When chemical contaminants enter the body of a person, they circulate around the body in the blood. Different contaminants have different chemical properties and specific contaminants tend to accumulate in specific parts of the body, called target tissues, or in substances produced in the body such as breast milk (Table 3).
Table 3: Some common pollutants and their target tissues
|Pollutant||Target tissues or substances|
|lead||bone, teeth, nervous tissue|
|mercury||nervous tissue, particularly the brain|
|organochlorine pesticides, polychlorinated biphenyls (PCBs)||fatty tissue, breast milk|
The affinity of specific pollutants for specific target tissues is related to a very important aspect of ecotoxicology, called bioaccumulation. This refers to the fact that, having been released into the environment, a pollutant is not randomly or evenly dispersed, but becomes concentrated into particular components of ecosystems. For example, DDT is accumulated in the fat reserves of birds, where it can reach quite high levels. (DDT, dichloro-diphenyl-trichloroethane, was the first widely used synthetic pesticide and has been used to kill agricultural and domestic insect pests since 1939; see Section 4.2.) This has two important effects. In the affected bird it means that, if it uses its fat reserves to provide energy for some specific activity, such as reproduction or migration, a large dose of DDT is released into its blood over a short time. Every time a predator eats such a bird, it too receives a large dose which, in turn, is stored in its fat. The consequence of bioaccumulation is that contaminants that may be quite safe to wildlife, or humans, when encountered at the kind of concentrations at which they are released into water, can become concentrated at particular points in the food-chain at levels that are not safe (Figure 8).