Polychlorinated biphenyls (PCBs)
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The manufacture of Polychlorinated biphenyls (PCBs) started in 1929 and peaked in the 1960s. From 1973 their use 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
Open uses were banned in the United Kingdom. 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.
