Banana equivalent dose (BED) is an informalunit of measurement ofionizing radiation exposure, intended as a general educational example to compare a dose of radioactivity to the dose one is exposed to by eating one average-sizedbanana. Bananas contain naturally occurringradioactive isotopes, particularlypotassium-40 (40K), one of several naturally occurringisotopes of potassium. One BED is often correlated to 10−7sievert (0.1 μSv); however, in practice, this dose is notcumulative, as the potassium in foods is excreted in urine to maintainhomeostasis.[1] The BED is only meant as an educational exercise and is not a formally adopted dose measurement.
The origins of the concept are uncertain, but one early mention can be found on theRadSafe nuclear safety mailing list in 1995, where Gary Mansfield of theLawrence Livermore National Laboratory mentions that he has found the "banana equivalent dose" to be "very useful in attempting to explain infinitesimal doses (and corresponding infinitesimal risks) to members of the public".[3] A value of 9.82×10−8sieverts or about 0.1microsieverts (10 μrem) was suggested for consuming a 150-gram (5.3 oz) banana.[citation needed]
The banana equivalent dose is an informal measurement, so any equivalences are necessarily approximate, but it has been found useful by some as a way to inform the public about relative radiation risks.[3]
The radiation exposure from consuming a banana is approximately 1% of the average daily exposure to radiation, which is 100 banana equivalent doses (BED). The maximum permitted radiation leakage for a nuclear power plant is equivalent to 2,500 BED (250 μSv) per year, while a chestCT scan delivers 70,000 BED (7 mSv). An acutelethal dose of radiation is approximately 35,000,000 BED (3.5 Sv, 350 rem). A person living 16 kilometres (10 mi) from theThree Mile Island nuclear reactor received an average of 800 BED of exposure to radiation during the 1979Three Mile Island accident.[5]
The major natural source of radioactivity in plant tissue ispotassium: 0.0117% of the naturally occurring potassium is the unstableisotope potassium-40. This isotopedecays with ahalf-life of about 1.25 billion years (4×1016 seconds), and therefore theradioactivity of natural potassium is about 31becquerel/gram (Bq/g), meaning that, in one gram of the element, about 31atoms will decay every second.[a][6] Plants naturally contain radioactivecarbon-14 (14C), but in a banana containing 15 grams of carbon this would give off only about 3 to 5 low-energybeta rays per second. Since a typical banana contains about half a gram of potassium,[7] it will have an activity of roughly 15 Bq.[8] Although the amount in a single banana is small in environmental and medical terms, the radioactivity from a truckload of bananas is capable of causing afalse alarm when passed through aRadiation Portal Monitor used to detect possiblesmuggling ofnuclear material at U.S. ports.[9]
The dose uptake from ingested material is defined ascommitted dose, and in the case of the overall effect on the human body of the radioactive content of a banana, it will be the "committed effective dose". This is typically given as the net dose over a period of 50 years resulting from the intake of radioactive material.
According to theUS Environmental Protection Agency (EPA), isotopically pure potassium-40 will give a committed dose equivalent of 5.02 nSv over 50 years per becquerel ingested by an average adult.[10] Using this factor, one banana equivalent dose comes out as about 5.02 nSv/Bq × 31 Bq/g × 0.5 g ≈ 78 nSv = 0.078 μSv. In informal publications, one often sees this estimate rounded up to 0.1 μSv.[4] The International Commission on Radiological Protection estimates a coefficient of 6.2 nSv/Bq for the ingestion of potassium-40,[11] with this datum the calculated BED would be 0.096 μSv, closer to the standard value of 0.1 μSv.
Several sources point out that the banana equivalent dose is a flawed concept because consuming a banana does not increase one's exposure to radioactive potassium.[12][13][1]
The committed dose in the human body due to bananas is not cumulative because the amount of potassium (and therefore of40K) in the human body is fairly constant due tohomeostasis,[14][15] so that any excess absorbed from food is quickly compensated by the elimination of an equal amount.[3][12]
It follows that theadditional radiation exposure due to eating a banana lasts only for a few hours after ingestion, i.e. the time it takes for the normal potassium content of the body to be restored by the kidneys. The EPA conversion factor, on the other hand, is based on the mean time needed for the isotopic mix of potassium isotopes in the body to return to the natural ratio after being disturbed by the ingestion of pure40K, which was assumed by EPA to be 30 days.[14] If the assumed time of residence in the body is reduced by a factor of ten, for example, the estimated equivalent absorbed dose due to the banana will be reduced in the same proportion.
These amounts may be compared to the exposure due to the normalpotassium content of the human body of 2.5 grams per kilogram,[16] or 175 grams in a 70 kg adult. This potassium will naturally generate 175 g × 31 Bq/g ≈ 5400 Bq of radioactive decays, constantly through the person's adult lifetime.
Other foods rich in potassium (and therefore in40K) includepotatoes,kidney beans,sunflower seeds, andnuts.[17][18]
Brazil nuts in particular (in addition to being rich in40K) may also contain significant amounts of radium, which have been measured at up to 444 Bq/kg (12 nCi/kg).[19][20]
Tobacco contains traces ofthorium,polonium anduranium.[21][22] The process of drying and then smoking the solid matter concentrates those radionuclides further, creating in essencetechnologically enhanced naturally occurring radioactive material.
...The average radiation dose to people living within 10 miles of the plant was 0.08 millisieverts...
...the average banana contains about 422mg of potassium...
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 from40K in the body is constant.
