Introduction
A product or substance used in the control of pests such as vermin, mosquitoes, moulds, and weeds, all of which may affect public health or attack useful resources. (The term subsumes insecticides with which it is often used interchangeably.) There are three main classes: Chlorinated hydrocarbons (e.g. DDT) which are long-lived and capable of being concentrated biologically. organophosphates which are short-lived and degrade to ‘harmless’ end-products. Artificial pyrethrins originally based on natural sources from pyrethrum flower heads but now being synthesized in very large amounts. Widely used pesticides are: Chlorinated hydrocarbons (persistent in the environment).