The unwanted or undesirable presence of a substance that may have an undesirable effect such as be infectious or harmful in some way to humans or the environment. It may be in the form of deposition, absorption, or adsorption of radioactive material, (p. 79) or involve biological and chemical agents such as in food, soil, skins. Bacteria in drinking water, radioactive material in soil, oil in water, or water in oil, are all examples of contaminants. Compare pollution.