Banana equivalent dose
Encyclopedia
A banana equivalent dose is a whimsical unit of radiation
exposure, informally defined as the additional dose a person will absorb from eating one banana
. It may be sometimes abbreviated as BED.
The concept is based on the fact that bananas, like most organic material, naturally contain a certain amount of radioactive isotopes—even in the absence of any artificial pollution
or contamination. The banana equivalent dose was meant to express the severity of exposure to radiation, such as resulting from nuclear power
, nuclear weapon
s or medical procedures, in terms that would make sense to most people.
s and informal publications since then..
unit; that is, a unit for measuring potentially damaging radiation absorbed by body tissue, rather than the total radiation (of any kind) emitted by a source or absorbed by matter. The corresponding SI unit is the sievert
(Sv), defined as a radiation dose biologically equivalent to one joule
of absorbed gamma-ray energy per kilogram
of tissue. In the U.S., an older unit, the roentgen equivalent man (rem), equal to 0.01 sieverts, is sometimes used.
The major natural source of radioactivity in plant tissue is potassium
, which in nature contains 0.0117% of the unstable isotope
potassium-40
(40K). This isotope decays with a half-life
of about 1.25 billion years (4×1016 seconds), and therefore the activity of natural potassium is about 31 Bq
/g
— meaning that, in one gram of the element, about 31 atom
s will decay per second
. Plants naturally contain other radioactive isotopes, such as carbon-14
(14C), but their contribution to the total activity is much smaller. Since a typical banana contains about half a gram of potassium, it will have an activity of roughly 15 Bq. (Although small in environmental and medical terms, the radioactivity of a few bananas is sufficient to trigger radiation sensors used to detect possible smuggling
of nuclear material
at U.S. ports.)
The absorbed radiation dose depends on the type and energy
of the emitted particle
s, as well as on the location of the source in the body (external, inhaled, ingested, etc.). According to the US Environmental Protection Agency (EPA), the conversion factor (CEDE) is 5.02 nanosieverts over 50 years for each becquerel of isotopically pure potassium-40 ingested by an average adult. Using this factor, one banana equivalent dose comes out as about 5.02 nSv/Bq × 31 Bq/g × 0.5 g ≈ 78 nanosieverts = 0.078 μSv. In informal publications one often sees this estimate rounded up to 0.1 μSv.
, so that any excess absorbed from food is quickly compensated by the elimination of an equal amount.
It follows that the additional radiation exposure due to eating a banana lasts only for a few hours after ingestion, namely the time it takes for the normal potassium contents 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 pure 40K; which was assumed by EPA to be 30 days. 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 normal potassium content of the human body
, 2.5 g per kg
, or 175 grams in a 70 kg adult. This potassium will naturally generate 175 g × 31 Bq/g ≈ 5400 Bq of radiation, through the person's lifetime.
es, kidney beans, sunflower seed
s, and nut
s; especially brazil nut
s, which may have up to 444 Bq/kg (12 nCi/kg) — four times the radioactivity of bananas.
By the same reasoning originally used to arrive at the banana equivalent dose, one may conclude that all the food consumed in one year results in a total exposure of about 0.4 mSv (40 mrem), which is more than 10% of the total dose absorbed from all natural and man-made sources in that same period. However, the same criticisms raised against the BED may apply to this estimate as well.
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
exposure, informally defined as the additional dose a person will absorb from eating one banana
Banana
Banana is the common name for herbaceous plants of the genus Musa and for the fruit they produce. Bananas come in a variety of sizes and colors when ripe, including yellow, purple, and red....
. It may be sometimes abbreviated as BED.
The concept is based on the fact that bananas, like most organic material, naturally contain a certain amount of radioactive isotopes—even in the absence of any artificial pollution
Pollution
Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
or contamination. The banana equivalent dose was meant to express the severity of exposure to radiation, such as resulting from nuclear power
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...
, nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
s or medical procedures, in terms that would make sense to most people.
History
The concept probably originated on the RadSafe nuclear safety mailing list in 1995, where a value of 9.82×10−8 sieverts or about 0.1 μSv was suggested. It has been used in many other internet forumInternet forum
An Internet forum, or message board, is an online discussion site where people can hold conversations in the form of posted messages. They differ from chat rooms in that messages are at least temporarily archived...
s and informal publications since then..
Relationship with standard units
The BED is supposed to be a radiation dose equivalentEquivalent dose
The equivalent absorbed radiation dose, usually shortened to equivalent dose, is a computed average measure of the radiation absorbed by a fixed mass of biological tissue, that attempts to account for the different biological damage potential of different types of ionizing radiation...
unit; that is, a unit for measuring potentially damaging radiation absorbed by body tissue, rather than the total radiation (of any kind) emitted by a source or absorbed by matter. The corresponding SI unit is the sievert
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...
(Sv), defined as a radiation dose biologically equivalent to one joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
of absorbed gamma-ray energy per kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
of tissue. In the U.S., an older unit, the roentgen equivalent man (rem), equal to 0.01 sieverts, is sometimes used.
The major natural source of radioactivity in plant tissue is potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, which in nature contains 0.0117% of the unstable isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
potassium-40
Potassium-40
Potassium-40 is a radioactive isotope of potassium which has a very long half-life of 1.248 years, or about 39.38 seconds.Potassium-40 is a rare example of an isotope which undergoes all three types of beta decay. About 89.28% of the time, it decays to calcium-40 with emission of a beta particle...
(40K). This isotope decays with a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of about 1.25 billion years (4×1016 seconds), and therefore the activity of natural potassium is about 31 Bq
Becquerel
The becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...
/g
Gram
The gram is a metric system unit of mass....
— meaning that, in one gram of the element, about 31 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s will decay per second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
. Plants naturally contain other radioactive isotopes, such as carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...
(14C), but their contribution to the total activity is much smaller. Since a typical banana contains about half a gram of potassium, it will have an activity of roughly 15 Bq. (Although small in environmental and medical terms, the radioactivity of a few bananas is sufficient to trigger radiation sensors used to detect possible smuggling
Smuggling
Smuggling is the clandestine transportation of goods or persons, such as out of a building, into a prison, or across an international border, in violation of applicable laws or other regulations.There are various motivations to smuggle...
of nuclear material
Nuclear material
Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", consisting of enriched uranium , uranium-233, and...
at U.S. ports.)
The absorbed radiation dose depends on the type and energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
of the emitted particle
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...
s, as well as on the location of the source in the body (external, inhaled, ingested, etc.). According to the US Environmental Protection Agency (EPA), the conversion factor (CEDE) is 5.02 nanosieverts over 50 years for each becquerel of isotopically pure potassium-40 ingested by an average adult. Using this factor, one banana equivalent dose comes out as about 5.02 nSv/Bq × 31 Bq/g × 0.5 g ≈ 78 nanosieverts = 0.078 μSv. In informal publications one often sees this estimate rounded up to 0.1 μSv.
Criticism
The validity of the banana equivalent dose concept has been challenged. Critics, including the EPA, pointed out that the amount of potassium (and therefore of 40K) in the human body is fairly constant because of homeostasisHomeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
, so that any excess absorbed from food is quickly compensated by the elimination of an equal amount.
It follows that the additional radiation exposure due to eating a banana lasts only for a few hours after ingestion, namely the time it takes for the normal potassium contents 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 pure 40K; which was assumed by EPA to be 30 days. 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 normal potassium content of the human body
Composition of the human body
The composition of the human body can be looked at from several different points of view.By mass, human cells consist of 65–90% water . Oxygen therefore contributes a majority of a human body's mass...
, 2.5 g per kg
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
, or 175 grams in a 70 kg adult. This potassium will naturally generate 175 g × 31 Bq/g ≈ 5400 Bq of radiation, through the person's lifetime.
Radiation from other foods
Besides bananas, other foods that are rich in potassium (and therefore in 40K) are potatoPotato
The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...
es, kidney beans, sunflower seed
Sunflower seed
The sunflower seed is the fruit of the sunflower . The term "sunflower seed" is actually a misnomer when applied to the seed in its pericarp . Botanically speaking, it is more properly referred to as an achene. When dehulled, the edible remainder is called the sunflower kernel.There are three types...
s, and nut
Nut (fruit)
A nut is a hard-shelled fruit of some plants having an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts...
s; especially brazil nut
Brazil Nut
The Brazil nut is a South American tree in the family Lecythidaceae, and also the name of the tree's commercially harvested edible seed.- Order :...
s, which may have up to 444 Bq/kg (12 nCi/kg) — four times the radioactivity of bananas.
By the same reasoning originally used to arrive at the banana equivalent dose, one may conclude that all the food consumed in one year results in a total exposure of about 0.4 mSv (40 mrem), which is more than 10% of the total dose absorbed from all natural and man-made sources in that same period. However, the same criticisms raised against the BED may apply to this estimate as well.
External links
- Radioactivity in food: your questions answered, Food Standards AgencyFood Standards AgencyThe Food Standards Agency is a non-ministerial government department of the Government of the United Kingdom. It is responsible for protecting public health in relation to food throughout the United Kingdom and is led by a board appointed to act in the public interest...