperkeratosis, and blepharitis resulting
from hypertrophy of the Meibomian glands; hyperplasia
of the epithelial lining of the extrahepatic bile duct,
the gall bladder, and urinary tract; hepatomegaly and
liver damage accompanied by necrosis, hemorrhage, and
intrahepatic bile duct hyperplasia; hepatotoxicity also
manifested by the development of porphyria and altered
metabolism of porphyrins; teratogenesis, developmental
and reproductive toxicity observed in several animal
species: Carcinogenesis caused by PCBs in laboratory
animals is primarily associated with their effects as
promoters. Endocrine and reproductive dysfunction, i.e.,altered plasma levels of steroid and thyroid hormones
with menstrual irregularities, reduced conception rate,
early abortion, excessive menstrual and postconceptional
hemorrhage, and anovulation in females, and testicular
atrophy and decreased spermatogenesis in males have
also been reported in some species.
