Learn More About PCBs: Polychlorinated Biphenyls

Polychlorinated biphenyls (PCBs) are a class of organic chemical compounds with 1 to 10 chlorine atoms attached to biphenyl and a general structure of C₁₂H_10-xCl_x. Most PCB congeners are colorless, odorless crystals.

The commercial mixtures are clear viscous liquids (the more highly chlorinated mixtures are more viscous, for example, Aroclor 1260 is a "sticky resin"). Although the physical and chemical properties vary widely across the class, PCBs have low water solubilities and low vapor pressures. They are soluble in most organic solvents, oils, and fats.

PCBs are very stable compounds and do not degrade easily. However, under certain conditions they may be destroyed by chemical, thermal, and biochemical processes. These processes may occur intentionally (e.g., incineration), unintentionally, or metabolically. Because of their high thermodynamic stability, all degradation mechanisms are difficult. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or a catalysis. Environmental and metabolic degradation generally proceeds quite slowly relative to most other compounds.

PCBs were commercially produced as complex mixtures containing multiple isomers at different degrees of chlorination. These were sold under trade names followed by a 4 digit number. The first two digits refer to the number of carbon atoms in the biphenyl skeleton (for PCBs this is 12), the second two numbers indicate the percentage of chlorine by mass in the mixture. Thus Aroclor 1260 has 12 carbon atoms and contains 60% chlorine by mass. PCB mixtures have been used for a variety of applications, including dielectric fluids for capacitors and transformers, heat transfer fluids, hydraulic fluids, lubricating and cutting oils, and as additives in pesticides, paints, carbonless copy ("NCR") paper, adhesives, sealants, plastics, and as a fixative for microscopy. The major producer, Monsanto, marketed PCBs under the trade name Aroclor from 1930 to 1977. Their commercial utility was based largely on their chemical stability, including low flammability, and desirable physical properties, including electrical insulating properties. Their chemical and physical stability has also been responsible for their continuing low-level persistence in the environment, and the lingering interest decades after regulations were imposed to control environmental contamination.

In the 1970s, their use declined and essentially terminated because of environmental concerns. PCBs are persistent organic pollutants and have entered the environment through both use and disposal. The environmental transport of PCBs is complex and global. The public, legal, and scientific concerns about PCBs arose from research indicating they were environmental contaminants that had a potential to adversely impact the environment, and, therefore, were undesirable as commercial products. The extent to which PCBs are toxic remains controversial. Despite active research spanning five decades, extensive regulatory actions, and an effective ban on their production since the 1970s, PCBs remain a focus of environmental attention.

PCB Health Effects

The most commonly observed health effects in people exposed to large amounts of PCBs are skin conditions such as chloracne and rashes. Studies in exposed workers have shown changes in blood and urine that may indicate liver damage. PCB exposures in the general population are not likely to result in skin and liver effects. Most of the studies of health effects of PCBs in the general population examined children of mothers who were exposed to PCBs.

Animals that ate food containing large amounts of PCBs for short periods of time had mild liver damage and some died. Animals that ate smaller amounts of PCBs in food over several weeks or months developed various kinds of health effects, including anemia; acne-like skin conditions; and liver, stomach, and thyroid gland injuries. Other effects of PCBs in animals include changes in the immune system, behavioral alterations, and impaired reproduction. PCBs are not known to cause birth defects.

Few studies of workers indicate that PCBs were associated with certain kinds of cancer in humans, such as cancer of the liver and biliary tract. Rats that ate food containing high levels of PCBs for two years developed liver cancer. The Department of Health and Human Services (DHHS) has concluded that PCBs may reasonably be anticipated to be carcinogens. The EPA and the International Agency for Research on Cancer (IARC) have determined that PCBs are probably carcinogenic to humans.

Women who were exposed to relatively high levels of PCBs in the workplace or ate large amounts of fish contaminated with PCBs had babies that weighed slightly less than babies from women who did not have these exposures. Babies born to women who ate PCB-contaminated fish also showed abnormal responses in tests of infant behavior. Some of these behaviors, such as problems with motor skills and a decrease in short-term memory, lasted for several years. Other studies suggest that the immune system was affected in children born to and nursed by mothers exposed to increased levels of PCBs. The most likely way infants will be exposed to PCBs is from breast milk. Transplacental transfers of PCBs were also reported. Because an infant will receive more than ten times the amount of PCBs from breast milk than it will for the rest of its life, It is being debated whether the benefits of breast-feeding outweigh the risks from exposure to PCBs.

Studies have shown that PCBs alter estrogen levels in the body and contribute to reproduction problems. In the womb, males can be feminized or the baby may be intersex, neither a male or a female. Also, both sets of reproductive organs may develop. More instances of this are being reported. Biological magnification of PCBs has also led to polar bears and whales that have both male and female sex organs and males that cannot reproduce. This effect is also known as endocrine disruption. Endorcrine Disrupting Chemicals (EDCs) pose a serious threat to reproduction in top-level predators.

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see also:

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