PCBs were too stable for their own good, and now they sink to ocean trenches
Polychlorinated biphenyls barely burn, resist acids and heat, and refuse to break down, which made them perfect for cooling transformers. Those same traits turned them into one of the most persistent pollutants ever made, still turning up decades after the United States banned most uses in 1978.
PCBs are built from biphenyl, two linked benzene rings, with between one and ten hydrogen atoms swapped for chlorine. That allows 209 distinct compounds, around 130 of which appeared in commercial products. They are pale yellow, oily liquids that shun water but dissolve readily in fats and oils, conduct heat well, insulate electrically and have high flash points. In an electrical arc they give off gases that will not burn. Industry used them in transformers and capacitors, old fluorescent light fittings, hydraulic fluids, carbonless copy paper, paints, sealants and cable coatings.
Monsanto, the only North American producer, sold them from 1930 to 1977 as Aroclor, with a four-digit code whose last two digits gave the chlorine content by weight; Aroclor 1260 is 60 percent chlorine. Estimates of total world production range from about 1 million to 1.5 million tonnes, with the United States making over 600,000 tonnes. A 1988 estimate put 370,000 tons loose in the environment and 780,000 tons still in products, landfills and storage. Chemical processes still create some as unintended by-products.
The trouble is toxicity. The International Agency for Research on Cancer classes PCBs as definite human carcinogens. Some resemble dioxins in shape and action; others disrupt thyroid hormones or damage nerves and immune systems, with developing animals, humans included, most at risk. Partial burning can even produce highly toxic dioxins and furans, so destroying unwanted stocks needs intense heat or catalysts. International production is now banned under the Stockholm Convention.
Once released, PCBs travel. The atmosphere carries lighter forms around the globe, and ventilation of contaminated indoor air appears to be a major source. Oceans are the main reservoir, and under deep-sea pressure PCBs become denser than water and collect in the deepest trenches. Because organisms retain them, concentrations build up the food chain.
Source: Polychlorinated biphenyl
