Toxic materials are poisonous. Some quickly break down into harmless substances in the environment. Others are persistent and do not break down. Instead, they accumulate in the food chain and damage the organisms in it, especially the top predators. Mercury and DDT are two persistent toxic materials.

Mercury

Mercury compounds were used until quite recently to make insecticides (chemicals that kill the insects that damage crops) and special paints that stop barnacles growing on the hulls of ships.

Unfortunately, when it gets into the food chain mercury damages the nervous systems and reproductive systems of mammals, including humans. The diagram shows how mercury can accumulate in the food chain.

Tiny plankton in the sea 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.

The small fish are eaten by larger fish, and these are eaten by still larger ones, such as tuna fish. 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

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. Concentrations of DDT in birds causes weakness in their eggs, and reduces their population. DDT is now banned because of this.