The 4 natural oxidation states of
selenium are elemental
selenium (0), selenide (-2),
selenite (+4), and
selenate (+6). Inorganic
selenate and
selenite predominate in water whereas organic
selenium compounds (
selenomethionine,
selenocysteine) are the major
selenium species in cereal and in vegetables. The principal applications of
selenium include the manufacture of ceramics, glass, photoelectric cells, pigments, rectifiers,
semiconductors, and steel as well as use in photography,
pharmaceutical production, and rubber vulcanizing. High concentrations of
selenium in surface and in ground water usually occur in farm areas where irrigation water drains from
soils with high
selenium content (Kesterson Reservoir, California) or in lakes receiving condenser cooling water from
coal-fired electric power plants (Belews Lake, North Carolina). For the general population, the primary pathway of exposure to
selenium is food, followed by water and air. Both
selenite and
selenate possess substantial bioavailability. However, plants preferentially absorb selenates and convert them to organic compounds. Aquatic organisms (e.g., bivalves) can accumulate and magnify
selenium in the food chain.
Selenium is an essential component of
glutathione peroxidase, which is an important
enzyme for processes that protect
lipids in polyunsaturated membranes from oxidative degradation. Inadequate concentrations of
selenium in the Chinese diet account, at least in part, for the illness called
Keshan disease.
Selenium deficiency occurs in the geographic areas where
Balkan nephropathy appears, but there is no direct evidence that
selenium deficiency contributes to the development of this chronic, progressive
kidney disease. Several lines of scientific inquiry suggest that an increased risk of
cancer occurs as a result of low concentrations of
selenium in the diet; however, insufficient evidence exists at the present time to recommend the use of
selenium supplements for the prevention of
cancer. The toxicity of most forms of
selenium is low and the toxicity depends on the chemical form of
selenium. The acute ingestion of
selenious acid is almost invariably fatal, preceded by stupor,
hypotension, and
respiratory depression. Chronic
selenium poisoning has been reported in China where changes in the hair and nails resulted from excessive environmental exposures to
selenium. Garlic odor on the breath is an indication of excessive
selenium exposure as a result of the expiration of
dimethyl selenide. The US National Toxicology Program lists
selenium sulfide as an animal
carcinogen, but there is no evidence that other
selenium compounds are
carcinogens.