Page 185 - Soil and water contamination, 2nd edition

172 Soil and Water Contamination

In humans and mammals, DDT , DDE, or DDD enters the body mainly via ingestion
of contaminated food. Studies in animals fed with DDT have shown that DDT may
cause liver cancer, but studies in DDT-exposed workers have not shown increased
incidences of cancer (ATSDR, 2013). Nevertheless, the US Environmental Protection
Agency classifies both DDT and its breakdown products DDE and DDD as probable
human carcinogens.
9.5.5 Polychlorinated biphenyls (PCBs )

Polychlorinated biphenyls (PCBs ; C H Cl ; cf. Figure 9.Ib) are mixtures of up to
12 10-(x+y) (x+y)
209 individual man-made chlorinated compounds. They are either oily liquids or solids
that are colourless to light yellow and have no odour. Some PCBs can exist as a vapour in
air. The vapour pressure and water solubility of PCBs decreases with an increasing degree
of chlorination. PCBs have a very high chemical stability, low flammability, low electrical
conductivity, and good heat-conducting properties. For this reason, PCBs were widely used
as coolants and lubricants in transformers and capacitors, heat exchangers, and hydraulic
systems. They are also used as plastics solvents and paint strippers. Many commercial PCB
mixtures are known by the trade name Aroclor. The production of PCBs worldwide had
almost ceased by 1977. However, they still occur in old equipment and the destruction
of these residues remains an issue. PCBs enter the environment via leaks from and fires in
apparatus containing PCBs, leakage from landfill sites, and discharges of sewage effluents
PCBs are very persistent in the environment; the more chlorinated PCBs are more
persistent than less chlorinated ones. PCBs can be broken down by microorganisms or
photochemically under the influence of UV light. They can travel long distances through
the atmosphere and be deposited in areas far from where they were released. In water, PCBs
are strongly bound to organic particles and bottom sediments . Only a small amount of
PCBs may remain dissolved. In soil, PCBs are fixed strongly to organic matter and to clay
minerals. Because of their strong affinity with soil, PCBs are mostly fixed in the topsoil. In
sandy soils poor in organic matter, however, PCBs can be transported to deeper horizons.
Because the different PCB compounds have different solubility and volatility, they can be
spatially separated due to differential volatilisation of PCBs or downward percolation of
soil moisture. As a result, the more persistent compounds remain in the topsoil, whereas
the relatively mobile compounds are either volatilised or transported downward in the soil
profile .
The strong fixation of PCBs to soil particles means that there is very limited plant
uptake of PCBs from soil. However, algae and fish can take up PCBs in considerable
amounts and via this pathway, PCBs have a high potential to biomagnify in the food chain.
In birds and mammals, PCBs accumulate primarily in fat tissues. The toxic effects of PCBs
include damage to the liver and skin. Animals that ate food containing large amounts of
PCBs for short periods of time had mild liver damage and some died. Rats that were fed
with high levels of PCBs for two years developed liver cancer. Animals that ate smaller
amounts of PCBs in food over several weeks or months developed various kinds of health
effects, including anaemia, acne-like skin conditions, and damage to the liver, stomach, and
thyroid gland. Other effects of PCBs in animals comprise changes in the immune system,
altered behaviour, and impaired reproduction. At normal levels of exposure, PCBs are not
very toxic to humans. The most commonly observed health effects in humans exposed to
large amounts of PCBs are skin conditions such as acne and rashes. Studies on exposed
workers have shown changes in blood and urine that may indicate liver damage. Few studies
of workers have indicated that PCBs are associated with liver cancer in humans (ATSDR,
2013).

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