Polychlorinated biphenyls (PCBs)
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The manufacture of Polychlorinated biphenyls (PCBs) began in the 1920s and was taken over by Monsanto Co. from Swann Chemical Company in 1929. Manufacture peaked in the 1960s, by which time the electrical industry had lobbied congress to make them mandatory safety equipment, knowing all the while that they were extremely toxic. In 1966, they were "discovered" by Swedish chemist Dr. Soren Jensen as an environmental contaminant, and according to a 1994 article in Sierra, it was Dr. Jensen who named them. Previously, they had been called "phenols" or referred to by various trade names, such as Aroclor, Kennechlor, Pyrenol and others.
From 1973 their use was banned in "open" or "dissipative" sources, such as:
- plasticisers in paints and cements
- casting agents
- fire retardant fabric treatments
- adhesives
- paints and water-proofing
- railway sleepers
--> However, they were allowed in "totally enclosed uses" such as transformers and capacitors, which however tended to explode on a fairly regular basis. It was Ward B. Stone of the New York State Department of Environmental Conservation who first published his findings in the early 1970s that PCBs were leaking from transformers and had contaminated the soil at the bottom of utility poles.
Combustion of disused railway sleepers to cremate infected livestock during the 2005 foot and mouth outbreak evaporated large quantities of PCBs both from the sleepers and from the animals fatty tissues. Some PCBs were oxidised in the fires to PCDFs (polychlorinated dibenzofurans), such as dioxins, creating a local health problem.
The use of PCBs in "closed" uses include:
- capacitors
- dielectric fluids in transformers
- vacuum pump fluids
- hydraulic fluids
Closed uses were also banned in the UK from 1981, when nearly all UK PCB synthesis ceased.
Globally, probably 1 million tonnes of PCBs were manufactured in total. They were synthesised in such large amounts mainly because of their incompressibility, extreme resistance to combustion, very poor electrical conductivity and high specific heat capacity. They now pose an environmental hazard because they are persistent pollutants: they are not easily degraded in the environment.
[edit] Toxicity
The toxicity of PCBs to animals was first noticed in the 1970s when emaciated seabird corpses with very high PCB body burdens were washed up on beaches. The source(s) of the PCBs was (were) unknown though, because seabirds may die at sea and be washed ashore from a very wide area. Where they were found was no reliable indicator of where they had died.
