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PCBs, polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs)

Uses

Polychlorinated biphenyls (PCBs) have been used since 1929 in a variety of applications, including as heat transfer fluids in large transformers and as dielectric fluids in capacitors. Though their use has now ceased, they are still present in many older electrical installations. A typical PCB is made up from a mixture of congeners, each having different numbers and positions of the chlorine substituents:

PCDDs (or 'dioxins') and PCDFs are different from the other chemicals described in these pages because they are not manufactured deliberately. They can be produced during incineration, paper manufacture, and in the production of chlorinated aromatics such as 2,4,5-trichlorophenol (an intermediate in the manufacture of the herbicide 2,4,5-T). Like PCBs, many congeners of PCDD exist, but one of the most researched is 2,3,7,8-tetrachlorodibenzodioxin (TCDD).

Persistence and oestrogenicity

PCBs

PCBs are very persistent in the environment, since they are very resistant to biodegradation. In addition, they are fat soluble, and tend to accumulate in organisms, with those highest in the food chain being most affected. Levels of up to 195 ppm have been found in the blubber of dolphins, and up to 232 ppm in the milk of Arctic seals (Alloway and Ayres, 1993). Some level of PCB contamination can usually be found in fat from any organism in the world, including humans.

It appears that the main oestrogenic effect of PCBs may be due to their hydroxylated metabolites, which are produced when the body attempts to break them down, in addition some PCB congeners may be anti-oestrogenic (IEH, 1995). Those metabolites with a para-hydroxylation on one of the rings are particularly effective at mimicking oestradiol (McKinney and Waller, 1994), though others are also oestrogenic (Soto et al, 1995).

Many PCDDs are known to be toxic and carcinogenic. PCDDs appear to be anti-oestrogenic (IEH, 1995).

Exposure to PCBs in food has been linked to delayed brain development and reduced IQ in children (Jacobson and Jacobson, 1997).

Dioxins

Dioxins are persistent and bioaccumulative, and the most researched dioxin, TCDD, is proven to cause cancer in humans (McGregor et al, 1998). Dioxins are also endocrine (hormone) disrupters, and alter the immune system (Grassman et al, 1998). The World Health Organisation is cutting its recommended safe limit for dioxins and furans from 10 pg/kg/day to 1-4 pg/kg/day. Research by the UK Ministry of Agriculture has shown that adults are already taking in enough dioxin (including dioxin-like PCBs) in their food to breach this new limit, and children are taking in even more (ENDS, 1998).

Further information and policy

There is much more information about dioxin on other web sites. The dioxin homepage is a good place to start. A detailed study on EU dioxin exposure and health, published in October 1999, is available on the Commission's web site here.

Both dioxin and PCBs are included in the global POPs negotiations which are currently underway, see the policy page for more details.


This page was last updated in December 1999
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References

Alloway, B. J. and Ayres, D. C. 1993. Chemical principles of environmental pollution. Blackie Academic & Professional, London.

ENDS, 1998. WHO to slash dioxin safety limits. ENDS Report 281, p4-5.

Grassman, J. A., Masten, S. A., Walker, N. J. and Lucier, G. W. 1998. Animal models of human response to dioxins. Environmental Health Perspectives 106 Suppl. 2, p761-775.

IEH 1995. Environmental oestrogens: Consequences to human health and wildlife. Institute for Environment and Health, University of Leicester, Leicester, UK.

Jacobson, J. L. and Jacobson, S. W. 1997. Evidence for PCBs as neurodevelopmental toxicants in humans. NeuroToxicology 18: 415-424.

McGregor, D. B., Partensky, C., Wilbourn, J. and Rice, J. M. 1998. An IARC evaluation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans as risk factors in human carcinogenesis. Environmental Health Perspectives 106 Suppl. 2, p755-760.

McKinney, J. D. and Waller, C. L. 1994. Polychlorinated biphenyls as hormonally active structural analogues. Environ. Health Persp. 102: 290-297.

Soto, A. M., Sonnenschein, C., Chung, K. L., Fernandez, M. F., Olea, N. and Serrano, F. O. 1995. The E-SCREEN assay as a tool to identify estrogens: An update on estrogenic environmental pollutants. Environ. Health Persp. 103 (Suppl. 7): 113-122.


URL: http://website.lineone.net/~mwarhurst/pcb.html