Selenium pollution can arise from mining and industrial activities, also from the improper use or disposal of products containing selenium, which may include insecticides, electronic components and radiographic materials. Certain plants are known to accumulate selenium when grown in soils containing a high natural content of this element or where selenium has entered the soil from industrial waste.
Selenium is a chemical element closely allied in physical and chemical properties with sulphur. It occurs in rocks and soils all over the world. There are no true deposits of selenium anywhere, and it cannot be economically recovered from the earth directly. The highest concentrations known are in native sulphur from volcanoes, which contains up to 8.350 ppm. Selenium does, however, occur together with tellurium in the sediments and sludges left from electrolytic copper refining and the chief world supplies are from the copper refining industries of Canada, the USA, and Zimbabwe where the slimes contain up to 15% selenium.The manufacture of selenium rectifiers, which convert alternating current to direct current, accounts for over half the world's production of selenium. It is also used for decolourizing green glass and making ruby glass, for allying with stainless steel and copper, as an additive in the natural and synthetic rubber industries, as an insecticide, as 75-Se for the radioactive scanning of the pancreas and for photostat and X-ray xerography.
In certain areas (such as parts of the mid-Western USA) some plants (for example, loco-weed) accumulate selenium to such an extent that grazing animals display toxic symptoms.
Cereals such as wheat can accumulate up to 30mg/kg without impairment of growth. It is thus possible for food crops to be contaminated by selenium.