Selenium is a rare earth element that is an essential nutrient for humans and other animals. Selenium supplements are available as a remedy for a variety of ailments. In large enough quantities, though, the substance can be toxic, so caution in taking supplements is advisable. There are many industrial applications for selenium compounds, including uses in electronics and metallurgy.
Properties
Selenium has the chemical symbol Se and is a metalloid, meaning it has properties of both metals and nonmetals. It occurs naturally in compounds with heavy metals like mercury and lead to form some common minerals. Selenium occurs fairly rarely in the earth's crust, with an overall concentration of only about one part in 10 million. In sea water, the concentration is about 100 times less than that. Most available elemental selenium is a byproduct of the processing of mined copper.
Dietary Requirement
Selenium plays a number of important roles in human health, from the formation of red blood cells to adding elasticity to the skin. Compounds of selenium occur naturally in fish and grains. Though selenium deficiencies are rare, studies have linked a deficiency to several diseases, including cancer, heart disease and arthritis. The recommended intake of selenium is only tens of micrograms per day, depending on a person's age. Due to the belief that selenium might help the body fight cancer, some people in the United States take selenium dietary supplements.
Toxicity
You must take care in using selenium supplements because ingestion of levels even moderately higher than normal can be toxic. Your body needs only about 50 mcg per day; ingesting 10 times that amount could lead to severe adverse effects. A single overdose may cause kidney failure, and chronic excessive intake can result in hair loss, fatigue and vomiting. Because many grassy plants contain large amounts of selenium, excessive intake is a threat to grazing animals like cattle and sheep. Livestock can die as a result of ingesting a few milligrams a day from plants high in the element.
Industrial Uses
Selenium also has some unique electrical properties. Its resistance changes with the amount of visible light striking it, which makes it useful in photocells. Light also causes some selenium compounds to produce electric current, so it is a major component in photoelectric solar cells. The metal is sometimes a component of stainless steel and printer toner and is used as a dye in glass and enamel coatings.
References
- Jefferson Lab: It's Elemental: The Element Selenium
- Encyclopedia Britannica: Selenium
- Mineral Information Institute: The Role of Elements in Life Processes
- National Institutes of Health: Office of Dietary Supplements: Selenium
- American Cancer Society: Complementary and Alternative Medicine: Selenium
- Cornell University Department of Animal Science: Plants Poisonous to Livestock
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