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Organic pollutants 173
Cl
Cl O Cl Cl O Cl
Cl O Cl Cl O Cl
2,3,7,8-tetrachlorodibenzo-p-diozin 1,2,3,7,8-tetrachlorodibenzo-p-diozin 6642 6642 6642
Figure 9.2 Examples of dioxins: 2,3,7,8-tetrachlorodibenzo-p-diozin and 1,2,3,7,8-pentachlorodibenzo-p-diozin.
9.6 DIOXINS
Chlorinated dibenzo-p-dioxins (CDDs) are a class of 75 chemically related compounds that
contain two benzene rings with a varying number of chlorine atoms attached to them and
which are linked by two bridging oxygen atoms (Figure 9.2). They are commonly known
as chlorinated dioxins. One of the most hazardous dioxins is 2,3,7,8-tetrachlorodibenzo-p-
diozin (2,3,7,8-TCDD; see Figure 9.2). In pure form, CDDs are crystals or colourless solids.
2,3,7,8-TCDD is odourless and the odours of the other CDDs are not known.
CDDs enter the environment as mixtures containing a number of individual
components. They are not intentionally manufactured by industry, except for research
purposes. They (2,3,7,8-TCDD in particular) may be formed during the chlorine bleaching
process at pulp and paper mills or during chlorination at plants that treat waste or drinking
water. CDDs can occur as contaminants in the manufacture of certain organic chemicals and
are released into the atmosphere during the incineration of industrial and municipal solid
waste. They may also be released during industrial accidents, such as the Séveso accident in
Italy in 1976. This accident led to the release of a cloud, which contaminated over 1800 ha
of land and affected the vegetation and many birds. CDDs were also an ingredient of Agent
Orange, a defoliation herbicide widely used by the US Air Force during the Vietnam War in
the 1960s (Alloway and Ayres, 1997).
In the atmosphere, CDDs may be transported over long distances, which is why dioxins
are ubiquitous in the global environment. When released in wastewaters, some CDDs are
broken down by sunlight, some evaporate to air, but most attach strongly to solid particles
and settle to the bottom sediments . CDD has a high potential to biomagnify in the food
chain, resulting in measurable levels in animals. In animals, exposure to low levels of
2,3,7,8-TCDD can induce various toxic effects, including damage to the liver, weight loss,
disruption of the immune system, reduced reproductive ability, and birth defects including
skeletal deformities, kidney defects, and weakened immune responses. Human exposure
to large amounts of 2,3,7,8-TCDD can cause chloracne, a severe skin disease with acne-
like lesions on the face and upper body. Other skin effects noted in people exposed to
large doses of 2,3,7,8-TCDD include skin rashes, discoloration, and excessive body hair.
Exposure to high concentrations of CDDs may also cause liver damage and changes in
glucose metabolism and hormonal levels (ATSDR, 2013). According to the World Health
Organisation, 2,3,7,8-TCDD is a human carcinogen.
9.7 EMERGING SUBSTANCES OF CONCERN
The term ‘ Emerging Substances of Concern’, or ESOC, has recently been introduced
for a wide range of chemicals – mostly organic – that are currently causing concern to
environmental toxicologists. These substances are termed ‘emerging’ because they have
only recently been introduced into the environment, have only recently been detected in
the environment as a result of improved chemical analytical technologies, or their potential
toxicity has only recently been recognised (Ternes and Von Gunten, 2010; Stuart et al.,
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