Chemicals such as DDT and other organochlorine pesticides are not very soluble in water but do dissolve in fat. Therefore in animals, including humans, that are exposed to these substances the chemicals tend to be distributed into tissues containing fat. Although DDT is metabolized to some extent and some is excreted (see Chapter 2), some of it remains in animals. If exposure is repeated or continuous, accumulation occurs. For example, fish or other organisms swimming in contaminated rivers may accumulate substances like DDT directly from the surrounding water or from sediments. This is called bioaccumulation. Even the level after accumulation may not be enough to be hazardous to organisms at the bottom of the food chain such as plankton or small fish. But the larger animals eating these smaller fry and further up, or at the top of, the food chain could acquire sufficient levels of DDT for it to be lethal. Thus the concentration in the fat in these animals can be much higher. For example, in Lake Michigan it was found that although the sediment had DDT levels of 0.02 ppm, herring gulls had levels of 98 ppm. Therefore a biomagnification of more than i,ooo-fold had occurred (see Figure ii). It has been estimated that there is an accumulation factor of 5-15 between a predatory bird and its prey. Add to this the possible greater sensitivity of certain species and one can see how a substance such as DDT can cause the death of certain animals. In humans exposure occurs mainly if we eat food containing DDT or similar substances, but a constant diet contaminated with DDT could lead to accumulation if the level of intake exceeds the detoxication and elimination. Consequently food is monitored for such chemicals. Because breast milk has a high fat content, these substances will appear in the milk and be
ii. The accumulation and concentration of chemicals in the environment via the food chain. This is known as bioaccumulation and biomagnification, and applies especially to chemicals which are soluble in fat such as the organochlorine pesticides. The animals (e.g. fish-eating birds) at the top of the food chain acquire the highest concentration of chemical.
transferred to the suckling baby, which can then be exposed to a greater dose on the basis of its size. For example, lactating mothers exposed to DDT at 0.0005 mg per kg body weight per day were found to produce milk containing 0.08 ppm DDT, with the result that their infants were exposed to 0.0112 mg per kg body weight per day, twenty times greater than the mothers.
This principle is further illustrated by an example at Clear Lake in the USA. Here a very similar insecticide, DDD (also known as TDE), a breakdown product of DDT, was used, because it is less poisonous to fish than DDT. DDD was used to control midges which were a serious nuisance on the lake. Spraying over the water in which the larvae of these insects lived, in 1949, had a dramatic effect on the midge population for some years. There were no apparent effects on fish or, initially, on other wildlife. In later years more DDD was used but then effects on wildlife were detected, in particular on the bird the Western Grebe, whose breeding numbers were seriously reduced. Then in 1954 large numbers of dead grebes were found. Eventually it was discovered that the grebes had high levels of DDD in their fat.
Although the level of DDD used was low, it accumulated in organisms, each animal in the food chain gaining higher levels, until the animal at the top of the chain, the final predator, acquired levels that were lethal. This worried scientists when it was discovered. The same phenomenon applies to DDT. These examples illustrate what happens when excessive or even just larger than necessary amounts are used. There was already mounting public concern when, in 1963, Rachel Carson's book Silent Spring was published, which documented these occurrences and laid the blame on DDT. It had an enormous impact and the eventual result was that DDT was banned in many countries.
In the UK, however, it continued to be used for some time, perhaps more carefully. In 1963 it was used to control the pine looper caterpillar, a serious pest in pine forests, and large areas were sprayed with about 1 lb per acre. The caterpillar was successfully controlled and, although DDT could be detected in birds from the area, there were no deaths of birds nor was their population affected. This example illustrates how it is possible to use insecticides such as DDT carefully and avoid adverse effects on non-target species.
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