What are PCB’s and are they bad?

Polychlorinated biphenyls, PCB’s, are a class of 209 molecules widely used in industry as coolants, plasticizers, waterproofing materials, and other uses. Producing PCB’s entailed reacting biphenyl in the presence of chlorine, so the product contained a mixture of a wide variety of PCB’s. These molecules were appealing because they are inert, inexpensive, have low vapor pressures, and insulate electricity well. Unfortunately, the explosive use and careless disposal of PCB’s throughout the 20th century did not raise red flags until the 80’s , at which point their bioaccumulation had been well underway and their environmental persistence is high. In the United States PCB production was ended in 1977, with other countries eventually ending PCB synthesis, too. However, their use in old machines is not completely irradiated and their resistance to degradation in the environment causes them to still be a major contaminant. Another problem with PCBs is the production of dibenzofurans (furans) from the heating of the molecules. Determining the toxicity of PCB’s and furans is relatively inconclusive; but, lab animals have died or developed cancer for PCB exposure and there have been cases of human PCB poisoning.   Prenatal studies determined that PCB’s affect brain development and body size.

Any number of chlorines can be added to the numbered carbon positions.

In the body, PCB’s are metabolized to phenols, producing arene oxides as intermediates. These intermediates have the capability to bind to nucleophilic cell components, such as proteins, DNA, and RNA. In short, this is bad because binding to DNA can induce strand breaks and broken DNA is less than ideal for an organism.

Farmed salmon and PCB’s

This study measured the amount of PCB’s in 700 wild and farmed salmon and determined that the amount of the contaminants in farmed fish is significantly higher than that of wild salmon (36.63 ppb in farmed versus 4.75 in wild). Dr. Barbara Knuth, a researcher in the study, suggests that farmed salmon consumption should be limited, which is especially important considering that the salmon consumption increases 10-20% per year according to the executive director of Salmon of the Americas.

The Food and Drug Administration (FDA) advises “consumers not to alter their consumption of farmed or wild salmon”, stating that the PCB content of both salmon groups is well below their 2000 ppb tolerance level. Additionally, the FDA emphasizes that the majority of the PCBs were found in the skin and fat of the fish which is not popularly consumed. Dr, Terry Troxell, a director of the FDA speaking of the salmon study says “when salmon is cooked, you lyse a considerable amount of fat, and so the levels go down quite a bit”.

How did the salmon end up with PCB’s and other contaminants? Farmed fish eat fish meal containing oils from other fish. At each step in the food chain, contaminants build up in fatty tissues; so, theoretically a vegetarian fish would have lower levels than a fish who eats a vegetarian fish.

Bioaccumulation of PCB’s

Trent states the salmon industry is looking to reduce PCB content in their farmed fish by switching the fish oil for a vegetable alternative. He also adds that meat and dairy likely contain worrisome PCB levels – emphasizing the bioaccumulation of contaminants.

An additional, apparently unpublished study by the Environmental Working Group measured PCB’s in salmon and determined “they were so high that people should eat farmed salmon no more than once a month”.

The article then goes into discuss the dangers of PCB’s and concludes that, while mixtures of PCB’s cause cancer in laboratory rodents, no link has been seen in humans. There has been a proven health risk with the mixture of PCB’s and two other compounds – dieldrin and toxaphene. However, the study did not measure the there compounds and the FDA refutes the idea that the three work together and have deleterious health effects without “the science to show they’re based on the same mechanism of action”. Discourse continues between the scientific community and the FDA and salmon industry. Without hard evidence for a mechanism for PCB danger in humans, the FDA and industry are skeptical to admit that the higher level of the contaminant in farmed fish poses any threat to consumers.

The unconvincing argument for not being concerned about PCB’s in salmon.

Regardless of the lack of determined mechanism for deleterious effects of PCB’s, I think it is undeniable that we do not want them in our food. They are resistant to biological breakdown, suggesting our bodies would not be readily capable of destroying them. And, their water insolubility means that they would not be “flushed out”, literally. Although the study did not look at furan contamination in the farmed and wild fish, it is likely that they additionally contain the compounds. Adverse biological effects have been observed and it appears that the FDA and salmon industry do not have the public’s best interest in mind when suggesting we do not reduce salmon consumption. At the same time the industry states the intent to reduce farmed salmon PCB content, implying that they are not convinced that it is not an issue. With the proposed mechanism for metabolized PCB intermediates binding to DNA, I am additionally not convinced that we should not be concerned.

They are a “probable” human carcinogen, according to the International Agency for Research on Cancer- lab animals have been seen to have an increase in a variety of cancers. However PCB’s cannot be fed to humans intentionally to see if they are carcinogenic… For obvious ethical reasons. Nevertheless lab animals are a relatively good model for human biology and I would air on the side of declaring PCB’s as something to be avoided before bioaccumulation of the contaminant continues and cancer rates skyrocket.

Considering the world’s agreement for stopping production of PCB’s, basically everyone acknowledges that PCB’s are bad, to say the least. It may be true that the levels, even in the farmed fish, are nothing to be terribly concerned about. Yet, it seems like the FDA and definitely the salmon industry have some hidden agenda. It is somewhat irresponsible that the FDA refuses to admit that farmed salmon may contain an alarming amount of PCB’s. I do not know about you; but, I definitely would like to avoid consuming a “probable human carcinogen”.