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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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