selenium (Se) /silē″nē·əm/ [Gk, selene, moon] , a metalloid element of the sulfur group. Its atomic number is 34, and its atomic mass is 78.96. Selenium occurs mainly in iron, copper, lead, and nickel ores in the form of metallic selenides. One of the chief commercial sources is the flue dust produced by the burning of pyrites to make sulfuric acid. Selenium occurs as a trace element in foods, and research continues to determine the most effective daily allowances for different age groups. Dietary experts say that the estimated safe, adequate intake of selenium for infants 6 months of age is 0.04 mg, for adults 0.05 to 0.2 mg. Although selenium deficiency can result in liver problems and degeneration of muscles in some animals, in humans its need has not yet been clearly defined. The bright orange insoluble powder selenium sulfide is used externally in the control of seborrheic dermatitis, dandruff, and other forms of dermatosis. Selenium sulfide, used as a lotion, is used in some therapeutic shampoos as 2.5% of selenium sulfide in a detergent vehicle; it is sold without prescription as a 1% detergent suspension in a scented detergent vehicle. Adverse effects may include conjunctivitis if the preparation enters the eyes, increased oiliness or dryness of the hair, and orange tinting of gray hair. The antidandruff effectiveness of selenium sulfide is thought to stem from its antimitotic activity and its residual adherence to the hair after shampooing. Normal skin absorbs very little of the drug, but inflamed or damaged skin absorbs it readily. Selenium is used in the nuclear medicine compound selenomethionine for diagnosing parathyroid tumors. The element is also used as a photoconductive layer of xeroradiographic plates. Burns and dermatitis venenata may result from prolonged skin contact.
