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A little bit of selenium is usually plenty to meet your daily requirements. Over the long term, routinely getting unsafe levels could lead to selenium toxicity, a condition linked to breathing issues, kidney failure, and heart problems. At high enough levels, selenium toxicity could even be fatal.

These supplements usually come in the form of capsules or tablets. But whole foods are the best sources of selenium, as the mineral may be destroyed when it's processed. Unless your doctor tells you to take a supplement, get selenium from foods to avoid taking too much.

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A review of 69 studies that included over 350,000 people found that having a high blood level of selenium was associated with a lower risk of certain types of cancer, including breast, lung, colon, and prostate cancers (9).

Summary Higher blood levels of selenium may protect against certain cancers,

while supplementing with selenium may help improve quality of life in people

undergoing radiation therapy.

For example, a review of 16 controlled studies including over 433,000 people with heart disease showed that taking selenium supplements decreased levels of the inflammatory marker C-reactive protein (CRP).

This indicates that selenium may help lower heart disease risk by reducing inflammation and oxidative stress in your body. Oxidative stress and inflammation have been linked to atherosclerosis, or the buildup of plaque in arteries.

Summary Selenium is crucial for the health and proper functioning of your

immune system. Higher levels of selenium may help boost the immune systems of

people with HIV, influenza, tuberculosis, and hepatitis C.

For example, one study showed that the selenium concentration in Brazil nuts varied widely by region. While a single Brazil nut from one region provided up to 288% of the recommended intake, others provided only 11% (35).

Selenium is found in metal sulfide ores, where it partially replaces the sulfur. Commercially, selenium is produced as a byproduct in the refining of these ores, most often during production. Minerals that are pure selenide or selenate compounds are known but rare. The chief commercial uses for selenium today are glassmaking and pigments. Selenium is a semiconductor and is used in photocells. Applications in electronics, once important, have been mostly replaced with silicon semiconductor devices. Selenium is still used in a few types of DC power surge protectors and one type of fluorescent quantum dot.

Although trace amounts of selenium are necessary for cellular function in many animals, including humans, both elemental selenium and (especially) selenium salts are toxic in even small doses, causing selenosis.[8] Selenium is listed as an ingredient in many multivitamins and other dietary supplements, as well as in infant formula, and is a component of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) as well as in three deiodinase enzymes. Selenium requirements in plants differ by species, with some plants requiring relatively large amounts and others apparently not requiring any.[9]

Selenium forms several allotropes that interconvert with temperature changes, depending somewhat on the rate of temperature change. When prepared in chemical reactions, selenium is usually an amorphous, brick-red powder. When rapidly melted, it forms the black, vitreous form, usually sold commercially as beads.[10] The structure of black selenium is irregular and complex and consists of polymeric rings with up to 1000 atoms per ring. Black selenium is a brittle, lustrous solid that is slightly soluble in CS2. Upon heating, it softens at 50 C and converts to gray selenium at 180 C; the transformation temperature is reduced by presence of halogens and amines.[7]

It is a polymeric solid that forms monomeric SeO2 molecules in the gas phase. It dissolves in water to form selenous acid, H2SeO3. Selenous acid can also be made directly by oxidizing elemental selenium with nitric acid:[19]

In comparison with its sulfur counterpart (sulfur hexafluoride), selenium hexafluoride (SeF6) is more reactive and is a toxic pulmonary irritant.[23] With different stoichiometry, the elements form selenium tetrafluoride, a laboratory-scale fluorinating agent, instead.

The only stable chlorides are selenium tetrachloride (SeCl4) and selenium monochloride (Se2Cl2), which might be better known as selenium(I) chloride and is structurally analogous to disulfur dichloride. Metastable solutions of selenium dichloride can be prepared from sulfuryl chloride and selenium (reaction of the elements generates the tetrachloride instead), and constitute an important reagent in the preparation of selenium compounds (e.g. Se7). The corresponding bromides are all known, and recapitulate the same stability and structure as the chlorides.[24]

The iodides of selenium are not well known, and for a long time were believed not to exist.[25] There is limited spectroscopic evidence that the lower iodides may form in bi-elemental solutions with nonpolar solvents, such as carbon disulfide[26] and carbon tetrachloride;[25] but even these appear to decompose under illumination.[27]

Selenium, especially in the II oxidation state, forms a variety of organic derivatives. They are structurally analogous to the corresponding organosulfur compounds. Especially common are selenides (R2Se, analogues of thioethers), diselenides (R2Se2, analogues of disulfides), and selenols (RSeH, analogues of thiols). Representatives of selenides, diselenides, and selenols include respectively selenomethionine, diphenyldiselenide, and benzeneselenol. The sulfoxide in sulfur chemistry is represented in selenium chemistry by the selenoxides (formula RSe(O)R), which are intermediates in organic synthesis, as illustrated by the selenoxide elimination reaction. Consistent with trends indicated by the double bond rule, selenoketones, R(C=Se)R, and selenaldehydes, R(C=Se)H, are rarely observed.[32]

In 1873, Willoughby Smith found that the electrical conductivity of grey selenium was affected by light.[36][37] This led to its use as a cell for sensing light. The first commercial products using selenium were developed by Werner Siemens in the mid-1870s. The selenium cell was used in the photophone developed by Alexander Graham Bell in 1879. Selenium transmits an electric current proportional to the amount of light falling on its surface. This phenomenon was used in the design of light meters and similar devices. Selenium's semiconductor properties found numerous other applications in electronics.[38][39][40] The development of selenium rectifiers began during the early 1930s, and these replaced copper oxide rectifiers because they were more efficient.[41][42][43] These lasted in commercial applications until the 1970s, following which they were replaced with less expensive and even more efficient silicon rectifiers.

Selenium came to medical notice later because of its toxicity to industrial workers. Selenium was also recognized as an important veterinary toxin, which is seen in animals that have eaten high-selenium plants. In 1954, the first hints of specific biological functions of selenium were discovered in microorganisms by biochemist, Jane Pinsent.[44][45] It was discovered to be essential for mammalian life in 1957.[46][47] 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, selenocysteine was shown to be encoded by the codon UGA. The recoding mechanism was worked out first in bacteria and then in mammals (see SECIS element).[48]

Native (i.e., elemental) selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedra or tiny acicular (hair-like) crystals.[49] Isolation of selenium is often complicated by the presence of other compounds and elements.

Selenium occurs naturally in a number of inorganic forms, including selenide, selenate, and selenite, but these minerals are rare. The common mineral selenite is not a selenium mineral, and contains no selenite ion, but is rather a type of gypsum (calcium sulfate hydrate) named like selenium for the moon well before the discovery of selenium. Selenium is most commonly found as an impurity, replacing a small part of the sulfur in sulfide ores of many metals.[50][51]

In living systems, selenium is found in the amino acids selenomethionine, selenocysteine, and methylselenocysteine. In these compounds, selenium plays a role analogous to that of sulfur. Another naturally occurring organoselenium compound is dimethyl selenide.[52][53]

Certain soils are selenium-rich, and selenium can be bioconcentrated by some plants. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff.[50][51] Ocean water contains significant amounts of selenium.[54][55]

Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, nickel, or lead. Electrolytic metal refining is particularly productive of selenium as a byproduct, obtained from the anode mud of copper refineries. Another source was the mud from the lead chambers of sulfuric acid plants, a process that is no longer used. Selenium can be refined from these muds by a number of methods. However, most elemental selenium comes as a byproduct of refining copper or producing sulfuric acid.[58][59] Since its invention, solvent extraction and electrowinning (SX/EW) production of copper produces an increasing share of the worldwide copper supply.[60] This changes the availability of selenium because only a comparably small part of the selenium in the ore is leached with the copper.[61]

Industrial production of selenium usually involves the extraction of selenium dioxide from residues obtained during the purification of copper. Common production from the residue then begins by oxidation with sodium carbonate to produce selenium dioxide, which is mixed with water and acidified to form selenous acid (oxidation step). Selenous acid is bubbled with sulfur dioxide (reduction step) to give elemental selenium.[62][63] ff782bc1db