*IARC: International Agency for Research on Cancer.
Polychlorinated biphenyls, (PCBs) are a group of highly toxic chlorinated industrial chemicals used as dielectrics, coolants and lubricants in electrical transformers and other electrical equipment, weatherproofers, and to prolong residual activity of pesticides. PCBs are usually released to the environment as a mixture in which other chemicals are also present. PCBs are fire-resistant, have a low volatility, and are relatively stable and persistent, making them well-suited for industrial use but also problematic in the environment. PCBs had been in use for more than 25 years when in 1966 attention focussed on PCB poisoning birds and people. By the late 1970's, evidence of the extreme persistence and adverse health effects of PCBs had resulted in bans on their manufacture in some industrialised countries. Many industrialised countries have taken, now, steps to control and restrict the flow of PCBs into the environment. The most influential force leading to these restrictions has probably been a 1973 recommendation from the Organisation for Economic Co-operation and Development (OECD), (WHO, 1993). Although they are no longer manufactured or imported into many countries, there remain seizable quantities in storage. In addition, PCB fluids are present in many older transformers, fluorescent lighting fixtures, and other electrical devices and appliances. These are vulnerable to release into the environment, as older components can leach. Other sources of PCB contamination come from improper disposal or incineration of PCBs and PCB-containing waste sites.(WFPHA, World Federation of Public Health Associations, 2000).
Because of their high persistence, and their other physical and chemical properties, PCBs are present in the environment all over the world. In past years, many thousands of samples of different foodstuffs have been analysed in several countries for contaminants including PCBs. The levels for the most important PCB-containing food items were: animal fat, 20-240 micrograms/kg of fat; cow's milk, 5-200 micrograms/kg; butter, 30-80 micrograms/kg; fish, 10-500 micrograms/kg, on a fat basis. The major foods in which contamination with PCBs needs consideration are fish, shellfish, meat, milk, and other dairy products. Median levels in fish, reported in various countries, are of the order of 100 micrograms/kg (on a fat basis). (WHO, 1993). PCBs are affecting more heavily population of cold areas of the planet. A study in the late 1980s showed that PCB levels in milk of Inuit women from the east coast of Hudson Bay in northern Quebec were approximately five times higher than in women of southern Canada. The U.S. Food and Drug Administration requires that dairy, poultry, seafood and infant products not contain PCBs in more that 0.2-3 ppm. Nonetheless, average human exposure may exceed the U.S. EPA and FDA's regulatory guidelines in many foods. (WFPHA, 2000).
PCBs is a category, or family, of chemical compounds formed by the addition of chlorine (Cl2) to biphenyl (C12H10), which is a dual-ring structure comprising two 6-carbon benzene rings linked by a single carbon-carbon bond. The nature of an "aromatic" (benzene) ring allows a single attachment to each carbon. This means that there are 10 possible positions for chlorine substitution (replacing the hydrogens in the original biphenyl). Species with a single chlorine substituent are called "monochlorobiphenyl" (or just "chlorobiphenyl"). Species with two chlorines are called "dichlorobiphenyl", and the those with three through ten chlorines, in order, are called: "tri...", "tetra...", "penta...", "hexa...", "hepta...", "octa...", "nona...", and "decachlorobiphenyl". The positions of the chlorine substituents on the rings are denoted by numbers assigned to each of the carbon atoms, with the carbons supporting the bond between the rings being designated 1 and 1'. Any single, unique, well-defined chemical compound in the PCB category is called a "congener". The name of a congener specifies the total number of chlorine substituents and the position of each chlorine. For example: 4,4'-dichlorobiphenyl is a congener comprising the biphenyl structure with two chlorine substituents, one on each of the two carbons at the "4" (also called "para") positions of the two rings. There are 209 PCB congeners. (U.S. EPA web page on PCBs names for congeners).
Acute effects of PCB exposure in humans were documented following ingestion
of contaminated rice oil in Japan in 1968 and Taiwan in 1979. Long term
studies of the more than 2,000 people who were exposed during these events
revealed increased mortality due to PCB intake. A positive association
was established between PCB dosing and acute liver damage, with liver
disease being the cause of death in a significant number of exposed people.
Acute exposure to PCBs ash also caused chloracne, a chemically induced
acneform eruption. Human fetal exposures of PCBs are associated with neural
and developmental changes, lower psychomotor scores, short-term memory
and spatial learning effects, and long-term effects on intellectual function.
Neurological dysfunction had been associated with perinatal PCB exposure
in several Dutch studies.
PCBs have a long and documented history of adverse effects in wildlife.
They have been associated with poor reproductive success and impaired
immune function of captive harbour seals in the Arctic. After a major
flood in the Saginaw River basin in Michigan in 1986 allowed PCB contaminants
to spread through the ecosystem, the following year's hatch rate of Caspian
terns in the area dropped by more than 70 percent. Hatching chicks showed
developmental deformities, and none survived more than five days, Hatch
ability of this Caspian tern colony did not show recovery after three
more breeding seasons. (WFPHA, 2000).
Several factors determine the degree of bioaccumulation in adipose tissues:
the duration and level of exposure, the chemical structure of the compound,
and the position an pattern of substitution. In general, the higher chlorinated
congeners are accumulated more readily. Experimentally determined bioconcentration
factors of various PCBs in aquatic species (fish, shrimp, oyster) range
from 200 up to 70 000 or more. In the open ocean, there is bioaccumulation
of PCBs in higher trophic levels with an increased proportion of higher
chlorinated biphenyls in higher ranking predators.
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