PCBs, polychlorinated dibenzo-p-dioxins
(PCDDs) and polychlorinated dibenzofurans (PCDFs)
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
Persistence and oestrogenicity
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,
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 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
to the hormone disrupting chemicals home page
Alloway, B. J. and Ayres, D. C. 1993. Chemical
principles of environmental pollution. Blackie Academic &
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.
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
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):