The speed with which pesticides break down into harmless constituents varies greatly between products, and is affected by factors such as climate and soil type. Some pesticides, notably the organochlorine insecticides, are broken down relatively slowly and hence have time to spread to areas far distant from their places of application. Despite their low solubility in water, these insecticides and their metabolites become widely dispersed in freshwater and marine environments. Since organisms of economic significance are very susceptible to small amounts of these insecticides, and since the insecticides can accumulate

in food chains, great care should be taken to prevent unnecessary contamination of water by them.

Studies on organochlorine insecticides in the environment led indirectly to the discovery that certain industrial substances used in plastics, lubricants etc., the polychlorinaled biphenyls, can also become widely distributed in freshwater and marine ecosystems. Recent studies on the ecological significance of this form of pollution are discussed.