The book "Silent Spring" by Rachel Carson, published in the early 1960s, raised concerns about how pesticides could build up through food chains.

Organochlorine pesticides, like DDT, were found to pass from animal to animal so that the last in the chain - birds of prey, were the ones most affected.

Carson demonstrated that these chemicals were "persistent". The substances may go from view, but they are still somewhere, somewhere they may "accumulate" - ie add up to levels which may be a risk (see Unit 2).

Miles from their source, such chemicals could accumulate in plankton, with more accumulating in the fish that eat the plankton, and still more in the animals that eat the fish.

Residues were found in birds of prey. Their eggshells became so thin few chicks hatched.

She also showed that these chemicals did not just travel thousands of miles, they could also "cross generations". The chemicals crossed into the foetus in the body, or through the genes into the next generation.