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PCBs, or polychlorinated biphenyls are a class of organic chemicals consisting of two joined benzene rings (hence "biphenyl") and any number of their ten hydrogen atoms replaced with chlorine (hence "polychlorinated"). They have have had many industrial uses, notably in electronics where they can act as fluids in transformers or capacitors.
The toxic properties of PCB compounds were noted as early as the 1930s but didn't come to prominence until the 1970s when dead seabirds were found with high concentrations of PCBs in them. Effects of exposure to humans are well known, and include skin irritation and liver damage. According to International Agency for Research on Cancer (IARC), "There is sufficient evidence in humans for the carcinogenicity of 2,3,7,8-tetra-chlorodibenzo-para-dioxin." 2,3,7,8-tetra-chlorodibenzo-para-dioxin, also known as TCDD or dioxin, is the most potent rodent carcinogen in the Carcinogenic Potency Database. Based on this evidence and broad mechanistic evidence, 3,4,5,3’,4’-Pentachlorobiphenyl (PCB-126) is considered to be a known human carcinogen. IARC also considers dioxin-like PCBs (numbers 77, 81, 105, 114, 118, 123, 126, 156, 157, 167, 169, and 189) to be known human carcinogens based on mechanistic data.
Generally, PCBs aren't soluble in water, are persistent in the environment (due to fat solubility), are highly volatile and quite flammable. Some types of PCBs have similar toxic properties to dioxins. Basically, this is nasty shit that you don't want to be pouring into rivers and lakes… oh, wait.
Prior to their ban in the United States in 1979, PCBs were widely flushed into rivers and waters without much thought. General Electric is believed to have flushed nearly 600 metric tons of the stuff into the Hudson river in the thirty years leading up to the ban — this also led to a fishing ban on the Hudson as health and environmental effects became known.
In popular culture
Zodiac by Neal Stephenson goes into great detail about the accursed stuff.