香蕉等效劑量

香蕉等效劑量（Banana Equivalent Dose，BED）是一種非正式的電離輻射暴露劑量單位，旨在向大眾比較通常的輻射劑量與吃下一個平均尺寸香蕉所受到的天然放射性核素輻射劑量。香蕉包含著天然放射性的鉀-40 (⁴⁰K)。

1香蕉等效劑量=0.0778微西弗。計算的依據主要是由於香蕉含有大量的鉀元素，天然鉀當中約有百萬分之117(0.0117%)的天然放射性的同位素鉀40。鉀40由於半衰期長達12億5千萬年（地球壽命為45億年）。每克天然鉀的放射性活度，等於其中⁴⁰K的原子數量除以⁴⁰K原子以秒計量的平均壽命。每克天然鉀中⁴⁰K的原子數量是阿伏加德羅常數 6.022×10²³除以天然鉀的原子量39.0983克/摩爾再乘以⁴⁰K的豐度0.000117，結果為1.80×10¹⁸每克。對於指數衰減，平均壽命等於半衰期(3.94 × 10¹⁶秒)除以2的自然對數，結果為5.684×10¹⁶秒^[1]因此每1克天然鉀中，放射性活度約31貝克勒。而1根香蕉中，平均而言約含有0.5克的天然鉀，^[2]所以每根香蕉約含有放射性活度15.5貝克勒的放射性鉀40。依據美國環境保護署提供的轉換因數，每貝克的鉀40平均對每個成年人在50年期間造成的等效劑量為5.02納西弗。所以，1根香蕉的等效劑量=(15.5貝克勒)(5.02納西弗/貝克勒)=0.0778微西弗。1根香蕉中包含約15克碳中的碳-14(¹⁴C)提供了3-5次貝塔衰變每秒。

由於人體對鉀的新陳代謝保持了鉀的總量平穩，因此從香蕉等食物攝入的鉀並不會在人體內蓄積。因此攝入鉀-40的放射性劑量也不會在人體內蓄積。^[3]^[4] 因此任何從食品中過量吸收的鉀都會有等量的鉀被腎臟排出體外。^[5]^[6]

1根香蕉的輻射劑量，相當於每日的天然本底輻射的百分之一。即100根香蕉等效劑量。核電站對周邊環境的最大允許輻射暴露是每年2500根香蕉等效劑量(250 μSv)。胸部CT掃描是7萬根香蕉等效劑量(7 mSv)。致死劑量是3千5百萬根香蕉等效劑量(3500 mSv)。距離1979年三里島核事故10英里的人承受了800根香蕉等效劑量。^[7]

其他含鉀非常豐富的食物（如葵花籽、腰豆、馬鈴薯、巴西豆等）也有類似的等效劑量概念。

歷史

有據可查的香蕉等效劑量最早出現在1995年RadSafe（英語：RadSafe）核安全郵件列表中，勞倫斯利佛摩國家實驗室的Gary Mansfield提到了香蕉等效劑量在向公眾解釋極小劑量帶來極小風險時非常有用。^[5] ^[8]^[9]^[10]^[11]

參考文獻

^ Bin Samat, Supian; Green, Stuart; Beddoe, Alun H. The ⁴⁰K activity of one gram of potassium. Physics in Medicine and Biology. 1997, 42 (2): 407. Bibcode:1997PMB....42..407S. doi:10.1088/0031-9155/42/2/012.
^ Bananas & Potassium. [2011-07-28]. （原始內容存檔於2011-08-14）. ...the average banana contains about 422 mg of potassium... （頁面存檔備份，存於網際網路檔案館）
^ U. S. Environmental Protection Agency (1999), Federal Guidance Report 13 （頁面存檔備份，存於網際網路檔案館）, page 16: "For example, the ingestion coefficient risk for 40K would not be appropriate for an application to ingestion of ⁴⁰K in conjunction with an elevated intake of natural potassiumm. This is because the biokinetic model for potassium used in this document represents the relatively slow removal of potassium (biological half-time 30 days) that is estimated to occur for typical intakes of potassium, whereas an elevated intake of potassium would result in excretion of a nearly equal mass of natural potassium, and hence of ⁴⁰K, over a short period."
^ Eisenbud, Merril; Gesell, Thomas F. Environmental radioactivity: from natural, industrial, and military sources. Academic Press. 1997: 171–172. ISBN 978-0-12-235154-9. It is important to recognize that the potassium content of the body is under strict homeostatic control and is not influenced by variations in environmental levels. For this reason, the dose from ⁴⁰K in the body is constant.
^ ^5.0 ^5.1 RadSafe mailing list: original posting （頁面存檔備份，存於網際網路檔案館） and follow up thread （頁面存檔備份，存於網際網路檔案館）. FGR11 discussed.
^ Maggie Koerth-Baker. Bananas are radioactive—But they aren't a good way to explain radiation exposure. Aug 27, 2010 [2017-09-26]. （原始內容存檔於2011-05-28）. （頁面存檔備份，存於網際網路檔案館） (Accessed 25 May 2011). Attributes the title statement to Geoff Meggitt, former UK Atomic Energy Authority.
^ Three Mile Island Accident. [2015-10-25]. （原始內容存檔於2016-01-28）. ...The average radiation dose to people living within 10 miles of the plant was 0.08 millisieverts... （頁面存檔備份，存於網際網路檔案館）
^ Federal Guidance Report #11 （頁面存檔備份，存於網際網路檔案館） (table 2.2, page 156) Lists conversion factor of 5.02×10⁻⁹ Sv/Bq for committed effective dose equivalent of ingested pure potassium-40 (not of natural potassium).
^ Environmental and Background Radiation （頁面存檔備份，存於網際網路檔案館）, Health Physics Society （頁面存檔備份，存於網際網路檔案館）.
^ Radiation chart. [2017-09-26]. （原始內容存檔於2011-07-05）. （頁面存檔備份，存於網際網路檔案館）
^ Tom Watson. Radioactive Banana! Peeling Away the Mystery. Feb 26, 2012 [2017-09-26]. （原始內容存檔於2012-04-08）. （頁面存檔備份，存於網際網路檔案館） (Accessed 14 March 2012).

外部連結

Radioactivity in food: your questions answered （頁面存檔備份，存於網際網路檔案館）, Food Standards Agency

[Samat-1] Bin Samat, Supian; Green, Stuart; Beddoe, Alun H. The ⁴⁰K activity of one gram of potassium. Physics in Medicine and Biology. 1997, 42 (2): 407. Bibcode:1997PMB....42..407S. doi:10.1088/0031-9155/42/2/012.

[2] Bananas & Potassium. [2011-07-28]. （原始內容存檔於2011-08-14）. ...the average banana contains about 422 mg of potassium... （頁面存檔備份，存於網際網路檔案館）

[fgr13-3] U. S. Environmental Protection Agency (1999), Federal Guidance Report 13 （頁面存檔備份，存於網際網路檔案館）, page 16: "For example, the ingestion coefficient risk for 40K would not be appropriate for an application to ingestion of ⁴⁰K in conjunction with an elevated intake of natural potassiumm. This is because the biokinetic model for potassium used in this document represents the relatively slow removal of potassium (biological half-time 30 days) that is estimated to occur for typical intakes of potassium, whereas an elevated intake of potassium would result in excretion of a nearly equal mass of natural potassium, and hence of ⁴⁰K, over a short period."

[homeostasis-4] Eisenbud, Merril; Gesell, Thomas F. Environmental radioactivity: from natural, industrial, and military sources. Academic Press. 1997: 171–172. ISBN 978-0-12-235154-9. It is important to recognize that the potassium content of the body is under strict homeostatic control and is not influenced by variations in environmental levels. For this reason, the dose from ⁴⁰K in the body is constant.

[radsafelist-5] 5.0 ^5.1 RadSafe mailing list: original posting （頁面存檔備份，存於網際網路檔案館） and follow up thread （頁面存檔備份，存於網際網路檔案館）. FGR11 discussed.

[meggitt-6] Maggie Koerth-Baker. Bananas are radioactive—But they aren't a good way to explain radiation exposure. Aug 27, 2010 [2017-09-26]. （原始內容存檔於2011-05-28）. （頁面存檔備份，存於網際網路檔案館） (Accessed 25 May 2011). Attributes the title statement to Geoff Meggitt, former UK Atomic Energy Authority.

[7] Three Mile Island Accident. [2015-10-25]. （原始內容存檔於2016-01-28）. ...The average radiation dose to people living within 10 miles of the plant was 0.08 millisieverts... （頁面存檔備份，存於網際網路檔案館）

[fgr11-8] Federal Guidance Report #11 （頁面存檔備份，存於網際網路檔案館） (table 2.2, page 156) Lists conversion factor of 5.02×10⁻⁹ Sv/Bq for committed effective dose equivalent of ingested pure potassium-40 (not of natural potassium).

[hps.org2-9] Environmental and Background Radiation （頁面存檔備份，存於網際網路檔案館）, Health Physics Society （頁面存檔備份，存於網際網路檔案館）.

[xkcd-10] Radiation chart. [2017-09-26]. （原始內容存檔於2011-07-05）. （頁面存檔備份，存於網際網路檔案館）

[apcal-11] Tom Watson. Radioactive Banana! Peeling Away the Mystery. Feb 26, 2012 [2017-09-26]. （原始內容存檔於2012-04-08）. （頁面存檔備份，存於網際網路檔案館） (Accessed 14 March 2012).

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