Are the PCB compounds really super-poisons?
In 1999, Belgian chickens started to die and failed to reproduce. Since the symptoms were characteristic of dioxins, analyses were conducted, and indeed high concentrations were found both in the chickens and the chicken feed. Somewhat later also PCB-compounds were analysed, and much more of these were found. This sparked the food scandal of the turn of millennium in Europe, and countries are still suffering from the administrative after-shocks, although not from the health consequences.
It was later found out that some fool had emptied PCB oils from a transformer to a fat recycling tank where fry oils and other fats were collected for recycling prior to being used for animal feed by the feed industries. Along with the recycled oils, tens of kilograms of PCB oils were also delivered to the feed manufacturers, including their dioxin impurities. No definite health effects could be shown, because the incident fortunately was uncovered within a few months, and in that timeframe it was not likely to see human effects when consuming the products.
A much more serious PCB accident took place in Fukuoka, Japan, in 1968. Rice oil was contaminated from a leaking heat exchanger over a prolonged period, and at least 1200 people suffered symptoms of poisoning. The syndrome is known as Yusho disease. In 1979, a similar accident took place for the same reason in Taiwan and it is known as Yu-Cheng. In contrast to the Belgian incident, in these Asian cases, the rice oil was intended for human consumption, and therefore human intake was much higher.
What are PCB-compounds?
Polychlorinated biphenyls or PCBs are oily persistent chemicals that were introduced at the beginning of the 1930s for a wide range of technical purposes such as cooling oils in electrical appliances, plastic softeners in different products, pump oils, heat exchanger oils, flame retardants, sealants etc. There are theoretically 209 different PCB compounds, and all technical preparations are mixtures of dozens of congeners. They seemed ideal because they were stable and non-flammable. More than a million tonnes were used from the 1930s to 1970s. However, because of their persistence and bioaccumulation in aquatic organisms, their use started to be banned in different countries from the 1970s onwards.
The effects of very high exposures are well known because of the Yusho and Yu- Cheng catastrophes. Unborn children were affected in the most dramatic manner. Poisoning resulted in stillbirths, lowered birth weight, dark pigmentation of the skin and other skin disorders, premature teeth at birth, a variety of tooth and gum changes, as well as neurological defects and delayed development. Since the oils are such complicated mixtures, it was not clear at once, how much was due to PCB compounds, and how much was attributable to the much more toxic dibenzo-p-dioxins and dibenzofuranes present at much lower concentrations. At their worst, the concentrations of PCBs during the poisoning were about 100,000 times higher than concentrations to which the "normal" population is exposed.
PCB compounds are not super-poisons. In fact their toxicity is rather low with the exception of half a dozen of "dioxin-like PCB compounds." These dioxin-like compounds are present at much lower concentrations, in fact within the same range as the dioxins themselves. It seems possible, and after some recent re-evaluations even likely, that almost all PCB effects are due to dioxin-like impurities and these dioxin-like PCBs rather than the "bulk PCBs".
One of the possible non-dioxin-like risks of PCBs may be disturbance of thyroid hormone balance. Some of the metabolites of PCBs resemble thyroid hormone, and compete for its carrier molecules in rats causing a relative lack of hormone. Whether this mechanism is associated with the alleged delays in human brain development remains to be seen.
Persistence in the environment
The biggest problem with PCB compounds is their persistence in the environment and their biomagnification. They are fat soluble and very poorly water soluble. Therefore in nature they are attracted towards living organisms, especially to their fat. The consequence is bioconcentration and biomagnification in the food chain which means that the highest concentrations will be found at the uppermost levels of the food chain.
The Baltic Sea and the Great Lakes of North America are examples of environments which are contaminated with PCB compounds. Since their use was discontinued, the concentrations of PCBs have been slowly declining, especially in animal organisms. In the Baltic Sea, the concentrations have decreased perhaps to about one fifth of their peak levels in the 1970s. On the other hand, there is a huge amount stored in the bottom sediments, and these compounds will probably stay there for centuries, maybe millennia. They continue to be environmental problems to some degree even if they no longer cause health problems.
PCBs are classic example of a group of chemicals that seemed to be too good to be true – and weren't. The technical properties which led to their popularity later turned out to be their fundamental problem: oils with stability, heat resistance, non-flammability, and microbiological persistence are ideal for bioaccumulation. Perhaps very well purified PCBs would have been a more viable group of chemicals for limited use in specific purposes.
Notes and references
One level up: Is man defiled by what goes to his mouth?
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