Dioxins and Polychlorinated Biphenyls

Fish can also contain other organic pollutants, such as PCBs and dioxins. Whereas mercury accumulates in the muscles of larger predatory fish, PCBs and dioxins are found in the fatty tissues of fish. Most human exposure to PCBs and dioxins comes from dietary sources because they accumulate in the lipid fractions of meat, fish, milk and milk products, eggs, grains, and oils.

Table 5 Concentrations of methylmercury in surveyed fish in the United Kingdom


Methylmercury (mg/kg)







Fresh tuna


Canned tuna




Pink shrimps












PCBs and dioxins have been linked with increased rates of some cancers in studies of individuals exposed to high amounts through either vocational exposure or accidental environmental contamination. Prenatal exposure to large amounts of these pollutants (e.g., through contaminated fish) has been associated with neurobehavioral alterations in newborn children. Some studies have also suggested that exposure to smaller quantities of PCBs and dioxins in utero may lead to more subtle cognitive and motor developmental delays, although a favorable home environment appears to counteract any effect. However, the difficulty of separating the effects of PCBs and dioxins from potentially confounding factors (e.g., exposure to other contaminants, breast-feeding, smoking, and maternal education) makes it difficult to reach firm conclusions. Further research is also needed to ascertain whether any cognitive changes are temporary or persist into later life.

The potency of dioxins is expressed as toxic equivalents (TEQs), which have been internationally accepted. In the United Kingdom, the tolerable daily intake (TDI) recommended by COT is 2 pg TEQ/kg body weight, which is in line with recommendations of other international and European expert committees. In common with the United States and the European Union, approximately one-third of the UK population may exceed the TDI in their daily diet. The TEQ, therefore, provides a target to reduce dioxins and PCBs in the environment internationally. Since the 1960s, following the prohibition of many dioxins and PCBs by governments, concentrations have been declining in breast milk, which is commonly used to determine exposure. For example, between 1982 and 1997, consumption of dioxins and PCBs in the United Kingdom decreased by 75%, and due to strict controls concerning production, use, and disposal of PCBs and dioxins, it is anticipated that intakes will decrease further.

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