selenium

A nonmetallic element, red in powder form, black in vitreous form, and metallic gray in crystalline form, resembling sulfur and obtained primarily as a byproduct of electrolytic copper refining. It is widely used in rectifiers, as a semiconductor, and in xerography. Its photovoltaic and photoconductive actions make it useful in photocells, photographic exposure meters, and solar cells. Atomic number 34; atomic weight 78.96; melting point (of gray selenium) 217°C; boiling point (gray) 684.9°C; specific gravity (gray) 4.79; (vitreous) 4.28; valence 2, 4, or 6.

[Greek selēnē, moon (from selas, light, brightness) + –IUM.]

A chemical element, Se, atomic number 34, atomic weight 78.96. The properties of this element are similar to those of tellurium. See also Periodic table.

Selenium burns in air with a blue flame to give selenium dioxide, SeO₂. The element also reacts directly with a variety of metals and nonmetals, including hydrogen and the halogens. Nonoxidizing acids fail to react with selenium, but nitric acid, concentrated sulfuric acid, and strong alkali hydroxides dissolve the element. The only important compound of selenium with hydrogen is hydrogen selenide, H₂Se, a colorless flammable gas possessing a distinctly unpleasant odor, and a toxicity greater and a thermal stability less than that of hydrogen sulfide. Selenium oxyhalide, SeOCl₂, is a colorless liquid widely used as a nonaqueous solvent. The oxybromide, SeOBr₂, is an orange solid having chemical properties similar to those of SeOCl₂. The oxyflouride, SeOF₂, a colorless liquid with a pungent smell, reacts with water, glass, and silicon, and also forms additional compounds. Compounds in which C-Se bonds appear are numerous and vary from the simple selenols, RSeH, to molecules exhibiting biological activity such as selenoamino acids and selenopeptides. See also Organoselenium compound.

The abundance of this widely distributed element in the Earth's crust is estimated to be about 7 × 10⁻⁵ % by weight, occurring as the selenides of heavy elements and to a limited extent as the free element in association with elementary sulfur. Examples of the variety of selenide minerals are berzelianite (Cu₂Se), eucairite (AgCuSe), and jermoite [As(S,Se)₂]. Selenium minerals do not occur in sufficient quantity to be useful as commercial sources of the element.

Major uses of selenium include the photocopying process of xerography, which depends on the light sensitivity of thin films of amorphous selenium, the decolorization of glasses tinted by the presence of iron compounds, and use as a pigment in plastics, paints, enamels, glass, ceramics, and inks. Selenium is also employed in photographic exposure meters and as a metallurgical additive to improve the machinability of certain steels. Minor uses include application as a nutritional additive for numerous animal species, use in photographic toning, metal-finishing operations, metal plating, high-temperature lubricants, and as catalytic agents, particularly in the isomerization of certain petroleum products.

The biological importance of selenium is well established, as all classes of organisms metabolize selenium. In humans and other mammals, serious diseases arise from either excessive or insufficient dietary selenium. The toxic effects of selenium have long been known, particularly for grazing animals. In soils with high selenium content, some plants accumulate large amounts of selenium. Animals that ingest these selenium-accumulating plants develop severe toxic reactions. See also Soil chemistry.

Although toxic at high levels, selenium is an essential micronutrient for mammalian species. The accepted minimum daily requirement of selenium for adult humans is 70 micrograms. Many types of food provide selenium, particularly seafood, meats, grains, and the onion family. Mammals and birds require selenium for production of the enzyme glutathione peroxidase, which protects against oxidation-induced cancers. Other seleno-proteins of unknown function are found in mammalian blood, various tissues, and spermatozoa. See also Amino acids.

A dietary essential mineral, which is part of enzymes glutathione peroxidase and thyroxine deiodinase. Through its role in glutathione peroxidase it acts as an antioxidant, and to some extent can compensate for vitamin E deficiency. Similarly, vitamin E can compensate for selenium deficiency to some extent.

Requirements are of the order of 50 μg/day; in parts of New Zealand, Finland, and China soils are especially poor in selenium and deficiency occurs. In China selenium deficiency is associated with Keshan disease. Rich sources include: fish and shellfish, mung (dahl) and red kidney beans, Brazil nuts, bread, kidney, lentils, liver, pork, rabbit, veal.

Selenium is toxic in excess; mild selenium intoxication results in production of foul-smelling hydrogen selenide, which is excreted on the breath and through the skin. Intakes above 450 μg/day are considered hazardous.

A trace element found in meat, seafood, and cereals. Selenium acts as an antioxidant and is a constituent of some enzymes. Its functions are closely related to those of vitamin E. Selenium and vitamin E often work with each other. In the UK, the adult Reference Nutrient Intake (RNI) is 60 micrograms per day for women and 75 micrograms for men. Supplements are not recommended because high levels of selenium can be toxic. The upper limit of safe intake is 6 micrograms of selenium per kilogram body weight per day (about 400-450 micrograms for most adults).

A trace element used in the treatment of seborrhea and dandruff of the scalp. Selenium is toxic in large amounts.

Brand names: Selepen®

Chemical formula:

Drug Forms:
• Selenium Oral tablet (below)
• Selenium Yeast Oral tablet

Español:
• Selenio, Tableta oral
• Levadura de selenio, Tableta oral

Selenium Oral tablet

What is this medicine?

SELENIUM is a mineral found in nature. It is added to a healthy diet to prevent or to treat low selenium levels.

This medicine may be used for other purposes; ask your health care provider or pharmacist if you have questions.

What should I tell my health care provider before I take this medicine?

They need to know if you have any of the following conditions:
•intestinal or stomach disease
•kidney disease
•an unusual or allergic reaction to selenium, other medicines, foods, dyes, or preservatives
•pregnant or trying to get pregnant
•breast-feeding

How should I use this medicine?

Take this medicine by mouth with a glass of water. Follow the directions on the package or prescription label. For best results take with food. Take your medicine at regular intervals. Do not take your medicine more often than directed.

Talk to your pediatrician regarding the use of this medicine in children. While this drug may be prescribed for selected conditions, precautions do apply.

Overdosage: If you think you have taken too much of this medicine contact a poison control center or emergency room at once.
NOTE: This medicine is only for you. Do not share this medicine with others.

What may interact with this medicine?

Interactions are not expected.

This list may not describe all possible interactions. Give your health care provider a list of all the medicines, herbs, non-prescription drugs, or dietary supplements you use. Also tell them if you smoke, drink alcohol, or use illegal drugs. Some items may interact with your medicine.

What should I watch for while using this medicine?

Follow a healthy diet. Taking a supplement does not replace the need for a balanced diet. Some foods that have selenium naturally are meat, poultry, grains, and seafood.

Too much of this supplement can be unsafe. Talk to your doctor or health care provider about how much is right for you.

What side effects may I notice from receiving this medicine?

Side effects that you should report to your doctor or health care professional as soon as possible:
•allergic reactions like skin rash, itching or hives, swelling of the face, lips, or tongue
•bad breath and body odor
•diarrhea with nausea, vomiting
•hair loss
•metallic taste
•unusually tired

This list may not describe all possible side effects. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Where should I keep my medicine?

Keep out of the reach of children.

Store at room temperature between 15 and 30 degrees C (59 and 86 degrees F). Keep container tightly closed. Throw away any unused medicine after the expiration date.

Last updated: 7/1/2002

Important Disclaimer: The drug information provided here is for educational purposes only. It is intended to supplement, not substitute for, the diagnosis, treatment and advice of a medical professional. This drug information does not cover all possible uses, precautions, side effects and interactions. It should not be construed to indicate that this or any drug is safe for you. Consult your medical professional for guidance before using any prescription or over the counter drugs.

Description

Selenium is a nonmetallic element with an atomic number of 34 and an atomic weight of 78.96. Its chemical symbol is Se. Selenium is most commonly found in nature in its inorganic form, sodium selenite. An organic form of selenium, selenomethionine, is found in foods.

General Use

The role of selenium in human nutrition and other therapeutic applications has provoked intense controversy over the past two decades. In contrast to such major minerals as magnesium and calcium, neither selenium's benefits nor its toxic aspects are yet fully understood. Until very recently, selenium was considered a toxic element that was not necessary to human health. In 1989, selenium was reclassified as an essential micronutrient in a balanced human diet when the National Research Council established the first recommended daily allowance (RDA) for it. It is considered a minor mineral, or a trace element, as distinct from a major mineral such as calcium or phosphorus, or an electrolyte such as sodium or chloride. There is less than 1 mg of selenium in the average human body. The selenium is concentrated in the liver, kidneys, and pancreas. In males, selenium is also found in the testes and seminal vesicles. Selenium currently has a variety of applications, ranging from standard external preparations for skin problems to experimental and theoretical applications in nutrition and internal medicine.

Alternative Medicine

Naturopaths use selenium supplements to treat asthma, acne, tendinitis, infertility problems in men, and postmenopausal disorders in women. Selenium is also considered an important component in naturopathic life extension (longevity) diets, because of its role in tissue repair and maintaining the youthful elasticity of skin.

Dermatology

Selenium has been used since the 1960s in dandruff shampoos and topical medications for such skin disorders as folliculitis ("hot tub" syndrome) and tinea versicolor, a mild infection of the skin caused by the yeast-like fungus Pityrosporum orbiculare. When selenium is compounded with sulfur to form a sulfide, it has antibiotic and antifungal properties. Selenium sulfide is absorbed by the outermost layer of skin cells, the epithelium. Inside the cells, the compound splits into selenium and sulfide ions. The selenium ions counteract the enzymes that are responsible for producing new epithelial cells, thus lowering the turnover of surface skin cells. As a result, itching and flaking of the skin associated with dandruff and tinea versicolor is reduced.

Nutrition

Prior to 1989, there were no established RDA values for selenium. In 1989, the National Research Council of the National Academy of Sciences defined the RDAs for selenium as follows: Males aged 15–18 years, 50 g; 19–24 years, 70 g; 25–50 years, 70 g; 51 years and older, 70 g. Females: aged 15–18 years, 50 g; 19–24 years, 55 g; 25–50 years, 55 g; 51 years and older, 55 g; pregnant, 65 g; lactating, 75 g. The generally higher levels for males are related to the importance of selenium in producing vigorous sperm.

The amount of selenium in the diet is influenced by its level in the soil. Most selenium is absorbed from food products, whether plants grown in the soil or animals that have eaten the plants. Much of the selenium in foods is lost during processing. About 60% of dietary selenium is absorbed as food passes through the intestines. Selenium leaves the body in the urine and feces; males also lose some selenium through ejaculation of sperm. Selenium levels in soil vary widely, not only in different countries but also across different regions. For example, in the United States the western states have higher levels of selenium in the soil than the eastern states. South Dakota has the highest rates of soil selenium in the United States, while Ohio has the lowest.

Foods that are high in selenium contain the element in an organic form, selenomethionine. This form of selenium is considerably less toxic than inorganic sodium selenite or elemental selenium. Good sources of selenium include brewer's yeast, wheat germ, wheat bran, kelp (seaweed), shellfish, Brazil nuts, barley, and oats. Onions, garlic, mushrooms, broccoli, and Swiss chard may contain high amounts of selenium if they are grown in selenium-rich soil. Selenium is also present in drinking water in some parts of the world and can be added to drinking water as a health measure. Nursing mothers should note that human milk is much richer in selenium than cow's milk.

There is no widely recognized deficiency syndrome for selenium, unlike the syndromes associated with calcium or magnesium (hypocalcemia and hypomagnesemia, respectively). However, many researchers who have investigated Keshan disease, a form of heart disease in children, believe that it is caused by selenium deficiency. The disease can be prevented but not cured with supplemental selenium; it responds to treatment with 50 g per day. The symptoms of Keshan disease, which is named for the region of China where it was discovered, include enlargement of the heart and congestive heart failure. The soil in the Keshan region is low in selenium. The researchers observed that the local Chinese treat Keshan disease with astragalus (Astragalus membranaceus), a plant that absorbs selenium from the soil.

Selenium toxicity is still a matter of controversy. It is a known fact that humans can tolerate higher levels of selenium in its organic form (selenomethionine) than in its inorganic forms. Humans can show symptoms of selenium toxicity after doses as low as 1 mg of sodium selenite. On the other hand, some researchers speculate that the organic forms of selenium may accumulate in the body and interfere with the functioning of sulfur molecules in the body, or that they may cause genetic mutations. These long-term questions await further research. In addition, researchers disagree about how much selenium will produce symptoms of toxicity. It has been suggested that toxicity can result from a daily intake of 2 mg in people who already have body stores of 2.5 mg of selenium or higher. Another measurement suggests that selenium toxicity may occur wherever the food or water regularly contains more than 5 or 10 parts per million of selenium. Patients with symptoms of selenium toxicity usually have blood plasma levels of 100 g/dl or higher, which is about four times the upper limit of normal levels.

The symptoms of selenium toxicity are not always clearly defined. People living in areas of selenium-rich soil sometimes develop heart, eye, or muscular problems. Eating foods containing high amounts of selenium over a long period of time increases the risk of tooth decay. It is thought that the selenium may compete with the fluoride in teeth, thus weakening their structure. Other symptoms associated with high levels of selenium include a metallic taste in the mouth, garlic-like breath odor, dizziness, nausea, skin inflammation, fatigue, and the loss of hair or nails. The symptoms of acute selenium poisoning include fever, kidney and liver damage, and eventual death.

Internal Medicine

Selenium is most widely recognized as a substance that speeds up the metabolism of fatty acids and works together with vitamin E (tocopherol) as an antioxidant. Antioxidants are organic substances that are able to counteract the damage done to human tissue by oxidation (the breakdown of fatty acids). Selenium's antioxidant properties have been studied with respect to several diseases and disorders. In addition to its antioxidant properties, selenium also appears to work as an anti-inflammatory agent in certain disorders.

CARDIOVASCULAR DISEASES. Low levels of selenium have been associated with high risk of heart attacks and strokes. It is thought that the antioxidant properties of selenium can help prevent atherosclerosis (narrowing and hardening of the arteries) by decreasing the formation of fatty deposits in the arteries. It does so by soothing the inflamed arterial walls and binding the free radicals that damage the tissues lining the arteries. Other studies indicate that selenium reduces the symptoms of angina pectoris.

CATARACTS. Cataracts in the eye contain only one-sixth as much selenium as normal lens tissue. The healthy lens requires adequate levels of three antioxidant enzymes: superoxide dismutase, catalase, and glutathione peroxidase. Glutathione peroxidase in the human eye is dependent on selenium, which suggests that a selenium deficiency speeds up the progression of cataracts.

CANCER. Low dietary levels of selenium have been associated with an increased incidence of cancer. Cancers of the respiratory system and the gastrointestinal tract seem to be especially sensitive to the level of selenium in the body. In a recent study, patients with histories of skin cancer were given 200 g of selenium per day. Results indicated that the patients had a reduced incidence of rectal, prostate, and lung cancers as well as a lower rate of mortality from all cancers. In addition, cervical dysplasias (abnormal growths of tissue) in women are associated with low levels of selenium in the patient's diet. In animal studies, as little as 1–4 parts per million of selenium added to the water or food supply is associated with a decreased incidence of cancer. It is not yet known, however, exactly how selenium protects against cancer. Some researchers believe that it may prevent mutations or decrease the rate of cell division, particularly on the outer surfaces of the body. A recent study of the effects of a selenium compound on mammary tissue indicates that selenium may inhibit the growth of tumors in deeper layers of tissue, not just cancers arising from the epithelium.

As of 2002, selenium is being studied as a possible chemopreventive for prostate cancer. The researchers hope to learn more about the mechanisms by which selenium slows the progress of an established cancer as well as discover a preventive strategy that makes use of selenium.

PERIODONTAL DISEASE. Selenium appears to speed up the healing of fragile gum tissue as well as opposing the actions of free radicals, which are extremely damaging to gum tissue.

RHEUMATOID ARTHRITIS. Selenium may be useful for treating several autoimmune diseases, especially lupus and rheumatoid arthritis (RA). It has been discovered that patients suffering from RA have low selenium levels. Selenium is necessary for production of the enzyme glutathione peroxidase, which reduces the production of inflammatory substances in the body (prostaglandins and leukotrienes) as well as opposing free radicals. Although supplemental selenium by itself has not been shown to cause improvement in RA, selenium taken together with vitamin E appears to have measurable positive results.

OSTEOARTHRITIS. Recent research in Germany indicates that selenium is beneficial in the prevention and treatment of osteoarthritis (OA), particularly OA resulting from physical wear and tear or structural problems in the patient's joints. Selenium supplements are even more effective when given together with vitamins in treating OA.

Preparations

Selenium is available in topical preparations and as a dietary supplement.

External Preparations

Selenium sulfide for the treatment of dandruff is available as over-the-counter (OTC) scalp preparations or shampoo containing 1% or 2.5% solutions of the drug. A topical 2.5% solution of selenium sulfide is available for the treatment of tinea versicolor. Common trade names include Exsel™, Selsun™, and Selsun Blue™.

Dietary Supplements

Selenium is widely available in vitamin/mineral dietary supplements and in nutritional antioxidant formulas. Although the average diet supplies enough selenium, some naturopaths recommend daily supplements of 100–200 g for adults and 30–150 g for children. Sexually active males are advised to take higher doses. Some naturopaths recommend taking selenium together with vitamin E on the grounds that their combined effect is greater than the sum of their individual effects. There are at present no definitive studies on the positive effects on health of selenium taken as a dietary supplement.

Precautions

Topical Preparations

Persons using selenium compounds to control dandruff or tinea versicolor should be careful to avoid applying the product to damaged or broken skin. In addition to irritating skin, selenium can enter the body through broken skin. This process is known as percutaneous absorption and can cause selenium toxicity if the preparation is used for a long period of time. Patients should wash their hands carefully after applying the selenium product to affected areas. Doing so will minimize absorption through small breaks in the skin of the hands.

Nutritional Supplements

It is difficult to assess the effectiveness of dietary supplements containing selenium because there is little agreement on standards for interpreting selenium levels in human blood. Depending on their intake, healthy adults may have blood plasma levels of selenium in the range of 8–25 g/dl. In addition, most of the selenium in the body is not carried in the blood but is stored in tissue. Analysis of hair has not been useful in measuring selenium. In the absence of a useful test, people who wish to take supplemental selenium should first find out whether they live in an area that already has high levels of selenium in the drinking water and soil. Most people will probably not need more selenium than is in standard vitamin/mineral supplements. In addition, the body seems to utilize selenium more efficiently when it is taken together with vitamin E.

Side Effects

The side effects of contact with compounds containing selenium sulfide include stinging of the skin; irritation of the lining of the eyes; hair discoloration or loss; and oily scalp. Both topical products and megadoses of selenium taken by mouth can cause selenium toxicity. The symptoms of selenium toxicity include nausea, vomiting, tiredness, abdominal pain, a garlicky breath odor, and the loss of hair and fingernails. These symptoms usually last 10–12 days after the selenium preparation is discontinued.

Interactions

Topical preparations containing selenium may interact with the metals in costume jewelry. Patients should remove all their jewelry before applying the shampoo or lotion.

With regard to dietary supplements, there is some evidence that vitamin C inactivates selenium within the digestive tract. Persons who are concerned about their selenium intake may prefer to take supplemental selenium in the absence of vitamin C.

Resources

Books

Baron, Robert B., M.D. "Nutrition." In Current Medical Diagnosis and Treatment, edited by Lawrence M. Tierney, M.D., et al. 39th ed. New York: McGraw-Hill, 2000.

Beers, Mark H., M.D., and Robert Berkow, M.D., eds. The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 2002.

Berger, Timothy G., M.D. "Skin, Hair, and Nails." In Current Medical Diagnosis and Treatment, edited by Lawrence M. Tierney, M.D., et al. 39th ed. New York: McGraw-Hill, 2000.

Burton Goldberg Group, comp. Alternative Medicine: The Definitive Guide. Fife, WA: Future Medicine Publishing, 1995.

Murray, Michael, N.D., and Joseph Pizzorno, N.D. Encyclopedia of Natural Medicine. Rocklin, CA: Prima Publishing, 1991.

Periodicals

Dong, Y., C. Ip, and H. Ganther. "Evidence of a Field Effect Associated with Mammary Cancer Chemoprevention by Methylseleninic Acid." Anticancer Research 22 (1A)(January-February 2002): 27-32.

Kurz, B., B. Jost, and M. Schunke. "Dietary Vitamins and Selenium Diminish the Development of Mechanically Induced Osteoarthritis and Increase the Expression of Antioxidative Enzymes in the Knee Joint of STR/1N Mice." Osteoarthritis and Cartilage 10 (February 2002): 119-126.

Nelson, M. A., M. Reid, A. J. Duffield-Lillico, and J. R. Marshall. "Prostate Cancer and Selenium." Urology Clinics of North America 29 (February 2002): 67-70.

Tan, J., W. Zhu, W. Wang, et al. "Selenium in Soil and Endemic Diseases in China." The Science of the Total Environment 4 (February 2002): 227-235.

Ujiie, S., and H. Kikuchi. "The Relation Between Serum Selenium Value and Cancer in Miyagi, Japan: 5-Year Follow-Up Study." Tohoku Journal of Experimental Medicine 196 (March 2002): 99-109.

[Article by: Rebecca J. Frey, PhD]

For more information on selenium, visit Britannica.com.

A trace element (see mineral) found in meat, seafood, and cereals. Selenium acts, often in association with vitamin E, as an antioxidant. It is also a component of many enzymes. The Reference Nutrient Intake for adults is 75 μg day⁻¹. Supplements are not recommended because high levels of selenium are toxic.

A chemical element, atomic number 34, atomic weight 78.96, symbol Se. It is an essential mineral nutrient and a very potent poison.

• s.-75 — a radioisotope of selenium having a half-life of 120 days and a principal gamma ray photon energy of 265 keV; used in the radiopharmaceutical selenomethionine. Symbol ⁷⁵Se.
• s. accumulator plants — see accumulator plants.
• s. converter plants — see converter plants.
• s. indicator plants — see indicator plants.
• s.-induced infection resistance — selenium deficiency in the diet may increase the patient's susceptibility to infection.
• s. nutritional deficiency — there are well-defined deficiency and poorly defined selenium responsive diseases. A nutritional deficiency of vitamin E has many of the same effects of selenium deficiency and it is often difficult to separate the two. See also enzootic muscular dystrophy, mulberry heart disease, hepatosis dietetica, iron poisoning, weaner illthrift.
• s. poisoning — can occur on pasture growing on selenium rich soil, especially if there are selenium indicator plants growing in it. Also when selenium supplementation is carried out carelessly and there is accidental access to poisonous amounts. Acute poisoning features diarrhea, dyspnea and death. Subacute poisoning is hallmarked by blind staggers (see dummy). Chronic poisoning produces a syndrome of stiff gait, loss of haircoat and separation, perhaps sloughing, of the hooves. Called also alkali disease.

  Coronary separation caused by selenium poisoning. By permission from Knottenbelt DC, Pascoe RR, Diseases and Disorders of the Horse, Saunders, 2003

• s.-responsive diseases — diseases such as reproductive inefficiency and illthrift in sheep and cattle respond in some situations to dietary supplementation with selenium but the diseases are not proven to be caused by nutritional deficiency of the element.
• s. sulfide, s. disulfide — a commonly used keratolytic in medicated shampoos employed in the treatment of skin disease in dogs and cats.
• s. weed — see neptunia amplexicaulis.

For the software testing framework, see Selenium (software).

34 arsenic ← selenium → bromine S ↑ Se ↓ Te Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	selenium, Se, 34
Element category	nonmetals
Group, Period, Block	16, 4, p
Appearance	gray-black, metallic luster
Standard atomic weight	78.96(3)  g·mol−1
Electron configuration	[Ar] 4s2 3d10 4p4
Electrons per shell	2, 8, 18, 6
Physical properties
Phase	solid
Density (near r.t.)	(gray) 4.81  g·cm−3
Density (near r.t.)	(alpha) 4.39  g·cm−3
Density (near r.t.)	(vitreous) 4.28  g·cm−3
Liquid density at m.p.	3.99  g·cm−3
Melting point	494 K (221 °C, 430 °F)
Boiling point	958 K (685 °C, 1265 °F)
Critical point	1766 K, 27.2 MPa
Heat of fusion	(gray) 6.69  kJ·mol−1
Heat of vaporization	95.48  kJ·mol−1
Specific heat capacity	(25 °C) 25.363  J·mol−1·K−1
Vapor pressure P(Pa) 1 10 100 1 k 10 k 100 k at T(K) 500 552 617 704 813 958
Atomic properties
Crystal structure	hexagonal
Oxidation states	6, 4, 2, 1,[1] -2 (strongly acidic oxide)
Electronegativity	2.55 (Pauling scale)
Ionization energies (more)	1st:  941.0  kJ·mol−1
	2nd:  2045  kJ·mol−1
	3rd:  2973.7  kJ·mol−1
Atomic radius	120  pm
Covalent radius	120±4  pm
Van der Waals radius	190 pm
Miscellaneous
Magnetic ordering	diamagnetic[2]
Thermal conductivity	(300 K) (amorphous) 0.519  W·m−1·K−1
Thermal expansion	(25 °C) (amorphous) 37  µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 3350 m/s
Young's modulus	10  GPa
Shear modulus	3.7  GPa
Bulk modulus	8.3  GPa
Poisson ratio	0.33
Mohs hardness	2.0
Brinell hardness	736  MPa
CAS registry number	7782-49-2
Most-stable isotopes
Main article: Isotopes of selenium iso NA half-life DM DE (MeV) DP 72Se syn 8.4 d ε - 72As γ 0.046 - 74Se 0.87% 74Se is stable with 40 neutrons 75Se syn 119.779 d ε - 75As γ 0.264, 0.136, 0.279 - 76Se 9.36% 76Se is stable with 42 neutrons 77Se 7.63% 77Se is stable with 43 neutrons 78Se 23.78% 78Se is stable with 44 neutrons 79Se syn 2.95×105 y β− 0.151 79Br 80Se 49.61% 80Se is stable with 46 neutrons 82Se 8.73% 1.08×1020 y β−β− 2.995 82Kr
References

Selenium (pronounced /səˈliːniəm/) is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature.

Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semi-metal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see allotropes section below). Selenium also exists in many non-conductive forms: a black glass-like allotrope, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter chemical cousin sulfur.

Selenium is found in economic quantities in sulfide ores such as pyrite, partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but all are rare. The chief commercial present uses for selenium are in glassmaking and in chemicals and pigments. Electronic uses for selenium, once important, have been supplanted by silicon semiconductor devices.

Selenium salts are toxic in large amounts, but trace amounts of the element are necessary for cellular function in most, if not all, animals, forming the active center of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known deiodinase enzymes (which convert one thyroid hormone to another). Selenium requirements in plants differ by species, with some plants apparently requiring none.^[3]

Contents 1 History and global demand 2 Occurrence 3 Production and allotropic forms 4 Isotopes 5 Health effects and nutrition 5.1 Selenium indicator plants 5.2 Toxicity 5.3 Deficiency 5.4 Controversial health effects 6 Non-biologic applications 7 Biologic applications 8 Evolution in biology 9 Compounds 10 References 11 External links

History and global demand

Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jakob Berzelius who found the element associated with tellurium (named for the Earth). It was discovered as a byproduct of sulfuric acid production.

It came to medical notice later because its toxicity to humans working in industry. It was also recognized as an important veterinary toxin. In 1954 first hints towards specific biological functions of selenium were discovered in microorganisms. Its essentiality for mammalian life was discovered in 1957. In the 1970s it was shown to be present in two independent sets of enzymes. This was followed by the discovery of selenocysteine in proteins. During the 1980s it was shown that selenocystine was encoded by the codon TGA. The recoding mechanism was worked out first in bacteria and then in mammals.

Growth in selenium consumption was historically driven by steady development of new uses, including applications in rubber compounding, steel alloying, and selenium rectifiers. Selenium is also an essential material in the drums of laser printers and copiers. By 1970, selenium in rectifiers had largely been replaced by silicon, but its use as a photoconductor in plain-paper copiers had become its leading application. During the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers using organic photoconductors were produced. Currently, the largest use of selenium worldwide is in glass manufacturing, followed by uses in chemicals and pigments. Electronics use, despite a number of continued applications, continues to decline.^[4]

In 1996, continuing research showed a positive correlation between selenium supplementation and cancer prevention in humans, but widespread direct application of this important finding would not add significantly to demand owing to the small doses required. In the late 1990s, the use of selenium (usually with bismuth) as an additive to plumbing brasses to meet no-lead environmental standards became important. At present, total world selenium production continues to increase modestly.

Occurrence

Native selenium

Selenium occurs naturally in a number of inorganic forms, including selenide, selenate and selenite. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff.

Selenium has a biological role, and it is found in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine. In these compounds selenium plays a role analogous to that of sulfur.

Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, silver, or lead. It is obtained as a byproduct of the processing of these ores, from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. These muds can be processed by a number of means to obtain free selenium.

Natural sources of selenium include certain selenium-rich soils, and selenium that has been bioconcentrated by certain plants. Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores.^[5]

See also Selenide minerals.

Production and allotropic forms

From left to right: Black, gray and red selenium

Structure of trigonal selenium

Native selenium is a rare mineral, which does not usually form good crystals, but when it does they are steep rhombohedrons or tiny acicular (hair-like) crystals. ^[6] Most elemental selenium comes as a byproduct of copper refining, or the production of sulfuric acid.^[7][8] Isolation of selenium is often complicated by the presence of other compounds and elements. Commonly, production begins by oxidation with sodium carbonate to produce selenium dioxide. The selenium dioxide is then mixed with water producing selenous acid. The selenous acid is finally bubbled with sulfur dioxide producing elemental red amorphous selenium.

Selenium produced in chemical reactions invariably appears as the amorphous red form—an insoluble, brick-red powder. When this form is rapidly melted, it forms the black, vitreous form which is usually sold industrially as beads. The most thermodynamically stable and dense form of selenium is the electrically conductive gray (trigonal) form, which is composed of long helical chains of selenium atoms (see figure).^[9] The conductivity of this form is notably light sensitive. Selenium also exists in three different deep-red crystalline monoclinic forms, which are composed of Se₈ molecules, similar to many allotropes of sulfur.^[10]

Isotopes

Main article: isotopes of selenium

Selenium has six naturally occurring isotopes, five of which are stable: ⁷⁴Se, ⁷⁶Se, ⁷⁷Se, ⁷⁸Se, and ⁸⁰Se. The last three also occur as fission products, along with ⁷⁹Se which has a half-life of 295,000 years. The final naturally occurring isotope, ⁸²Se, has a very long half-life (~10²⁰ yr, decaying via double beta decay to ⁸²Kr) and which for practical purposes can be considered to be stable. 23 other unstable isotopes have been characterized.

See also Selenium-79 for more information on recent changes in the half-life of this fission product, important for the dose calculations performed in the frame of the geological disposal of long-lived radioactive waste.

Health effects and nutrition

Although it is toxic in large doses, selenium is an essential micronutrient for animals. In plants, it occurs as a bystander mineral, sometimes in toxic proportions in forage (some plants may accumulate selenium as a defense against being eaten by animals, but other plants such as locoweed require selenium, and their growth indicates the presence of selenium in soil).^[3] It is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient which functions as cofactor for reduction of antioxidant enzymes such as glutathione peroxidases and certain forms of thioredoxin reductase found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium).

Glutathione peroxidase (GSH-Px) catalyzes certain reactions which remove reactive oxygen species such as peroxide:

2 GSH+ H₂O₂---------GSH-Px → GSSG + 2 H₂O

Selenium also plays a role in the functioning of the thyroid gland by participating as a cofactor for the three known thyroid hormone deiodinases.^[11]

Dietary selenium comes from nuts, cereals, meat, fish, and eggs. Brazil nuts are the richest ordinary dietary source (though this is soil-dependent, since the Brazil nut does not require high levels of the element for its own needs). High levels are found in kidney, tuna, crab and lobster, in that order.^[12][13]

Selenium indicator plants

Certain species of plants are considered indicators of high selenium content of the soil, since they require high levels of selenium in order to thrive. The main selenium indicator plants are Astragalus species (including some locoweeds), prince's plume (Stanleya sp.), woody asters (Xylorhiza sp.), and false goldenweed (Oonopsis sp.)^[14]

Toxicity

Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis.^[15] This 400 microgram Tolerable Upper Intake Level is primarily based on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow up study on the same five people in 1992.^[16] The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to not only avoid toxicity, but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries.^[17] The Chinese people who suffered from selenium toxicity ingested selenium by eating corn grown in extremely selenium-rich stony coal (carbonaceous shale). This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature.^[18]

Symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema and death.^[19] Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability. By contrast, selenates and selenites are very toxic, having an oxidant mode of action similar to that of arsenic trioxide. The chronic toxic dose of selenite for human beings is about 2400 to 3000 micrograms of selenium per day for a long time.^[20] Hydrogen selenide is an extremely toxic, corrosive gas.^[21] Selenium also occurs in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses. Nano-size selenium has equal efficacy, but much lower toxicity.^{[22][23][24][25][26][27][28]}

On April 19, 2009, twenty-one polo ponies began to die shortly before a match in the United States Polo Open. Three days later, a private pharmacy released a statement that the horses had received an incorrect dose of one of the ingredients used in a vitamin compound with which the horses had been injected. Such vitamin injections are common to promote recovery after a match. The pharmacy did not initially release the name of the specific ingredient due to ongoing law-enforcement and other investigations. Dr David Barber, an associate professor in the Center for Environmental and Human Toxicology at the University of Florida began conducting analysis of inorganic compounds of the vitamin supplement and discovered that selenium concentrations were ten to fifteen times higher than normal in the horses' blood samples and 15 to 20 times higher than normal in their liver samples. A pharmacy spokesperson later confirmed that selenium was the ingredient in question.^[29]

Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry undeveloped lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates. High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.^[30]

Deficiency

Main article: selenium deficiency

Selenium deficiency is relatively rare in healthy, well-nourished individuals. It can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition, and also^[31] on advanced-aged people (over 90). Alternatively, people dependent on food grown from selenium-deficient soil are also at risk. Interestingly, although New Zealand has low levels of selenium in its soil, adverse health effects have not been detected.^[32]

Controversial health effects

Cancer

Several studies have suggested a possible link between cancer and selenium deficiency,^{[33][34][35][36]} One study, known as the NPC, was conducted to test the effect of selenium supplementation on the recurrence of skin cancers on selenium-deficient men. It did not demonstrate a reduced rate of recurrence of skin cancers, but did show a reduced occurrence of total cancers, although without a statistically significant change in overall mortality.^[37] The preventative effect observed in the NPC was greatest in those with the lowest baseline selenium levels.^[38] In 2009 the 5.5 year SELECT study reported that selenium and vitamin E supplementation, both alone and together, did not significantly reduce the incidence of prostate cancer in 35,000 men who "generally were replete in selenium at baseline".^[38] The SELECT trial found that vitamin E did not reduce prostate cancer as it had in the Alpha-Tocopherol, Beta Carotene (ATBC) study, but the ATBC had a large percentage of smokers while the SELECT trial did not.^[38]

Dietary selenium prevents chemically induced carcinogenesis in many rodent studies.^[39] It has been proposed that selenium may help prevent cancer by acting as an antioxidant or by enhancing immune activity.

Not all studies agree on the cancer-fighting effects of selenium. One study of naturally occurring levels of selenium in over 60,000 participants did not show a significant correlation between those levels and cancer.^[40] The SU.VI.MAX study^[41] concluded that low-dose supplementation (with 120 mg of ascorbic acid, 30 mg of vitamin E, 6 mg of beta carotene, 100 µg of selenium, and 20 mg of zinc) resulted in a 30% reduction in the incidence of cancer and a 37% reduction in all-cause mortality in males, but did not get a significant result for females.^[42] However, there is evidence that selenium can help chemotherapy treatment by enhancing the efficacy of the treatment, reducing the toxicity of chemotherapeutic drugs, and preventing the body's resistance to the drugs.^[43] Studies of cancer cells in vitro showed that chemotherapeutic drugs, such as Taxol and Adriamycin, were more toxic to strains of cancer cells grown in culture when selenium was added.^[44][45]

In March 2009, a study from the Department of Cancer Biology at the University of Texas M. D. Anderson Cancer Center reports that Vitamin E (400 IU) and selenium (200 micrograms) supplements affect gene expression and can act as a tumor suppressor.^[46] Eric Klein, MD from the Glickman Urological and Kidney Institute in Ohio said the new study “lend credence to the previous evidence that selenium and vitamin E might be active as cancer preventatives”.^[47] In an attempt to rationalise the differences between epidemiological and in vitro studies and randomised trials like SELECT, Klein said that randomized controlled trials “do not always validate what we believe biology indicates and that our model systems are imperfect measures of clinical outcomes in the real world”.^[47]

HIV/AIDS

Some research has indicated a geographical link between regions of selenium-deficient soils and peak incidences of HIV/AIDS infection. For example, much of sub-Saharan Africa is low in selenium. However, Senegal is not, and also has a significantly lower level of AIDS infection than the rest of the continent. AIDS appears to involve a slow and progressive decline in levels of selenium in the body. Whether this decline in selenium levels is a direct result of the replication of HIV^[48] or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated.

Low selenium levels in AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death.^[49] Selenium normally acts as an antioxidant, so low levels of it may increase oxidative stress on the immune system leading to more rapid decline of the immune system. Others have argued that HIV encodes for the human selenoenzyme glutathione peroxidase, which depletes the victim's selenium levels. Depleted selenium levels in turn lead to a decline in CD4 helper T-cells, further weakening the immune system.^[50]

Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly correlate with the progression of the disease and the risk of death.^[51][52][53]

Tuberculosis

Some research has suggested that selenium supplementation, along with other nutrients, can help prevent the recurrence of tuberculosis.^[54]

Diabetes

A well-controlled study showed that selenium intake is positively correlated with the risk of developing type II diabetes. Because high serum selenium levels are positively associated with the prevalence of diabetes, and because selenium deficiency is rare, supplementation is not recommended in well-nourished populations such as the U.S.^[55]

Mercury

Experimental findings have demonstrated a protective effect of selenium on methylmercury toxicity, but epidemiological studies have been inconclusive in linking selenium to protection against the adverse effects of methylmercury.^[56]

Non-biologic applications

Chemistry

Selenium is a catalyst in many chemical reactions and is widely used in various industrial and laboratory syntheses, especially Organoselenium chemistry. It is also widely used in structure determination of proteins and nucleic acids by X-ray crystallography (incorporation of one or more Se atoms helps with MAD and SAD phasing.)

Manufacturing and materials use

The largest use of selenium worldwide is in glass and ceramic manufacturing, where it is used to give a red color to glasses, enamels and glazes as well as to remove color from glass by counteracting the green tint imparted by ferrous impurities.

Selenium is used with bismuth in brasses to replace more toxic lead. It is also used to improve abrasion resistance in vulcanized rubbers.

Electronics

Because of its photovoltaic and photoconductive properties, selenium is used in photocopying, photocells, light meters and solar cells. It was once widely used in rectifiers. These uses have mostly been replaced by silicon-based devices, or are in the process of being replaced. The most notable exception is in power DC surge protection, where the superior energy capabilities of selenium suppressors make them more desirable than metal oxide varistors.

Sheets of amorphous selenium convert x-ray images to patterns of charge in xeroradiography and in solid-state, flat-panel x-ray cameras.

Photography

Selenium is used in the toning of photographic prints, and it is sold as a toner by numerous photographic manufacturers including Kodak and Fotospeed. Its use intensifies and extends the tonal range of black and white photographic images as well as improving the permanence of prints.

Biologic applications

Medical use

The substance loosely called selenium sulfide, SeS₂, actually selenium disulfide or selenium (IV) sulfide, is the active ingredient in some dandruff shampoos.^[57] The effect of the active ingredient is to kill the scalp fungus Malassezia which causes shedding of dry skin fragments. The ingredient is also used in body lotions to treat Tinea versicolor due to infection by a different species of Malassezia fungus.^[58]

Nutrition

Selenium is used widely in vitamin preparations and other dietary supplements, in small doses (typically 50 to 200 micrograms per day for adult humans). Some livestock feeds are fortified with selenium as well.

Evolution in biology

Over three billion years ago, blue-green algae were the most primitive oxygenic photosynthetic organisms and are ancestors of multicellular eukaryotic algae.^[59] Algae that contain the highest amount of antioxidant selenium, iodide, and peroxidase enzymes were the first living cells to produce poisonous oxygen in the atmosphere. Venturi et al.^[59][60] suggested that algal cells required a protective antioxidant action, in which selenium and iodides, through peroxidase enzymes, have had this specific role. Selenium, which acts synergistically with iodine,^[61] is a primitive mineral antioxidant, greatly present in the sea and prokaryotic cells, where it is an essential component of the family of glutathione peroxidase antioxidant enzymes (GSH-Px). In fact, seaweeds accumulate high quantity of selenium and iodine.^[59] In 2008, Küpper et al.,^[62] showed that iodide also scavenges reactive oxygen species (ROS) in algae, and that its biological role is that of an inorganic antioxidant, the first to be described in a living system, active also in today’s humans.

From about three billion years ago, prokaryotic selenoprotein families drive selenocysteine evolution. Selenium is incorporated into several prokaryotic selenoprotein families in bacteria, archaea and eukaryotes as selenocysteine,^[63] where selenoprotein peroxiredoxins protect bacterial and eukaryotic cells against oxidative damage. Selenoprotein families of GSH-Px and deiodinase of eukaryotic cells seem to have a bacterial phylogenetic origin. The selenocysteine-containing form occurred in green algae, diatoms, sea urchin, fish and chicken, too. One family of selenium-containing molecules as glutathione peroxidases repairs damaged cell membranes, while another (glutathione S-transferases) repairs damaged DNA and prevents mutations.^[64]

When about 500 Mya, plants and animals began to transfer from the sea to rivers and land, the environmental deficiency of marine mineral antioxidants (as selenium, iodine, etc.) was a challenge to the evolution of terrestrial life.^[59] Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e. selenium, vanadium, magnesium, copper and zinc, may have been lacking in some terrestrial mineral-deficient areas.^[63] Marine organisms apparently retained and sometimes expanded their seleno-proteomes, whereas the seleno-proteomes of some terrestrial organisms were reduced or completely lost. These findings suggest that, with the exception of vertebrates, aquatic life supports selenium utilization, whereas terrestrial habitats lead to reduced use of this trace element.^[65] Marine fishes and vertebrate thyroid glands have the highest concentration of selenium and iodine. From about 500 Mya, freshwater and terrestrial plants slowly optimized the production of “new” endogenous antioxidants such as ascorbic acid (Vitamin C), polyphenols, flavonoids, tocopherols, etc. A few of these appeared more recently, in the last 50-200 million years, in fruits and flowers of angiosperm plants. In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late Jurassic period.

The deiodinase isoenzymes constituted the second family of eukaryotic selenoproteins with identified enzyme function. Deiodinases are able to extract electrons from iodides, and iodides from iodothyronines; so, they are involved in thyroid-hormone regulation, participating in the protection of thyrocytes from damage by H₂O₂ produced for thyroid-hormone biosynthesis.^[59][60] About 200 Mya, new selenoproteins were developed as mammalian GSH-Px enzymes.^{[66][67][68][69]}

Compounds

• Copper indium gallium selenide Cu(Ga,In)Se₂
• Mercury selenide (HgSe)
• Hydrogen selenide (H₂Se)
• Lead selenide (PbSe)
• Selenium dioxide (SeO₂)
• Selenic acid (H₂SeO₄)
• Selenous acid (H₂SeO₃)
• Selenium sulfides: Se₄S₄, SeS₂, Se₂S₆
• Sodium selenite (Na₂SeO₃)
• Zinc selenide (ZnSe)

Selenium occurs in the 0,+2,+4,+6 and -2 valence states. See also Selenium compounds and organoselenium chemistry.

References

1. ^ "Selenium : Selenium(I) chloride compound data". WebElements.com. http://www.webelements.com/webelements/compounds/text/Se/Cl2Se2-10025680.html. Retrieved on 2007-12-10. 
2. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81th edition, CRC press.
3. ^ ^{a b} Ruyle, George. "Poisonous Plants on Arizona Rangelands" (PDF). The University of Arizona. http://cals.arizona.edu/arec/pubs/rmg/1%20rangelandmanagement/2%20poisonousplants93.pdf. Retrieved on 2009-01-05. 
4. ^ Brown, Jr., Robert D.. "Mineral Commodity Survey 1997: Selenium". United States Geological Survey. http://minerals.usgs.gov/minerals/pubs/commodity/selenium/830398.pdf. Retrieved on 2009-01-05. 
5. ^ "Public Health Statement: Selenium" (PDF). Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/toxprofiles/tp92-c1.pdf. Retrieved on 2009-01-05. 
6. ^ "Native Selenium". Webminerals. http://www.galleries.com/minerals/elements/selenium/selenium.htm. Retrieved on 2009-06-06. 
7. ^ "Public Health Statement: Selenium - Production, Import/Export, Use, and Disposal" (PDF). Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/toxprofiles/tp92-c5.pdf. Retrieved on 2009-01-05. 
8. ^ "Chemistry : Periodic Table : selenium : key information". webelements. http://www.webelements.com/webelements/elements/text/Se/key.html. Retrieved on 2009-01-06. 
9. ^ Kotkata M.F. et al. (1992). "Structural studies of glassy and crystalline selenium-sulphur compounds". J. Mater. Sci. 27: 1785-1794. 
10. ^ Masters, Anthony F.. "Allotropes - Group 13, Group 14, Group 15, Group 16". Chemistry Explained. http://www.chemistryexplained.com/A-Ar/Allotropes.html. Retrieved on 2009-01-06. 
11. ^ "Selenium". Linus Pauling Institute at Oregon State University. http://lpi.oregonstate.edu/infocenter/minerals/selenium/. Retrieved on 2009-01-05. 
12. ^ Barclay, Margaret N. I.; Allan MacPherson, James Dixon (1995). "Selenium content of a range of UK food". Journal of food composition and analysis 8: 307–318. doi:10.1006/jfca.1995.1025. ISSN 0889-1575. 
13. ^ A list of selenium-rich foods can be found on The Office of Dietary Supplements Selenium Fact Sheet.
14. ^ T. Zane Davis (2008). "Selenium in Plants". p.8. http://www.ars.usda.gov/SP2UserFiles/Place/54282000/PPClassPPSlides/3-27-08DavisSelenium.pdf. Retrieved on 2008-12-05. 
15. ^ "Dietary Supplement Fact Sheet: Selenium". National Institutes of Health; Office of Dietary Supplements. http://ods.od.nih.gov/factsheets/selenium.asp#h7. Retrieved on 2009-01-05. 
16. ^ a report of the Panel on Dietary Antioxidants and Related Compounds, Subcommittees on Upper Reference Leves of Nutrients and of Interpretation and Use of Dietary Reference Intakes, and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine. (August 15, 2000). Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Institute of Medicine. pp. 314–315. ISBN 0309069491. http://www.nap.edu/openbook.php?record_id=9810&page=315. 
17. ^ Yang, G.; Zhou, R. (1994). Further Observations on the Human Maximum Safe Dietary Selenium Intake in a Seleniferous Area of China. 8. Department of Trace Element Nutrition, Institute of Nutrition and Food Hygiene, Chinese Academy of Preventive Medicine. pp. 159–165. 
18. ^ Yang, Guang-Qi; Xia, Yi-Ming (1995). "Studies on Human Dietary Requirements and Safe Range of Dietary Intakes of Selenium in China and Their Application in the Prevention of Related Endemic Diseases". Biomedical and Environmental Sciences (Beijing 100050, China: Department of Trace Element Nutrition, Institute of Nutrition and Food Hygiene, Chinese Academy of Preventive Medicine) 8: 187–201. 
19. ^ "Public Health Statement: Health Effects" (PDF). Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/toxprofiles/tp92-c3.pdf. Retrieved on 2009-01-05. 
20. ^ Wilber, C. G. (1980). "Toxicology of selenium". Clinical Toxicology 17: 171–230. 
21. ^ Olson, O.E. (1986). "Selenium Toxicity in Animals with Emphasis on Man". International Journal of Toxicology 5: 45. doi:10.3109/10915818609140736. 
22. ^ Zhang J, Wang X, Xu T (2008). "Elemental selenium at nano size (Nano-Se) as a potential chemopreventive agent with reduced risk of selenium toxicity: comparison with se-methylselenocysteine in mice". Toxicol. Sci. 101 (1): 22–31. doi:10.1093/toxsci/kfm221. PMID 17728283. 
23. ^ Gao X, Zhang J, Zhang L (2000). "[Acute toxicity and bioavailability of nano red elemental selenium]" (in Chinese). Wei Sheng Yan Jiu 29 (1): 57–8. PMID 12725047. 
24. ^ Zhang JS, Gao XY, Zhang LD, Bao YP (2001). "Biological effects of a nano red elemental selenium". Biofactors 15 (1): 27–38. doi:10.1002/biof.5520150103. PMID 11673642. 
25. ^ Zhang J, Wang H, Yan X, Zhang L (2005). "Comparison of short-term toxicity between Nano-Se and selenite in mice". Life Sci. 76 (10): 1099–109. doi:10.1016/j.lfs.2004.08.015. PMID 15620574. 
26. ^ Jia X, Li N, Chen J (2005). "A subchronic toxicity study of elemental Nano-Se in Sprague-Dawley rats". Life Sci. 76 (17): 1989–2003. doi:10.1016/j.lfs.2004.09.026. PMID 15707881. 
27. ^ Wang H, Zhang J, Yu H (2007). "Elemental selenium at nano size possesses lower toxicity without compromising the fundamental effect on selenoenzymes: comparison with selenomethionine in mice". Free Radic. Biol. Med. 42 (10): 1524–33. doi:10.1016/j.freeradbiomed.2007.02.013. PMID 17448899. 
28. ^ Peng D, Zhang J, Liu Q, Taylor EW (2007). "Size effect of elemental selenium nanoparticles (Nano-Se) at supranutritional levels on selenium accumulation and glutathione S-transferase activity". J. Inorg. Biochem. 101 (10): 1457–63. doi:10.1016/j.jinorgbio.2007.06.021. PMID 17664013. 
29. ^ "Polo pony selenium levels up to 20 times higher than normal". http://www.horsetalk.co.nz/news/2009/05/033.shtml. Retrieved on 2009-05-05. 
30. ^ Ohlendorf, H. M. (2003). "Ecotoxicology of selenium". Handbook of ecotoxicology. Boca Raton: Lewis Publishers. pp. 466–491. ISBN 9781566705462. http://books.google.de/books?hl=de&lr=&id=qN0I3husm50C&oi=fnd&pg=PA477. 
31. ^ Ravaglia et al. (01 Feb 2000). "Effect of micronutrient status on natural killer cell immune function in healthy free-living subjects aged >=90 y1". American Journal of Clinical Nutrition 71 (2): 590–598. PMID 10648276. http://www.ajcn.org/cgi/content/full/71/2/590. 
32. ^ MedSafe Editorial Team. "Selenium". Prescriber Update Articles. New Zealand Medicines and Medical Devices Safety Authority. http://www.medsafe.govt.nz/Profs/PUarticles/Sel.htm. Retrieved on 2009-07-13. 
33. ^ Russo, M. W.; Murray, S. C.; Wurzelmann, J. I.; Woosley, J. T.; Sandler R. S.; (1997). "Plasma selenium levels and the risk of colorectal adenomas". Nutr Cancer. 28 (2): 125–129.. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=%209290116. 
34. ^ P. Knekt et al. (1998). "Is low selenium status a risk factor for lung cancer ?". Am J Epidemiol 148: 975–82. http://www.ncbi.nlm.nih.gov/pubmed/9829869?dopt=Abstract. 
35. ^ Young KJ, Lee PN. (1999). "Intervention studies on cancer.". Eur J Cancer Prev 8: 91. doi:10.1097/00008469-199904000-00003. http://www.ncbi.nlm.nih.gov/pubmed/10335455?dopt=Abstract. 
36. ^ Burguera JL, Burguera M, Gallignani M, Alarcón OM, Burguera JA. (1990). "Blood serum selenium in the province of Mérida, Venezuela, related to sex, cancer incidence and soil selenium content". J Trace Elem Electrolytes Health Dis. 4: 73. http://www.ncbi.nlm.nih.gov/pubmed/2136228?dopt=Abstract. 
37. ^ L. C. Clark et al. (1996). "Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. A randomized controlled trial. Nutritional Prevention of Cancer Study Group". JAMA 276: 1957. doi:10.1001/jama.276.24.1957. http://www.ncbi.nlm.nih.gov/pubmed/8971064?dopt=Abstract. 
38. ^ ^{a b c} Lippman SM, Klein EA, Goodman PJ, et al. (January 2009). "Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT)". JAMA 301 (1): 39–51. doi:10.1001/jama.2008.864. PMID 19066370. 
39. ^ "Chemoprevention Database". http://www.inra.fr/reseau-nacre/sci-memb/corpet/indexan.html. Retrieved on 2009-05-05. 
40. ^ Garland M et al. (1995). "Prospective study of toenail selenium levels and cancer among women". J Natl Cancer Inst 87: 497. doi:10.1093/jnci/87.7.497. PMID 7707436. http://www.ncbi.nlm.nih.gov/pubmed/7707436?dopt=Abstract. 
41. ^ Hercberg S et al. (1998). "Background and rationale behind the SU.VI.MAX Study, a prevention trial using nutritional doses of a combination of antioxidant vitamins and minerals to reduce cardiovascular diseases and cancers". Int J Vitam Nutr Res 68: 3. http://www.ncbi.nlm.nih.gov/pubmed/9503043?dopt=Abstract. 
42. ^ Hercberg S 'et al. (2004). "The SU.VI.MAX Study: a randomized, placebo-controlled trial of the health effects of antioxidant vitamins and minerals". Arch Intern Med 164: 2335. doi:10.1001/archinte.164.21.2335. PMID 15557412. http://www.ncbi.nlm.nih.gov/pubmed/15557412?dopt=Citation. 
43. ^ "Selenium and Chemotherapy - Nutrition Health". http://nutrition-health.info/index.php/Selenium_and_Chemotherapeutic_Drugs. 
44. ^ "Selenium Cancer". http://nutrition-health.info/index.php/Selenium_Cancer_1. 
45. ^ Nilsonne G, Sun X, Nyström C, Rundlöf A-K, Fernandes A P, Björnstedt M, and Dobra K (2006). "Selenite induces apoptosis in sarcomatoid malignant mesothelioma cells through oxidative stress". Free Radical Biol. Med. 41: 874-885. doi:10.1016/j.freeradbiomed.2006.04.031. 
46. ^ D. Tsavachidou, T.J. McDonnell, S. Wen, X. Wang et al. (2009). "Selenium and Vitamin E: Cell Type- and Intervention-Specific Tissue Effects in Prostate Cancer". Journal of the National Cancer Institute 101: 306–320. doi:10.1093/jnci/djn512. PMID 19244175. 
47. ^ ^{a b} E.A. Klein, MD. (2009). "Selenium and Vitamin E: Interesting Biology and Dashed Hope". Journal of the National Cancer Institute 101: 283. doi:10.1093/jnci/djp009. PMID 19244172. 
48. ^ Patrick L (1999). "Nutrients and HIV: part one - beta carotene and selenium". Altern Med Rev 4: 403. http://www.ncbi.nlm.nih.gov/pubmed/10608913?dopt=Abstract. 
49. ^ "Dietary Supplement Fact Sheet: Selenium". http://ods.od.nih.gov/factsheets/selenium.asp#h6. Retrieved on 2009-05-05. 
50. ^ Taylor, Ethan Will (1995). "Selenium and cellular immunity". Biological Trace Element Research 49: 85. doi:10.1007/BF02788958. 
51. ^ Baum MK et al. (1997). "High risk of HIV-related mortality is associated with selenium deficiency". J Acquir Immune Defic Syndr Hum Retrovirol 15: 370. http://www.ncbi.nlm.nih.gov/pubmed/9342257?dopt=Abstract. 
52. ^ Campa A et al. (1999). "Mortality risk in selenium-deficient HIV-positive children". J Acquir Immune Defic Syndr Hum Retrovirol 20: 508. http://www.ncbi.nlm.nih.gov/pubmed/10225235?dopt=Abstract. 
53. ^ Baum MK, Shor-Posner G. (1998). "Micronutrient status in relationship to mortality in HIV-1 disease". Nutr Rev 56: S135. http://www.ncbi.nlm.nih.gov/pubmed/9481135?dopt=Abstract. 
54. ^ Villamor E, Mugusi F, Urassa W, et al. (June 2008). "A Trial of the Effect of Micronutrient Supplementation on Treatment Outcome, T Cell Counts, Morbidity, and Mortality in Adults with Pulmonary Tuberculosis". J. Infect. Dis. 197 (11): 1499–1505. doi:10.1086/587846. PMID 18471061. http://www.nutraingredients-usa.com/news/ng.asp?id=84945-micronutrients-tuberculosis-selenium. 
55. ^ Bleys, Joachim; Navas-Acien, Ana; Guallar, Eliseo (2007). "Serum Selenium and Diabetes in U.S. Adults". Diabetes Care 30 (4): 829–834. doi:10.2337/dc06-1726. http://care.diabetesjournals.org/cgi/content/abstract/30/4/829. 
56. ^ Watanabe C (2002). "Modification of mercury toxicity by selenium: practical importance?" (PDF). Tohoku J Exp Med 196 (2): 71–7. doi:10.1620/tjem.196.71. PMID 12498318. http://www.jstage.jst.go.jp/article/tjem/196/2/71/_pdf. 
57. ^ "Selenium(IV)_sulfide". Pharmacy Codes. http://pharmacycode.com/Selenium(IV)_sulfide.html. Retrieved on 2009-01-06. 
58. ^ "Selenium sulfide". DermNet NZ. http://dermnetnz.org/treatments/selenium.html. Retrieved on 2009-01-06. 
59. ^ ^{a b c d e} Venturi, S.; Donati, F. M.; Venturi, A.; Venturi, M. (2000). "Environmental iodine deficiency: A challenge to the evolution of terrestrial life?". Thyroid 10 (8): 727–729. doi:10.1089/10507250050137851. 
60. ^ ^{a b} Venturi S, Venturi M. Evolution of Dietary Antioxidant Defences. 2007. European Epi-Marker. 11, 3 :1-12. http://web.tiscali.it/iodio/
61. ^ Cocchi M, Venturi S. (2000). "Selenium and Iodide: ancient antioxidants of cellular membranes?". 7th Internat. Symp. on Selenium in Biology and Medicine. Venice (Italy) Abs. P-88: 134. 
62. ^ Küpper, F. C.; Carpenter, L. J.; McFiggans, G. B.; et al. (2008). "Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry". Proceedings of the National Academy of Sciences 105 (19): 6954–6958. doi:10.1073/pnas.0709959105. 
63. ^ ^{a b} Gladyshev, V. N.; Hatfield, D. L. (1999). "Selenocysteine-containing proteins in mammals". Journal of Biomedical Science 6: 151–160. doi:10.1007/BF02255899. 
64. ^ Stadtman, Thressa C. (1996). "Selenocysteine". Annual Review of Biochemistry 0 65: 83–100. doi:10.1146/annurev.bi.65.070196.000503. 
65. ^ Lobanov, Alexey V.;; Fomenko, Dmitri E.; Zhang, Yan; Sengupta, Aniruddha; Hatfield, Dolph L.; Gladyshev, Vadim N. (2007). "Evolutionary dynamics of eukaryotic selenoproteomes: large selenoproteomes may associate with aquatic life and small with terrestrial life". Genome Biology 8 (9): R198. doi:10.1186/gb-2007-8-9-r198. 
66. ^ Castellano S, Novoselov SV, Kryukov GV, et al. (2004). "Reconsidering the evolution of eukaryotic selenoproteins: a novel non-mammalian family with scattered phylogenetic distribution". EMBO Rep. 5: 71. doi:10.1038/sj.embor.7400036. 
67. ^ Kryukov, G. V.; Gladyshev, V. N. (2004). "The prokaryotic selenoproteome". EMBO Rep. 5: 538–543. doi:10.1038/sj.embor.7400126. 
68. ^ Wilting, R.; Schorling, S.; Persson, B. C.; Bock, A. (1997). "Selenoprotein synthesis in archaea: identification of an mRNA element of Methanococcus jannaschii probably directing selenocysteine insertion". Journal of Molecular Biology 266 (4): 637–641. doi:10.1006/jmbi.1996.0812. 
69. ^ Zhang, Y; Fomenko, D. E.; Gladyshev V. N.; (2005). "The microbial selenoproteome of the Sargasso Sea.". Genome Biol. 6 (4): R37. doi:10.1186/gb-2005-6-4-r37. 

External links

Wikimedia Commons has media related to: Selenium

Look up selenium in Wiktionary, the free dictionary.

• WebElements.com - Selenium
• National Institutes of Health page on Selenium
• ATSDR - Toxicological Profile: Selenium
• Peter van der Krogt elements site

v • d • e Periodic table
H																															He
Li	Be																									B	C	N	O	F	Ne
Na	Mg																									Al	Si	P	S	Cl	Ar
K	Ca															Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Rb	Sr															Y	Zr	Nb	Mo	Tc	Ru	Rh	Pd	Ag	Cd	In	Sn	Sb	Te	I	Xe
Cs	Ba	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg	Tl	Pb	Bi	Po	At	Rn
Fr	Ra	Ac	Th	Pa	U	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No	Lr	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Uub	Uut	Uuq	Uup	Uuh	Uus	Uuo
Alkali metals Alkaline earth metals Lanthanoids Actinoids Transition metals Other metals Metalloids Other nonmetals Halogens Noble gases

v • d • e   Selenium compounds SeCl4 · SeF4 · SeF6 · SeO2 · SeO3 · SeOBr2 · SeOCl2 · SeS2 · Se2S6

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)