Q. What is selenium?

Selenium is a trace element. Trace (or minor) elements are rare in nature, occurring in only 'trace' quantities. Other examples are antimony, arsenic, cadmium, chromium, molybdenum, thallium, uranium, and vanadium. While many are essential or beneficial for human health, in too high of a concentration they can become toxic. Selenium is a trace element of particular concern.

Selenium has an atomic number of 34 and atomic weight of 78. Because of its similar electrostatic balance and ionic radius, selenium behaves like sulfur and can substitute for it in many molecules. It was discovered in 1817 by Jons Berzelius: its name refers to the Greek 'Selene', moon.

In human terms, selenium is found in compounds that are essential for human nutrition and growth. Too much selenium, however, may lead to selenosis, the body's response to toxic doses.

Q. Is too much (or too little) selenium unhealthy?

Indications of selenium deficiency have not been fully described in humans, but very low levels of selenium are associated with Keshan Disease, a form of juvenile cardiomyopathy (a disease of the heart muscle), and Kaschin-Beck Disease, a form of chondrodystrophy (a type of abnormal bone growth). Both diseases have been recognized in rural China where levels of selenium in the soil are abnormally low.

Chronic selenium toxicity (too much selenium) is recognized by skin lesions, hair loss, or abnormalities of the skin and nails. Certain rural areas of China also have abnormally high levels of selenium in soils. Mostly inorganic forms of selenium are found in soils, but some plants such as Astragalus (milk vetch) convert these compounds to organic forms that can enter the food chain. Selenomethionine is the principal selenium compound found in foods.

Selenium is relatively rare in low-protein foods, but it is found in grains, fish, meat, Brazil nuts, and brewer's yeast. People who eat a balanced diet of food grown in the western United States probably get enough selenium from food.

Selenium compounds can be harmful at daily dietary levels 5-10 times higher than the daily requirement. Accidentally swallowing a large amount of selenium (for example, a very large quantity of selenium supplement pills) could be life-threatening without immediate medical treatment.

People exposed to very high levels of selenium in the atmosphere have reported dizziness, fatigue, irritation, collection of fluid in the lungs, and severe bronchitis. The exact levels at which these effects occur are not known. Upon contact with skin, selenium compounds have caused rashes, swelling, and pain.

Q. What is the effect of too much selenium on wildlife/animals?

In animals toxic levels may lead to death or birth deformities in offspring. In fish and birds, in addition to overt deformities, too much selenium can lead to reproductive failure and Cachexia -- starvation and wasting even though there is enough food. Note that because of bioaccumulation, the levels for protection of wildlife (5 ppb) are ten times lower than levels to protect humans (50 ppb).

Q. Why does selenium cause deformities?

Selenium may substitute for sulfur in the amino acids that make up proteins, and because selenium has a smaller ionic radius than sulfur, the proteins essential for transmitting information for growth and development become malformed. This is different from genetic mutation.

Q. Does selenium have therapeutic uses?

Medical research studies have claimed benefits including boosted immune function, male fertility, anti-cancer effects (prostate, colorectal, and lung only), anti-oxidant, treatment of psoriasis, and, when combined with other ingredients, Alzheimer's Disease.

Selenium sulfide is an ingredient in anti-dandruff shampoo. Selenium sulfide has not caused cancer in animals when it is placed on the skin, and the use of anti-dandruff shampoos containing selenium sulfide is considered safe. The Department of Health and Human Services (DHHS), however, has determined that selenium sulfide is reasonably anticipated to be a carcinogen. The compound has produced liver tumors in rats and mice and lung tumors in mice fed daily at very high levels. The EPA believes that other selenium compounds are not classifiable with regard to their carcinogenicity. Studies of laboratory animals and people show that most selenium compounds probably do not cause cancer.

Q. Are there other uses of selenium?

Other uses include glass manufacturing, chemicals and pigments, agriculture, metallurgy, and electronics.

Q. What are our sources of information?

The Selenium Forum

U.S. Department of Agriculture

U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry

U.S. Environmental Protection Agency

See the selenium links page for additional sources information.