Toxic materials are poisonous. Some quickly break down into harmless substances in the environment. Others are persistent (they stay in the environment and do not break down). These substances accumulate in the food chain and damage the organisms in it, particularly in the predators at the end of the chain. This is because accumulating compounds cannot be excreted.
Mercury compounds were used until recently to make insecticides (substances that kill the insects that damage crops), and special paints that stop barnacles growing on the hulls of ships.
Unfortunately, when mercury gets into a food chain, it damages the nervous systems and reproductive systems of mammals, including humans. The diagram shows how mercury can accumulate in the food chain.
In the sea, tiny animals and plants called plankton absorb the mercury compounds. When the plankton are eaten by small fish, the mercury they contain stays in the fish. As the fish need to eat a lot of plankton, the concentration of mercury in them becomes higher than its concentration in the plankton.
Larger fish eat the small fish, and larger ones still (such as tuna fish) eat them. This creates a high concentration of mercury in the tuna. People eating contaminated tuna may get mercury poisoning. Mercury is now banned from many chemical products and mercury use in industry is carefully regulated.
DDT is an insecticide that can pass up the food chain from insects to small birds, and then from the small birds to birds of prey, like hawks. It can accumulate in the birds of prey, giving them a large amount of DDT. High concentrations of DDT in birds cause weakness in the shells of their eggs, which leads to a reduction in their population. DDT is now banned because of this.