It is rarely found in pure form in the environment. When rocks decompose into soils, selenium often combines with sodium and oxygen to form sodium selenate, or with water to form sodium selenite, both of which are water soluble. Plants easily take up inorganic selenium compounds from water and change them to organic selenium compounds such as selenomethionine. The major man-made source of selenium in the environment is coal burning, especially the fly ash that results from burning coal. Most of the selenium that enters the body is excreted in the urine within 24 hours.
Selenium can build up in the body if exposure is high and occurs over a long period of time.
Body burdens of selenium primarily occur in the liver, kidneys, hair, and nails. Selenium is an essential dietary element for both human and animals in either the inorganic or organic form. Selenium has antioxidant effects which help prevent damage to tissues caused by oxygen. A selenium deficient diet can result in Keshan disease, the signs and symptoms of which may include muscle pain, cardiomyopathy, enlargement of the heart, increased red blood cell fragility, and pancreatic degeneration. No human populations in the U.S. have been reported to have chronic selenium toxicity, including populations in the western part of the country with naturally high levels of selenium in the soil and water.