Polychlorinated biphenyls (PCB)

DEFINITION: Chemical compounds with biphenyl molecules on which chlorine molecules substitute for two or more of the hydrogens

Polychlorinated biphenyls are among the most insidious of environmental pollutants. Although their production has been banned in many countries, widespread use has led to almost universal contamination by these compounds, which are resistant to biodegradation and have a tendency to bioaccumulate.

Polychlorinated biphenyls (PCBs) were first synthesized in 1881 and became readily available during the 1930s. A PCB is a biphenyl on which chlorine molecules substitute for two or more of the hydrogens. A related chemical, polybrominated biphenyl (PBB), contains bromine instead of chlorine. Many different kinds of PCBs form during their synthesis because each ring on the biphenyl molecule can have up to five chlorine atoms. Monsanto Company made PCBs in the United States and sold them under the trade name of Aroclor. Between 1930 and 1975, more than 570 million kilograms (1.26 billion pounds) of PCBs were made in the United States alone.

Some of the properties that made PCBs attractive chemicals were their high boiling points, low water solubility, and heat resistance. These physical properties meant that PCBs were hard to burn, resisted acids and bases, and were mostly inert. Consequently, PCBs were used in many manufactured products, including adhesives, fluorescent lights, insulation in transformers, high-pressure hydraulic fluids, plasticizers, varnishes and paints, and protective coatings for wood, metal, and concrete. Braided cotton and in electric wire insulation were also impregnated with PCBs.

The properties that made PCBs useful to industry also made them potentially long-lasting and widespread pollutants. However, PCBs were not suspected of being an environmental problem because they were mostly inert, were not deliberately spread, and were not acutely toxic. One exception was an incident in Japan in 1968 when rice bran oil was accidentally contaminated with PCBs. It was difficult to show chronic PCB toxicity, although symptoms such as chloracne and low birth weights in babies born to exposed mothers were known. The US Environmental Protection Agency (EPA) did not list PCBs as cancer-causing chemicals until they had been in use for a very long time.

In 1966 Sören Jensen, a Swedish chemist, reported observing traces of PCBs in animal tissue that dated as far back as 1944. Scientists in California corroborated his results, and subsequent analyses demonstrated that PCBs were everywhere in the environment: rainwater in England, river water in Japan, seals in Scotland, eagles in Sweden, Baltic Sea cod, mussels in the Netherlands, penguin eggs in Antarctica, pelican eggs in Panama, and, most disturbingly, human hair and fat. All evidence pointed to a global spread of PCBs. The most likely route of spread was determined to be the incineration of PCB-tainted wastes. It is now virtually impossible to find samples of material that do not contain some trace amounts of PCBs. Even foods grown in chemical-free fields can receive trace amounts of PCBs from atmospheric deposition of incinerator exhausts.

PCBs are not very soluble in water. They are, however, extremely soluble in lipids and rapidly adsorbed through the walls of the digestive system. PCBs are almost twice as soluble in fats as dichloro-diphenyl-trichloroethane (DDT) and thousands of times more likely than other environmental pollutants to bioconcentrate in animal tissue. Jensen’s results indicated that they were starting to bioaccumulate in the food chain. Scientists and environmentalists alike worried that PCBs might ultimately become a greater environmental problem than DDT because more of them had been produced, they lasted longer, and they apparently had wider distribution.

Declines in fish-eating bird populations and evidence of thin-shelled eggs began to increase after World War II. This coincided with a rapid rise in the use of chlorinated chemicals such as PCBs. PCBs cause birds to lay thin-shelled eggs, as does DDT, because these chemicals inhibit enzymes involved in calcium movement. The estrogen controls the calcium level in breeding female birds, and PCBs stimulate enzymes that make estrogen more soluble and readily excreted. When estrogen is low, calcium reserves are low, and little calcium is available for eggshell formation.

Several events ultimately led to the halt of PCB and PBB production in the United States by 1975. In 1974, thousands of cattle and other farm animals in Michigan were quarantined and destroyed because they were contaminated with PBBs. The contamination occurred because a chemical company accidentally mixed bags of a fire retardant containing PBBs with bags of a magnesium oxide mix used in cattle feed. The chemicals were mixed with feed that was distributed around the state. Farmers first began noticing in their farm animals in late 1973, and contaminated milk and eggs exposed several thousand farm families to PBBs.

It has proved difficult to remove PCBs once they get into the environment. Like all highly chlorinated compounds, PCBs are extremely resistant to biodegradation. In the late 1980s, scientists at the General Electric Research and Development facility in Schenectady, New York, began looking at sediment from sites in the upper Hudson River that were contaminated with up to 268,000 kilograms (591,000 pounds) of PCBs. The PCBs came from a capacitor manufacturing operation at Hudson Falls and Fort Edward, New York, that operated between 1952 and 1971. The scientists reported that highly-chlorinated PCBs changed to lower-chlorinated PCBs in sediments. They suggested that PCBs in the biodegrade in a two-step process. The researchers found that chlorine removal in anaerobic aquatic was followed by further in environments that eventually caused total PCB destruction. More important, chlorine removal detoxified carcinogenic PCB congeners known to persist in humans.

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