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Bismuth (pronounced /ˈbɪzməθ/, BIZ-məth) is a chemical element that has the symbol Bi and atomic number 83. This trivalent poor metal chemically resembles arsenic and antimony. Bismuth is heavy and brittle; it has a silvery white color with a pink tinge owing to the surface oxide. Bismuth is the most naturally diamagnetic of all metals, and only mercury has a lower thermal conductivity. It is generally considered to be the last naturally occurring stable, non-radioactive element on the periodic table, although it is actually slightly radioactive. Its only non-synthetic isotope bismuth-209 decays via alpha decay into thallium-205, with an extremely long half-life of 1.9 × 10¹⁹ years.

Bismuth compounds are used in cosmetics, medicines, and in medical procedures. As the toxicity of lead has become more apparent in recent years, alloy uses for bismuth metal as a replacement for lead have become an increasing part of bismuth's commercial importance.

Bismuth crystal with an iridescent oxide surface

Bismuth crystals

Bismuth is a brittle metal with a white, silver-pink hue, often occurring in its native form with an iridescent oxide tarnish showing many colors from yellow to blue. The spiral stair stepped structure of a bismuth crystal is the result of a higher growth rate around the outside edges than on the inside edges. The variations in the thickness of the oxide layer that forms on the surface of the crystal causes different wavelengths of light to interfere upon reflection, thus displaying a rainbow of colors. When combusted with oxygen, bismuth burns with a blue flame and its oxide forms yellow fumes. Its toxicity is much lower than that of its neighbors in the periodic table such as lead, tin, tellurium, antimony, and polonium.

Although ununpentium is theoretically more diamagnetic, no other metal is verified to be more naturally diamagnetic than bismuth.(Superdiamagnetism is a different physical phenomenon.) Of any metal, it has the second lowest thermal conductivity (after mercury) and the highest Hall coefficient. It has a high electrical resistance.When deposited in sufficiently thin layers on a substrate, bismuth is a semiconductor, rather than a poor metal.

Elemental bismuth is one of very few substances of which the liquid phase is denser than its solid phase (water being the best-known example). Bismuth expands 3.32% on solidification; therefore, it was long an important component of low-melting typesetting alloys, where it compensated for the contraction of the other alloying components

Though virtually unseen in nature, high-purity bismuth can form distinctive hopper crystals. These colorful laboratory creations are typically sold to collectors. Bismuth is relatively nontoxic and has a low melting point just above 271 °C, so crystals may be grown using a household stove, although the resulting crystals will tend to be lower quality than lab-grown crystals.

While bismuth was traditionally regarded as the element with the heaviest stable isotope, bismuth-209, it had long been suspected to be unstable on theoretical grounds. This was finally demonstrated in 2003 when researchers at the Institut d'Astrophysique Spatiale in Orsay, France, measured the alpha emission half-life of ²⁰⁹Bi to be 1.9 × 10¹⁹ years, over a billion times longer than the current estimated age of the universe. Owing to its extraordinarily long half-life, for all presently known medical and industrial applications bismuth can be treated as if it is stable and non-radioactive. The radioactivity is of academic interest, however, because bismuth is one of few elements whose radioactivity was suspected, and indeed theoretically predicted, before being detected in the laboratory.

Bismuth is unique in that it has the longest alpha decay half life (Tellurium-128 has a double beta decay half-life of over 2.2 × 10²⁴ years). Bismuth-209's half life is in fact a lot longer than estimated age of the universe. As such, while it is not technically stable, it is practically stable. It is created in supernovas via the r-process, and in stars via the s-process.

Bismuth (New Latin bisemutum from German Wismuth, perhaps from weiße Masse, "white mass") was confused in early times with tin and lead because of its resemblance to those elements. Bismuth has been known since ancient times, and so no one person is credited with its discovery. Agricola, in De Natura Fossilium states that bismuth is a distinct metal in a family of metals including tin and lead in 1546 based on observation of the metals and their physical properties. Claude François Geoffroy demonstrated in 1753 that this metal is distinct from lead and tin.

"Artificial bismuth" was commonly used in place of the actual metal. It was made by hammering tin into thin plates, and cementing them by a mixture of white tartar, saltpeter, and arsenic, stratified in a crucible over an open fire.^{[citation needed]}

Bismuth was also known to the Incas and used (along with the usual copper and tin) in a special bronze alloy for knives.

New York prices (2007)

Time
Price (USD/lb.)

December 2000
3.85–4.15

November 2002
2.70–3.10

December 2003
2.60–2.90

June 2004
3.65–4.00

September 2005
4.20–4.60

September 2006
4.50–4.75

November 2006
6.00–6.50

December 2006
7.30–7.80

March 2007
9.25–9.75

April 2007
10.50–11.00

June 2007
18.00–19.00

November 2007
13.50–15.00

Bismite mineral

Bismuth output in 2005

In the Earth's crust, bismuth is about twice as abundant as gold. It is not usually economical to mine it as a primary product. Rather, it is usually produced as a byproduct of the processing of other metal ores, especially lead, tungsten (China), tin, copper, and also silver (indirectly) or other metallic elements.

The most important ores of bismuth are bismuthinite and bismite. In 2005, China was the top producer of bismuth with at least 40% of the world share followed by Mexico and Peru, reports the British Geological Survey. Native bismuth is known from Australia, Bolivia, and China.

According to the USGS, world 2006 bismuth mine production was 5,700 tonnes, of which China produced 3,000 tonnes, Mexico 1,180 tonnes, Peru 950 tonnes, and the balance Canada, Kazakhstan and other nations. World 2006 bismuth refinery production was 12,000 tonnes, of which China produced 8,500 tonnes, Mexico 1,180 tonnes, Belgium 800 tonnes, Peru 600 tonnes, Japan 510 tonnes, and the balance Canada and other nations.

For the year 2007, the USGS Minerals Yearbook stated that world bismuth refinery production was of 15,000 tonnes, of which China was responsible for 77%, Mexico 8% and Belgium 5%.

The difference between world bismuth mine production and refinery production reflects bismuth's status as a byproduct metal. Bismuth travels in crude lead bullion (which can contain up to 10% bismuth) through several stages of refining, until it is removed by the Kroll-Betterton process or the Betts process. The Kroll-Betterton process uses a pyrometallurgical separation from molten lead of calcium-magnesium-bismuth drosses containing associated metals (silver, gold, zinc, some lead, copper, tellurium, and arsenic), which are removed by various fluxes and treatments to give high-purity bismuth metal (over 99% Bi). The Betts process takes cast anodes of lead bullion and electrolyzes them in a lead fluorosilicate-hydrofluorosilicic acid electrolyte to yield a pure lead cathode and an anode slime containing bismuth. Bismuth will behave similarly with another of its major metals, copper. Thus world bismuth production from refineries is a more complete and reliable statistic.

According to the Bismuth Advocate News, the price for bismuth metal from year-end 2000 to September 2005 was stuck in a range from $2.60 to $4.15 per lb., but after this period the price started rising rapidly as global bismuth demand as a lead replacement and other uses grew rapidly. New mines in Canada and Vietnam may relieve the shortages, but prices are likely to remain above their previous level for the foreseeable future. The Customer-Input price for bismuth is more oriented to the ultimate consumer; it started at US$39.40 per kilogram ($17.90 per pound) in January 2008 and reached US$35.55 per kg (US$16.15 per lb.) in September 2008.

While bismuth is most available today as a byproduct, its sustainability is more dependent on recycling. Bismuth is mostly a byproduct of lead smelting, along with silver, zinc, antimony, and other metals, and also of tungsten production, along with molybdenum and tin, and also of copper production. Recycling bismuth is difficult in many of its end uses, primarily because of scattering. Probably the easiest to recycle would be bismuth-containing fusible alloys in the form of larger objects, then larger soldered objects. Half of the world solder consumption is in electronics (i.e., circuit boards).^[11] As the soldered objects get smaller or contain little solder or little bismuth, the recovery gets progressively more difficult and less economic, although solder with a sizable silver content will be more worth recovering. Next in recycling feasibility would be sizeable catalysts with a fair bismuth content, perhaps as bismuth phosphomolybdate, and then bismuth used in galvanizing and as a free-machining metallurgical additive. Finally, the bismuth in the uses where it gets scattered the most, in stomach medicines (bismuth subsalicylate), paints bismuth vanadate on a dry surface, pearlescent cosmetics (bismuth oxychloride), and bismuth-containing bullets. The bismuth is so scattered in these uses as to be unrecoverable with present technology. Bismuth can also be available sustainably from greater efficiency of use or substitution, most likely stimulated by a rising price. For the stomach medicine, another active ingredient could be substituted for some or all of the bismuth compound^{[citation needed]}. It would be more difficult to find an alternative to bismuth oxychloride in cosmetics to give the pearlescent effect. However, there are many alloying formulas for solders and therefore many alternatives.

The most important sustainability fact about bismuth is its byproduct status, which can either improve sustainability (i.e., vanadium or manganese nodules) or, for bismuth from lead ore, constrain it; bismuth is constrained. The extent that the constraint on bismuth can be ameliorated or not is going to be tested by the future of the lead storage battery, since 90% of the world market for lead is in storage batteries for gasoline or diesel-powered motor vehicles.

The life-cycle assessment of bismuth will focus on solders, one of the major uses of bismuth, and the one with the most complete information. The average primary energy use for solders is around 200 MJ per kg, with the high-bismuth solder (58% Bi) only 20% of that value, and three low-bismuth solders (2% to 5% Bi) running very close to the average. The global warming potential averaged 10 to 14 kg carbon dioxide, with the high-bismuth solder about two-thirds of that and the low-bismuth solders about average. The acidification potential for the solders is around 0.9 to 1.1 kg sulfur dioxide equivalent, with the high-bismuth solder and one low-bismuth solder only one-tenth of the average and the other low-bismuth solders about average. There is very little life-cycle information on other bismuth alloys or compounds.

Bismuth forms trivalent and pentavalent compounds. The trivalent compounds are more common. Many of its chemical properties are similar to other elements in its group; namely, arsenic and antimony, although it is less toxic than those elements.

Bismuth is stable to both dry and moist air at ordinary temperatures. At elevated temperatures, the vapours of the metal combine rapidly with oxygen, forming the yellow trioxide, Bi₂O₃.^[13] On reaction with base, this oxide forms two series of oxyanions: BiO−2, which is polymeric and forms linear chains, and BiO3−3. The anion in Li₃BiO₃ is actually a cubic octameric anion, Bi₈O24−24, whereas the anion in Na₃BiO₃ is tetrameric.

Bismuth sulfide, Bi₂S₃, occurs naturally in bismuth ores.It is also produced by the combination of molten bismuth and sulfur.

Unlike earlier members of group 15 elements such as nitrogen, phosphorus, and arsenic, and similar to the previous group 15 element antimony, bismuth does not form a stable hydride analogous to ammonia and phosphine. Bismuth hydride, bismuthine (BiH₃), is an endothermic compound that spontaneously decomposes at room temperature. It is stable only below −60°C.

The halides of bismuth in low oxidation states have been shown to have unusual structures. What was originally thought to be bismuth(I) chloride, BiCl, turns out to be a complex compound consisting of Bi5+9 cations and BiCl2−5 and Bi₂Cl2−8 anions. The Bi5+9 cation has a distorted tricapped trigonal prismic molecular geometry, and is also found in Bi₁₀Hf₃Cl₁₈, which is prepared by reducing a mixture of hafnium(IV) chloride and bismuth chloride with elemental bismuth, having the structure [Bi⁺][Bi5+9][HfCl2−6]₃.^:50 Other polyatomic bismuth cations are also known, such as Bi2+8, found in Bi₈(AlCl₄)₂. Bismuth also forms a low-valence bromide with the same structure as "BiCl". There is a true monoiodide, BiI, which contains chains of Bi₄I₄ units. BiI decomposes upon heating to the triiodide, BiI₃, and elemental bismuth. A monobromide of the same structure also exists.

In oxidation state +3, bismuth forms trihalides with all of the halogens: BiF₃, BiCl₃, BiBr₃, and BiI₃. All of these, except BiF₃, are hydrolysed by water to form the bismuthyl cation, BiO⁺, a commonly encountered bismuth oxycation.Bismuth(III) chloride reacts with hydrogen chloride in ether solution to produce the acid HBiCl₄.

Bismuth dissolves in nitric acid to form bismuth(III) nitrate, Bi(NO₃)₃. In the presence of excess water or the addition of a base, the Bi³⁺ ion reacts with the water to form BiO⁺, which precipitates as (BiO)NO₃.

The oxidation state +5 is less frequently encountered. One such compound is BiF₅, a powerful oxidising and fluorinating agent. It is also a strong fluoride acceptor, reacting with xenon tetrafluoride to form the XeF+3 cation:

BiF₅ + XeF₄ → XeF+3BiF−6

The dark red bismuth(V) oxide, Bi₂O₅, is unstable, liberating O₂ gas upon heating.

In aqueous solution, the Bi³⁺ ion exists in various states of hydration, depending on the pH:

pH range
Species

<3
Bi(H₂O)3+6

0-4
Bi(H₂O)₅OH²⁺

1-5
Bi(H₂O)₄(OH)₂⁺

5-14
Bi(H₂O)₃(OH)₃

>11
Bi(H₂O)₂(OH)₄⁻

These mononuclear species are in equilibrium. Polynuclear species also exist, the most important of which is BiO⁺, which exists in hexameric form as the octahedral complex [Bi₆O₄(OH)₄]⁶⁺ (or 6 [BiO⁺]·2 H₂O).

Health

• Bismuth subsalicylate (the active ingredient in Pepto-Bismol and (modern) Kaopectate) is used as an antidiarrheal and to treat some other gastro-intestinal diseases (oligodynamic effect). The means by which this appears to work is still not well-documented. It is thought to be some combination of:

• Killing some bacteria that cause diarrhea
• Reducing inflammation/irritation of stomach and intestinal lining
• Retarding the expulsion of fluids into the digestive system by irritated tissues, by "coating" them.

• The product Bibrocathol is an organic molecule containing Bismuth and is used to treat eye infections.
• Bismuth subgallate (the active ingredient in Devrom) is used as an internal deodorant to treat malodor from flatulence (or gas) and faeces.
• Historically Bismuth compounds were used to treat Syphilis and today Bismuth subsalicylate and Bismuth subcitrate are used to treat the Peptic ulcer.

Other uses

• Strong permanent magnets can be made from the alloy Bismanol (BiMn).

• Bismuth has a potential role in electronic circuits and in manufacturing next-generation solar cells which would have a greater efficiency. Bismuth allows for the creation of new diodes that can reverse their direction of current flow.
• Many bismuth alloys have low melting points and are widely used for fire detection and suppression system safety devices.
• Bismuth is used as an alloying agent in production of malleable irons.
• It is also used as a thermocouple material.
• A carrier for U-235 or U-233 fuel in nuclear reactors.
• Bismuth is also used in solders. The fact that bismuth and many of its alloys expand slightly when they solidify make them ideal for this purpose.
• Bismuth subnitrate is a component of glazes that produces an iridescent luster finish.
• Bismuth telluride (a semiconductor) is an excellent thermoelectric material. Bi₂Te₃ diodes are used in mobile refrigerators and CPU coolers. Also used as detectors in Infra red spectrophotometers.
• A replacement propellant for xenon in Hall effect thrusters
• Bismuth is included in BSCCO (Bismuth Strontium Calcium Copper Oxide) which is a group of similar superconducting compounds discovered in 1988 among which is the highest temperature superconductor yet known with a transition temperature of 110K.
• Bi-213 can be produced by bombarding radium with bremsstrahlung photons from a linear particle accelerator. In 1997 an antibody conjugate with Bi-213, which has a 45 minute half-life, and decays with the emission of an alpha-particle, was used to treat patients with leukemia. This isotope has also been tried in cancer treatment, e.g. in the Targeted Alpha Therapy (TAT) program.
• The delta form of bismuth oxide when it exists at room temperature is a solid electrolyte for oxygen. This form normally only exists above and breaks down below a high temperature threshold, but can be electrodeposited well below this temperature in a highly alkaline solution.
• RoHS initiative for reduction of lead has broadened bismuth's use in electronics as a component of low-melting point solders, as a replacement for traditional tin-lead solders.

In the early 1990s, research began to evaluate bismuth as a nontoxic replacement for lead in various applications:

• As noted above, bismuth has been used in lead-free solders; its low toxicity will be especially important for solders to be used in food processing equipment and copper water pipes, although it can also be used in other applications including those in the automobile industry, in the EU for example.
• A pigment in artists' oil and acrylic paint (Bismuth Vanadate)
• Ingredient in free-machining brasses for plumbing applications, although it doesn't equal leaded steels' performance
• Ingredient in free-machining steels for precision machining properties
• A catalyst for making acrylic fibers
• Ingredient in lubricating greases
• Dense material for fishing sinkers
• In crackling microstars (dragon's eggs) in pyrotechnics, as the oxide, subcarbonate, or subnitrate
• Replacement for lead in shot and bullets. The Netherlands, the UK and U.S., and many other countries now prohibit the use of lead shot for the hunting of wetland birds, as many birds are prone to lead poisoning owing to mistaken ingestion of lead (instead of small stones and grit) to aid digestion, or even prohibit the use of lead for all hunting, such as in the Netherlands. Bismuth-tin alloy shot is one alternative that provides similar ballistic performance to lead. (Another less expensive but also more poorly performing alternative is "steel" shot, which is actually soft iron.) The lack of malleability does, however, make bismuth unsuitable for use in expanding hunting bullets.
• Fabrique Nationale de Herstal uses bismuth in the projectiles for its FN 303 less-lethal riot gun.

According to the USGS, U.S. bismuth consumption in 2006 totaled 2,050 tonnes, of which chemicals (including pharmaceuticals, pigments, and cosmetics) were 510 tonnes, bismuth alloys 591 tonnes, metallurgical additives 923 tonnes, and the balance other uses.

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Bewise Inc. www.tool-tool.com Reference source from the internet.

硒是一種化學元素，它的化 學符號是Se，它的原 子序數是34，是一種非金屬。

硒對生物同時具有必需性和毒性. 性質與硫及碲相似。它在有光時，導電性能較黑暗時好，故 用來做光 電池。

硒之英文全名為Selenium，取自希臘文Σελήνη（月 亮女神塞 勒涅的名字），為月 亮之意。因為它是一種固 體非 金屬，故此用石字部首，並賦予西字音譯。

硒是人體必需的微量礦物質營養素^[1]，多以氧 化態(Se²⁺、Se⁴⁺、和Se⁶⁺)存在，化學性質與硫相似，許多含硫胺基酸， 如甲 硫胺酸(Met)、半 胱氨酸(Cys)、胱氨酸也 可用硒取代硫。

硒在動物組織中最常以甲 硒胺酸（selenomethionine， 簡稱SeMet）和硒 半胱氨酸（selenocysteine，簡稱SeCys）的形態存在，其中甲硒胺酸無法由人體合成，僅能由植物合成後經攝食再經消化代謝而獲 得，故食材動植物來源組成將決定硒在飲食中的形式，此外，人體中甲硒胺酸可以取代甲硫胺酸；但硒半胱胺酸不能取代半胱胺酸。硒在生理上的功能除了抗氧化 外，還調控了甲狀腺的代謝和維他命C的 氧化還原態，也曾被提出和抗癌相關的可能性。在食材成分含量裡，同種植物性食材含硒成分變化相當大，乃因各原植物生長地的土壤中硒的濃度不同，當地的動物 也隨之反映相應情形，因此硒營養缺乏或過量情形常有地域性關係。

硒對生物同時具有必需性和毒性。

氧化硒離子和亞 氧化硒離子的毒性非常強，甚至具有類似砷的 毒性模式。氧 化硒更是具劇毒和腐蝕性的 氣體。

然而，純硒元素和金屬硒化物的毒性相對上不大，而且有些為重要的微 量元素之一。嚴重缺乏可引致克 山症和溪 山症。它們的病徵有：心肌壞死、萎縮、軟骨組織壞死。另外又與甲 狀腺腫、呆 小症和習 慣性流產有關。

[編 輯] 含量與分佈

人體本身的硒總含量為15mg。男性體內的硒多集中在睪丸及前列腺輸精管中，會隨精液一起排出體外。人體與動 物有二個硒儲存庫，一為身體蛋白質的甲 硒胺酸(SelenoMethionine，SeMet)， 它的儲存量視飲食中SeMet量而定，其提供硒的量，取決於甲硫胺酸的轉換率；二為肝臟酵素榖胱甘肽過氧化酶( glutathione peroxidase，GPX)的硒。

[編 輯] 食物來源

硒存在於土壤中，而世界各地的土壤硒含量皆不相同，各地植物所含的硒濃度也因此不同。一般而言，食物中的瘦肉、 柿子、蒜頭、海產、蔥、南瓜等含有多量的硒。動物製品的硒含量(約0.4-1.5μg/g)比植物體高；一般植物穀類的硒含量範圍可在<0.1μg /g─>0.8μg/g；在海洋生物中，硒類的含量也比植物多，但由於魚類(尤其是體內含汞的魚類)會形成汞─硒複合體，造成對硒的生物利用性極 低，故雖然硒在魚類的含量多但對於魚類本身的利用性極低；至於肉類會提供0.1-0.4μg/g；乳製品的硒含量則為<0.3μg/g。

另 外，全穀物和核果種子也是好的來源。在飲水中提供的硒攝取量十分有限，除非水流經含硒量高的土壤地區才可能有較高的含量。

植物中的硒是因 硒取代硫而進入植物體，硒型態有甲硒胺酸、硒胺酸與其代謝產物等。動物生長需要硒，在攝食植物時獲得甲硒胺酸。飲食中硒的形式取決於動植物食品的組合。

硒 的食物來源

食品名稱/重量
硒（μg）

鮪魚 / 3 oz
68

火腿（瘦肉）/ 3 oz
42

蛤蜊 / 3 oz
41

鮭魚 / 3 oz
40

意大利蛋麵 /1杯
35

沙 朗牛排 / 3 oz
28

雞胸肉 / 3 oz
20

Special K cereal
17

麩 燕麥片 / 1杯
14

全麥麵包 / 1片
10

燕麥糊 / 1/2杯
10

白麵包 / 1片
9

葡萄乾麥片 / 1杯
4

建議量

民眾的實際硒攝取量會因地而異，美國平 均每日81μg、加拿大每日113–220μg ，高於RDA。均飲食估計可提供約104-124 μg的硒。成人之上限攝取量(UL)訂為400μg。

硒的建議量在1980年只能根據估計而得，稱為Estimated safe and adequate dietary intake(ESADDI)；2000年則根據需要量之科學研究而訂定每日建議攝取量(RDA)。

過去曾有關於臺灣境內硒之飲食攝取量 的研究，分析結果六日飲食的硒攝取範圍在104~124μg（1.3~1.6μmol）/day，平均值為112μg（1.4μmol）/day，加上臺 灣非低硒區域，且食品貿易進出口抹去食品在硒含量上的地域性限制，推測臺灣境內應無硒營養缺乏的問題。

• 硒的營養來源:
  • 有機型式：甲 硒胺酸（selenomethione）、硒 半胱胺酸（selenocysteine）
  • 無機型式：硒 酸鹽（selenate）、亞 硒酸鹽（selenite ）
• 影響硒營養需求量的因素
  • 1.生物吸收率：見「吸收」。
  • 2.性別：早期來自中國研究報告， 當時硒缺乏現象比現在嚴重，在此情形顯示產齡女性較易罹患克山病（Keshan disease）；另外，過去20年報告顯示孩童不論男女有相同的比例罹患克山病；性別的影響必須在硒攝取量極低的情下才會顯現，假設考慮女性有較高機率 罹患克山病，硒對各年齡層的需求量將以男性參考體重為基準。

硒之膳食建議攝取量 (RDA)

年 齡
美國 (μg/day)
台灣 (μg/day)

0 個月~
AI=15
15

6個月~
AI=20
20

1歲~
20
20

4歲~
30
25

7歲~
30
30

10 歲~
40
40

13歲~
40
50

14歲~
55
50

孕婦
60
60

哺乳
70
70

• RDA（建議攝取量 Recommanded Dietary Allowances）：美國原始的飲食標準，代表同年齡層中，97~98％人的營養需求量。
• AI（足夠攝取量 Adequate Intake）：未能有足夠的實驗資訊建立EAR的情形下，所推估維持健康狀態的量，常用在一歲以下的嬰兒。

硒之上限攝 取量 (UL)

年齡
美國 (μg)
台灣 (μg)

0月~
45
35

3 月~
45
50

6月~
60
60

9月~
60
65

1歲~
90
90

4歲~
150
135

7歲~
150(4~8歲)
185

10歲~
280
280

13歲~
400(14歲~)
360

16歲~
400
400

19歲~
400
400

懷孕期
400
400

哺乳期
400
400

• UL（Tolerable Upper Intake Level 上限攝取量）：對於97~98％的人不可能產生不良健康影響之每日最大營養攝取量
  • 以 全靜脈注射營養（TPNTotal Parenteral Nutrition）為唯一營養來源者，需要硒的營養補充劑。
  • 有嚴重腸胃道疾病（例如：克 隆氏症）或曾移去一大段小腸者有硒營養缺乏的風險。
  • 碘營養缺乏者。研究指出硒缺乏會惡化碘缺乏的症狀，適當補充硒可以緩解 碘缺乏症狀以及在神經系統的影響。
  • 使用化療藥物者需要硒營養的補充。有研究指出，多種型態的硒可以減少化療藥物（例如：順鉑，cisplatin）所引發腎和骨髓的傷害。
  • 分 子特性
    • 硒蛋白質P(Selenoprotein P)- 是一種含有硒胺酸的血漿蛋白，也是一種運輸蛋白，主要是由肝臟合成，在血漿中大約有50％以上的硒是和含硒蛋白質P結合。含硒蛋白質P的結構最多可以帶有 十個硒胺酸殘基，當硒量下降時也會使殘基合成量下降。
    • α球蛋白（α-globulin）- 其中又分成α 1-globulin及α 2-globulin。兩者均為醣蛋白，亦皆可幫助脂質的運輸。其中α 2-globulin又有一些不同的功能：幫助血紅素的運輸、銅運輸、血液凝集以及調控氧化酶的活性。
    • β球蛋白（β- globulin）- 可以幫助脂質的運輸以及鐵和其他礦物質的運輸。
    • 榖 胱甘肽過氧化酶( glutathione peroxidase, GPX)
      • 這是研究最多的含硒酵素，因為最早發現硒的生化功能就是作為酵素榖胱甘肽過氧化酶的輔 基。榖胱甘肽過氧化酶有五種亞型，通常標記為GPX1, 2, 3, 4, 5，每一種的亞型存在於不同的組織，但是催化相同的反應。主要的功能是消除組織中的過 氧化氫(H₂O₂)和其他有機態過氧化物。還原過氧化物時，同時利用榖胱甘肽提供還原力(圖)。
    • 甲 狀腺素脫碘酶(Iodothyronine Deiodinases,IDI或DI)
      • 脫碘酶是含硒蛋白質，酵素的活性區是硒胺酸。已知有三種亞型。第一型存在肝臟、 腎臟和肌肉，第二型及第三型存在皮膚、腦下垂體、脂肪細胞和腦。主要功能是催化甲 狀腺素和相關代謝物脫去碘原子（圖）

        

        甲狀腺素脫碘酶的作用

        ，例如：5'- deiodinase(5'-DI)將T4型甲狀腺素脫碘轉換成T3型甲狀腺素，後者是體內活性最高的甲狀腺素，可調節代謝、生長及發育。去碘酶也會將 T4轉換反式T3(reverse T3)，催化產生反式T3的酵素是5-deiodinase。T3或是反式T3都可進一步脫碘產生T2或是3,3'-diiodothyronine，這 些都是沒有活性的代謝物。

    • 「硫氧化還原蛋白」還原酶(Thioredoxin Reductase，TrxR)
      • 酵素的活性區有硒胺酸，並含有FAD。此酵素存在血液、皮膚和肝臟等組織。主要反應是將氧 化態的「硫氧化還原蛋白」(thioredoxin) 中的雙硫鍵(disulfide bond)予以還原。還原態的「硫氧化還原蛋白」可以將氫原子提供給其他化合物。
    • 硒 代磷酸鹽合成酶(Selenophosphate Synthetase)
      • 硒代磷酸鹽合成酶有兩種亞型，其中一型含硒胺酸，催化硒離子磷酸化成硒代磷酸鹽的反應，這是合成 含硒蛋白質的必備原料。
    • 硒蛋白質P(Selenoprotein P)
      • 這 是硒的運輸蛋白質。有移除自由基的作用，具有抗氧化劑的功能。當體內的硒含量不足時，硒蛋白質P會優先獲得硒。
    • 硒蛋白質W(Selenoprotein W)
      • 含有硒胺酸，主要存在心肌、骨骼肌和其它組織的細胞質中，可能 扮演抗氧化劑的功能。
      • 步驟一：tRNASec與Serine經由Seryl-tRNA synthetase作用，生成Seryl-tRNASec。
      • 步驟二: 硒離子和ATP經由硒代磷酸鹽合成 酶反應生成硒代磷酸鹽。
      • 步驟三：Seryl-tRNASec和硒代磷酸鹽經由硒胺酸合成酶作用，產生含硒胺基酸殘基 Selenocysteyl-tRNASec。
      • 步驟四：轉譯時由SBP2和SECIS結合，繼而和tRNASec-eEFsec複 合物結合，再與核醣體作用而誘導硒胺酸插入蛋白質。
      • 動物缺硒

    硒蛋白的硒胺酸是在轉譯過程合成並直接利用的，稱為轉譯插入反應 （translational incorporation）。合成途徑需要的蛋白質有：硒 胺酸合成酶 selenocysteine synthase、硒半胱胺酸專用延長因子selenocysteine-specific elongation factor、selenocysteine-specific tRNA(tRNASec)、硒代磷酸鹽合成 酶 selenophosphate synthetase。硒胺酸對應的基因密碼是UGA，此密碼通常當做終止密碼，但若配合mRNA序列3』 端未轉譯區域具有獨特的二級結構SECIS(selenocysteine insertion sequence)，則成為轉譯硒胺酸的密碼。

    自然界中有許多細菌、植物或動物都能利用硒化氫(hydrogen selenide) 合成多種有機化合物，如大蒜中的 selenide garlic就是含有高單位的Se-methylselenocysteine。人類需要直接攝取有機的硒化物。自然界中甲硒胺酸(selenomethionine) 插入硒蛋白質中是直接取代甲硫胺酸(methionine)的位置而 得。也就是說在含甲硒胺酸的蛋白質合成過程先由甲硫胺酸編入，然後再接上硒成為甲硒胺酸並沒有特殊密碼。人類再利用甲硒胺酸釋出的硒，先合成磷酸硒 (selenophosphate)再生成硒半胱胺酸(selenocysteine, Sec)或其他小分子，再利用UGA的密碼將Sec編入人體的特殊蛋白硒蛋白質。人體硒的儲存者可能是硒蛋白質 P(selenoprotein p)，在已被發現的14種硒蛋白質中只有硒蛋白質 P含有10到12個Sec，其Sec數可以隨血中硒的濃度而改變。其他的硒蛋白質都只有單一個Sec。所以硒帶蛋白質 P可能是人類硒的儲存池，當食物中硒供應不足時硒蛋白質 P就會釋出硒供人體利用，但是 至今仍無法證實其功能。

    合成反應主要有四個步驟

    缺乏症候群

    硒缺乏會引起牲畜類動物疾病。硒缺乏造成硒蛋白質酵素活性下降。若硒以外之營養狀況良好，硒缺乏僅造成輕微的臨床症狀。若伴有營養不良、化學藥 物、感染等壓力，則會動物會出現嚴重病症。例如：硒缺乏加上維生素 E缺乏可導致大鼠與豬的之脂質過氧化與肝臟壞死，使豬、牛、羊的心臟損傷。在受感染的小鼠體內，硒缺乏可導致非致病性的coxsackie B3病毒轉變為具致病性的病毒，而造成小鼠的心肌炎。

  含硒胺基酸和無機態硒都會在組織中進行代 謝。從飲食而來的甲硒胺酸其利用情形和甲硫胺酸相似，可儲存在胺基酸代謝池中，用於合成蛋白質，也可代謝成硒胺酸和硒胱胺酸。

  硒胺酸可以 從飲食中直接得到，或是經由甲硒胺酸代謝而來。硒胺酸經由selenocysteine β-lyase作用之後產生游離態硒。游離態硒可以從榖胱甘肽(GSH)得到氫，然後生成硒化物(selenide)。硒化物有兩個代謝途徑，其一是經過 甲基化作用後藉由尿液排出體外，或是形成硒代磷酸鹽(selenophosphate)，這是體內重要含硒酵素的前驅物，例如5'-脫碘酶(5'- deiodinase)或榖胱甘肽過氧化酶(glutathione peroxidase)。

  從食物中得來的硒酸鹽在體內可轉換成亞硒 酸鹽，更進一步代謝成selenodiglutathione及硒離子，後者成為硒蛋白或酵素的原料。

  硒最主要的功能是作為各種en:selenoprotein硒 蛋白的組成分，進而影響其酵素活性或功能。

有機和無機形式的硒都可以很有效率的被吸收，只是發生在不同的腸道部位；吸收率並非調控動物體硒之恆態的機制。十二指腸是硒主要的吸收位置，空腸 和迴腸則有少量的吸收，但胃則沒有吸收硒之能力。甲硒胺酸的吸收效率比亞 硒酸鹽(selenite) 來的好。含有硒的胺基酸吸收是利用胺基酸運送系統，吸收率可達到 80％。甲硒胺酸的吸收率比硒胺酸好。在某些研究中亞硒酸鹽的吸收率可達到85％以上，因與腸道中物質的交互作用，吸收率較有變化。一但吸收後，保留程度 高於硒酸鹽。硒 酸鹽(selenate) 的吸收又比亞硒酸鹽好，幾乎被完全吸收；但併入組織前，大部分會由尿中排除。

維生素A、維生素C、維生素E都 會增加硒的吸收，當在小腸腔的榖 胱甘肽(glutathione, GSH)濃度低時也會增加吸收。重金屬(例：水銀)和植酸被認為會抑制硒的吸收。

高劑量的維生素C、鋅及重金屬（例如：汞）會減少硒的 吸收；但若在飲食中合併食用硒及維生素C，硒可以和飲食中的胺基酸形成保護結構而不影響其吸收。

小腸吸收之硒會和運輸蛋白結合經血液攜帶 至肝和其他組織。腎臟、肝臟、心臟、胰臟和肌肉都是硒含量較高的組織，肺臟、腦部、骨骼和紅血球也含有硒。目前如何調控硒從組織釋放到血漿裡或是組織從血 漿裡吸收的作用機制仍然不明。存在血漿中的硒，與許多不同分子結合成不同的形式存在着。其中最多的就是硒胺酸（Selenocysteine）：由硒原子 取代原本在Cysteine中的硫原子而存在，由硒蛋白質P（Selenoprotein P）這個運輸蛋白所攜帶，而這個運輸形式在血漿中也佔了一半以上。其它類型的運輸形式還有甲硒胺酸（Selenomethionine），由硒原子取代原 本在Methionine中的硫原子而存在，也是由硒蛋白質P所攜帶；除了這兩種有機硒之外，也有無機硒的運輸形式：硒酸鹽、亞硒酸鹽、氫化硒，與在人體 血液中α球蛋白及β球蛋白的硫氫基( sulfhydryl groups)結合，例如：極低密度脂蛋白(VLDL)和低密度脂蛋白(LDL)。

而 前述各種帶有硒且存在於血漿中的分子，均會被細胞所吸收。而細胞則釋放甲基化的硒化物至血漿中，再經由尿液將其排出體外。

人體缺硒

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      • • 克山病的主要病癥為心肌病變(cardiomyopathy)，包 括心律加快、心電圖異樣、充血性心臟衰竭、心臟組織的多病灶壞疽等，嚴重時會導致生命危險甚至死亡。
      • • 克山症（Keshan disease）^[16]是一 種因為飲食缺乏微量元素硒所造成的充血性心肌病變症。此病症的命名來自於中華人民共和國北方的黑龍江省克山縣，黑龍江省克山縣是此病高流行的地區，發現是 因為此地的土壤缺乏硒。克山症會造成心肌病變，好發於孩童和懷孕的婦女。補充硒可以改善病症，目前也發現此病症和病毒感染有關；特別是心肌病毒感染，如科 薩奇病毒引起的心肌炎或感染過敏性心肌炎。本病的發生除了黑龍江省之外，在吉林、遼寧、內蒙古、河北、河南、山東、山西、陝西、甘肅、四川、雲南、西藏等 地區都有病例，且病區多在荒僻山丘、高原及草原的農村，城鄉地區較少發病。
        • 克山病的症狀主要是造成擴張性心肌病變（Dilated Cardiomyopathy）。心肌呈變形、壞死、和疤痕形成。心臟擴張腫大，多數左心室擴張比右心室嚴重。心臟的切面可以看到大小不等黃色、灰白色壞 死、纖維化的疤痕；在顯微鏡下也可以觀察到心肌變性、肌纖維腫大、壞死的現象。適量的硒對缺硒造成的心肌損害有明顯的保護作用及抗氧化能力。硒是GSH- px的組成成分之一，該酶的主要作用是還原脂質過氧化物，清除自由基進而保護細胞膜的完整性。而低硒會造成GSH-px活性降低，造成心肌膜系統損傷。
        • 克山症的臨床症狀主要為急性和慢性心功能不全、心臟擴大、心律不整以及腦、肺、腎等栓塞，根據1982年中華人民共和國全國克山病 防治經驗交流會上的分形如下：
          • 急性:突然發病的狀況，在中華人民共和國北方，急型病多發生在冬季，會因寒冷、過勞、感染、暴飲暴食 或分娩等誘因而發病。重症者會出現心源性休克、急性肺水腫和嚴重心律失常的症狀。一開始可能感到頭暈、心窩部不適、反覆惡心嘔吐、吐黃水，繼而煩躁不安。 嚴重者可在數小時或數天內死亡。患者常會面色蒼白，四肢冰冷，血壓降低，呼吸減慢。心臟一般輕度大，心音弱，尤其第一心音減弱，舒張期和收縮期會出現雜 音。心律不整，主要為室性早搏、陣發性心動過速和房室傳導阻滯。急性心衰竭時肺部出現雜音，此外肝腫大和下肢水腫亦常見。
          • 亞急型： 發病不如急型快速。患者多為幼童，2～5歲佔85%。以春、夏季發病為多數。會出現心源性休克或充血性心力衰竭。發病初期表現為精神萎靡、咳嗽、呼吸急 促、食欲不振、面色灰暗和全身水腫。亦會出現心臟擴大、奔馬律和肝腫大。腦、肺、腎等處的栓塞並不少見。
          • 慢型：起病緩慢，很難被病 患所察覺，亦可由急型、亞急型或潛在型轉化而來。臨床表現主要為慢性充血性心力衰竭，有心悸、呼吸急促，勞累後加重，並會有少尿、水腫和腹水。體檢觀察發 現心臟向兩側明顯擴大，心音低，會聽到輕中度收縮期雜音和舒張期奔馬律，晚期可能出現右心衰竭的體徵如頸靜脈恕張、肝腫大和下肢浮腫等。嚴重者有胸、腹腔 積液，心源性肝硬化等症狀。心律不整的症狀如室性早搏、心動過速、傳導阻滯、心房顫動等。
          • 潛在型：可發生在平時看似健康的人，亦可 為其他型好轉的階段。前者常無症狀，可照常勞動或工作，而在普查中被發現，此屬穩定的潛在型。由其他型轉變而來者可有心悸、呼吸急促、頭昏、無力等症狀。心電圖會 有ST-T變化，QT間期延長和過早搏動。潛在型心臟雖受損，但心功能代償良好。心臟不增大或輕度增大。
        • 克山 症的預防措施首應注意環境衞生和個人衞生。保護水源，改善水質。改善營養條件，防止偏食，尤其對孕婦、產婦和兒童更應加強補充蛋白質，各種維生素及人體必 需的微量元素，包括鎂、碘等，並防治大骨節病、地方性甲狀腺病。 且流行區推廣預防性服藥 採用硒酸鈉作為預防性服藥，經多年推廣，證明可明顯降低發病率。通常採用每10天口服一次，1～5歲1mg，6～10歲2mg，11～15歲3mg，16 歲以上4mg。非發病季節可停服三個月。此外，流行區推薦使用含硒食鹽。農村使用含硒液浸過的種子種植。植物根部施加含硒肥料以提高農作物中含硒量。
      • • 溪山症的主要病徵為骨關節病變(osteoarthropathy)，包含骨關節、小腿、手臂的軟骨骺版退化與 壞死。此疾病為地域性、多發性、變形性骨關節病變，出現於亞洲低硒地區青春期前兒童與青少年。上述症狀僅發生於硒缺乏者，但改善硒營養狀況並無法完全避免 此疾病。
        • 兒童和全靜脈營養病人發生硒缺乏時，易導致關節僵硬、肌肉痛、頭髮和皮膚失去色素顏色、生長遲滯、指甲白化等症狀。生長遲 滯的現象與硒在甲狀腺素的代謝有關。

    人體攝取不足時，會造成克山病(Keshan disease)和溪山症(Kashin-Beck disease)。

• 化合物形式與毒性

元素態的硒和大部分的金屬硒化物毒性較小，因為生 物可用性(bioavailability)小。硒酸鹽和亞硒酸鹽的毒性較大，硒化氫(hydrogen selenide)的毒性最大，是一種氣狀的硒化合物。有機態硒化物如甲硒胺酸和硒胺酸與含硫胺基酸相似,因此毒性較無機態硒為低，但其吸收率高，雖不致 造成急性毒害，但長期大量攝取，會產生與無機硒相似的中毒症狀。

硒中毒(selenosis)可能發生在工人以及攝取過多硒的族群。目前 訂定硒的上限攝取量為400μg/day；硒的副作用發生最低量 (LOAEL)為910 μg。攝食過量時，極易導致毛髮異樣、指甲脫落、腳趾甲異樣等副作用，不過並無飲食硒中毒的案例。

中毒的嚴重程度與所攝取的硒含量成正比 的關係。中毒的症狀包含：反胃嘔吐、疲勞、腹瀉、頭髮與指甲損壞、異常刺痛感等，也會干擾硫的正常代謝以及抑制蛋白質合成。服用含有高量硒的藥物會造成急 性硒中毒，嚴重過量會導致肝硬化, 肺 水腫(pulmonary edema)，甚至喪命。治療硒在體內不平衡所造成的症狀目標：1.降低關節炎症狀；2.降低血壓；3.改善皮膚、毛髮及指（趾）甲問題。

食 物硒含量取決於土壤硒含量。美國雖有高硒地區，但農業部(USDA)已確認這些地區，並禁止飼養動物作為食物來源。美加地區食物運銷系統發達，可確保個人 不會只攝食到當地農產，保障民眾硒攝取量不致過高或過低。

• 硒中毒的生化指標

硒蛋白質含量在硒 需要量達到後，即呈現飽和狀態，不再隨硒攝取量增加而上升，因此無法被用於評估硒的毒性。測量組織（血液、血漿）的硒含量有助於評估硒中毒的危險性。尿液 硒排除量在特定控制之條件下，可作為硒毒性的指標。臨床症狀如毛髮、指甲易碎裂脫落等常被報道，是主要的評估終點。硒的甲基化代謝物因測量誤差大，且受許 多因素影響，不適用於硒中毒指標。

與其他營養素的關係

體內含鉛量增多時會有硒濃度下降的現象。銅不足會降低榖胱甘肽 過氧化酶和5'-脫碘酶的活性。硒與甲硫胺酸的利用有關。從食物攝取的硒有一部分是甲硒胺酸的形式，可作為合成蛋白質的材料。當甲硫胺酸供應不足時，甲硒 胺酸會成為它的替代物而用在蛋白質的合成，而不會代謝成為硒離子以供利用，間接引發硒的不足。鐵的缺乏會減少榖胱甘肽過氧化酶的合成，減少組織中的硒濃 度。維生素E和榖胱甘肽過氧化酶同樣有抗氧化的功能，在使細胞膜和DNA免於自由基的攻擊機制上，硒和維生素E常 一起作用，在功能上也有互補作用，其中一者濃度較高會減低另一濃度較低者所造成的影響。

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Selenium (pronounced /sɨˈlɛniəm/ si-LEN-ee-əm or /sɨˈliːniəm/ si-LEE-nee-əm) 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 section Allotropes 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 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 are rare. The chief commercial uses for selenium today are in glassmaking and in chemicals and pigments. Uses in electronics, 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, it seems, requiring none.

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 of its toxicity to humans working in industry. It was also recognized as an important veterinary toxin. In 1954, the first hints of 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 selenocysteine is encoded by the codon TGA. The recoding mechanism was worked out first in bacteria and then in mammals (see SECIS element).

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. At the current time, 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.

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.

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

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.Isolation of selenium is often complicated by the presence of other compounds and elements.

Most elemental selenium comes as a byproduct of refining copper or producing sulfuric acid.

Industrial production of selenium often involves the extraction of selenium dioxide from residues obtained during the purification of copper. Commonly, production begins by oxidation with sodium carbonate to produce selenium dioxide. The selenium dioxide is then mixed with water and the solution is acidified to form selenous acid (oxidation step). Selenous acid is bubbled with sulfur dioxide (reduction step) to give elemental selenium.

Elemental 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 densest form of selenium is the electrically conductive gray (trigonal) form, which is composed of long helical chains of selenium atoms (see figure). 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. However, selenium does not exhibit the unusual changes in viscosity that sulfur undergoes when gradually heated.

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), which, for practical purposes, can be considered to be stable. Twenty-three other unstable isotopes have been characterized.

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

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). It is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient that 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).

The glutathione peroxidase family of enzymes (GSH-Px) catalyze certain reactions that remove reactive oxygen species such as hydrogen peroxide and organic hydroperoxides:

2 GSH + H₂O₂----GSH-Px → GSSG + 2 H₂O

Selenium also plays a role in the functioning of the thyroid gland and in every cell that utilizes thyroid hormone, by participating as a cofactor for the three known thyroid hormone deiodinases, which activate and then deactivate various thyroid hormones and their metabolites. It may inhibit Hashimotos's disease, in which the body's own thyroid cells are attacked as alien. A reduction of 21% on TPO antibodies was reported with the dietary intake of 0.2 mg of selenium.

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). In descending order of concentration, high levels are also found in kidney, tuna, crab, and lobster.

The human body's burden of selenium is believed to be in the 13-20 milligram range.

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 This 400 microgram Tolerable Upper Intake Level is based primarily 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. 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. 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. A dose of selenium as small as 5 mg per day can be lethal for many humans.

Reference ranges for blood tests, showing selenium in purple in center

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. 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. Hydrogen selenide is an extremely toxic, corrosive gas. 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.

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 pharmacy released a statement explaining 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. Analysis of inorganic compounds of the vitamin supplement indicated 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. It was later confirmed that selenium was the ingredient in question.

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.

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 on advanced-aged people (over 90). Also, people dependent on food grown from selenium-deficient soil are also at risk. However, although New Zealand has low levels of selenium in its soil, adverse health effects have not been detected.

Selenium deficiency may only occur when a low selenium status is linked with an additional stress such as chemical exposure or increased oxidant stress due to vitamin E deficiency.

There are interactions between selenium and other nutrient such as iodine and vitamin E. The interaction is observed in the etiology of many deficiency diseases in animals and pure selenium deficiency is in fact rare. The effect of selenium deficiency on health remains uncertain, in particular, in relation to Kashin-Beck disease.

Cancer

Several studies have suggested a possible link between cancer and selenium deficiency. 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.The preventative effect observed in the NPC was greatest in those with the lowest baseline selenium levels.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". 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.. There was a slight trend toward more prostate cancer in the SELECT trial, but in the vitamin E only arm of the trial, where no selenium was given.

Dietary selenium prevents chemically induced carcinogenesis in many rodent studies. 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. The SU.VI.MAX study 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.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. 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.

In March 2009, Vitamin E (400 IU) and selenium (200 micrograms) supplements were reported to affect gene expression and can act as a tumor suppressor. 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”.In an attempt to rationalize the differences between epidemiological and in vitro studies and randomized 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”.

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 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. 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 T-cell associated genes encode selenoproteins similar to human glutathione peroxidase. Depleted selenium levels in turn lead to a decline in CD4 helper T-cells, further weakening the immune system.

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.

Tuberculosis

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

Diabetes

A well-controlled study showed that selenium intake is positively correlated with the risk of developing type 2 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.

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.

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.

Early photographic light meters used selenium but this application is now obsolete.

The substance loosely called selenium sulfide (approximate formula SeS₂) is the active ingredient in some anti-dandruff shampoos.The selenium compound kills 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.

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.

Selenium may be measured in blood, plasma, serum or urine to monitor excessive environmental or occupational exposure, confirm a diagnosis of poisoning in hospitalized victims or to assist in a forensic investigation in a case of fatal overdosage. Some analytical techniques are capable of distinguishing organic from inorganic forms of the element. Both organic and inorganic forms of selenium are largely converted to monosaccharide conjugates (selenosugars) in the body prior to being eliminated in the urine. Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations.

Main article: Evolution of dietary antioxidants

Over three billion years ago, blue-green algae were the most primitive oxygenic photosynthetic organisms and are ancestors of multicellular eukaryotic algae. 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. It has been 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, 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 (GSH-Px) antioxidant enzymes; seaweeds accumulate high quantity of selenium and iodine. In 2008, a study 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 an in vitro assay with the blood cells of 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, where selenoprotein peroxiredoxins protect bacterial and eukaryotic cells against oxidative damage. Selenoprotein families of GSH-Px and the deiodinases of eukaryotic cells seem to have a bacterial phylogenetic origin. The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchin, fish and chicken. Selenium enzymes are involved in utilization of the small reducing molecules glutathione and thioredoxin. One family of selenium-containing molecules (the glutathione peroxidases) destroy peroxide and repair damaged peroxidized cell membranes, using glutathione. Another selenium-containing enzyme in some plants and in animals (thioredoxin reductase) generates reduced thioredoxin, a dithiol that serves as an electron source for peroxidases and also the important reducing enzyme ribonucleotide reductase that makes DNA presursors from RNA precursors.

At 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. 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.Marine organisms 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 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 (including 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 constitute another family of eukaryotic selenoproteins with identified enzyme function. Deiodinases are able to extract electrons from iodides, and iodides from iodothyronines. They are, thus, involved in thyroid-hormone regulation, participating in the protection of thyrocytes from damage by H₂O₂ produced for thyroid-hormone biosynthesis.About 200 Mya, new selenoproteins were developed as mammalian GSH-Px enzymes.

Chalcogen compounds

Selenium forms two oxides: selenium dioxide (SeO₂) and selenium trioxide (SeO₃). Selenium dioxide is formed by the reaction of elemental selenium with oxygen:

Se₈ + 8 O₂ → 8 SeO₂

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 oxidising elemental selenium with nitric acid:

3 Se + 4 HNO₃ → 3 H₂SeO₃ + 4 NO

Salts of selenous acid are called selenites. These include silver selenite (Ag₂SeO₃) and sodium selenite (Na₂SeO₃).

Hydrogen sulfide reacts with aqueous selenous acid to produce selenium disulfide:

H₂SeO₃ + 2 H₂S → SeS₂ + 3 H₂O

Selenium disulfide consists of 8-membered rings of sulfur atoms with selenium replacing some of the sulfur atoms. It has an approximate composition of SeS₂, with individual rings varying in composition, such as Se₄S₄ and Se₂S₆. It has various applications, including use in shampoo as an anti-dandruff agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in fireworks.

Unlike sulfur, which forms a stable trioxide, selenium trioxide is unstable and decomposes to the dioxide above 185 °C:

2 SeO₃ → 2 SeO₂ + O₂ (ΔH = −54 kJ/mol)

Selenium trioxide may be synthesized by dehydrating selenic acid, H₂SeO₄, which is itself produced by the oxidation of selenium dioxide with hydrogen peroxide:

SeO₂ + H₂O₂ → H₂SeO₄

Hot, concentrated selenic acid is capable of dissolving gold, forming gold(III) selenate.

Selenium reacts with fluorine to form selenium hexafluoride:

Se₈ + 24 F₂ → 8 SeF₆

Unlike its sulfur counterpart, sulfur hexafluoride, however, SeF₆ is more reactive and is a toxic pulmonary irritant. It can cause frostbite and severe irritation on contact with skin.

Other selenium halides include SeF₄, Se₂Cl₂, SeCl₄, and Se₂Br₂. Selenium dichloride (SeCl₂), an important reagent in the study of selenium chemistry, may be prepared in pure form by reacting elemental selenium with SO₂Cl₂ in THF solution. Some of the selenium oxyhalides, such as SeOF₂, are useful as nonaqueous solvents.

Like oxygen and sulfur, selenium forms selenides with metals. For example, reaction with aluminum forms aluminum selenide:

3 Se₈ + 16 Al → 8 Al₂Se₃

Other selenides include mercury selenide (HgSe), lead selenide (PbSe), and zinc selenide (ZnSe). An important selenide is copper indium gallium selenide (Cu(Ga,In)Se₂), a semiconductor.

Selenium does not react directly with hydrogen; so hydrogen selenide, the analogue of hydrogen sulfide and water, is prepared by first reacting selenium with a metal to produce a selenide, and then protonating the selenide anion with an acid to produce H₂Se.

Tetraselenium tetranitride, Se₄N₄, is an explosive orange compound analogous to S₄N₄ It can be synthesized by the reaction of SeCl₄ with [((CH₃)₃Si)₂N]₂Se in dichloromethane solution at −78 °C.

Selenium reacts with cyanides to yield selenocyanates.For example:

8 KCN + Se₈ → 8 KSeCN

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（６）ダイヤモンド・エンド・ミ ル

（７）医療用品エ ンド・ミル設計

（８）自動車部品＆材料加工向けエンド・ミル設計

弊社の製品の供給調達機能は：

（１）生活産業～ハイテク工業までのエンド・ミル設計

（２）ミクロ・エ ンド・ミル～大型エンド・ミル供給

（３）小Ｌｏｔ生産～大量発注対応供給

（４）オートメーション整備調達

（５）スポット対応～流れ生産対応

弊社 の全般供給体制及び技術自慢の総合専門製造メーカーに貴方のご体験を御待ちしております。

Bewise Inc. talaşlı imalat sanayinde en fazla kullanılan ve üç eksende (x,y,z) talaş kaldırabilen freze takımlarından olan Parmak Freze imalatçısıdır. Çok geniş ürün yelpazesine sahip olan firmanın başlıca ürünlerini Karbür Parmak Frezeler, Kalıpçı Frezeleri, Kaba Talaş Frezeleri, Konik Alın Frezeler, Köşe Radyüs Frezeler, İki Ağızlı Kısa ve Uzun Küresel Frezeler, İç Bükey Frezeler vb. şeklinde sıralayabiliriz.

BW специализируется в научных исследованиях и разработках, и снабжаем самым высокотехнологичным карбидовым материалом для поставки режущих / фрезеровочных инструментов для почвы, воздушного пространства и электронной индустрии. В нашу основную продукцию входит твердый карбид / быстрорежущая сталь, а также двигатели, микроэлектрические дрели, IC картонорезальные машины, фрезы для гравирования, режущие пилы, фрезеры-расширители, фрезеры-расширители с резцом, дрели, резаки форм для шлицевого вала / звездочки роликовой цепи, и специальные нано инструменты. Пожалуйста, посетите сайт  www.tool-tool.com  для получения большей информации.

BW is specialized in R&D and sourcing the most advanced carbide material with high-tech coating to supply cutting / milling tool for mould & die, aero space and electronic industry. Our main products include solid carbide / HSS end mills, micro electronic drill, IC card cutter, engraving cutter, shell end mills, cutting saw, reamer, thread reamer, leading drill, involute gear cutter for spur wheel, rack and worm milling cutter, thread milling cutter, form cutters for spline shaft/roller chain sprocket, and special tool, with nano grade. Please visit our web  www.tool-tool.com  for more info.