DDT, BHC, PCBs etc. are non-biodegradable pollutants which are not degraded easily and long lasting in the environment. DDT shows the phenomenon of biological magnification or food chain concentration, i.e., the continuous increase in concentration of DDT at successive trophic level in a food chain. Fluorine occurs in nature as fluoride, excess fluoride in drinking water causes fluorisis. Cadmium poisoning is called itai-itai in Japan. Mercury poisoning causes a fatal disease called Minimata disease.