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 weapons or medical procedures, in terms that would make sense to most people.
Contents |
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.[1]
The BED is supposed to be a radiation dose equivalent 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 atoms will decay per second.[2][3] 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,[4] 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.[5])
The absorbed radiation dose depends on the type and energy of the emitted particles, 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.[6] 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.[7]
The validity of the banana equivalent dose concept has been challenged. Critics, including the EPA,[8] pointed out that the amount of potassium (and therefore of 40K) in the human body is fairly constant because of homeostasis,[9] so that any excess absorbed from food is quickly compensated by the elimination of an equal amount.[1][10]
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,[11] 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.
Besides bananas, other foods that are rich in potassium (and therefore in 40K) are potatoes, kidney beans, sunflower seeds, and nuts;[12][13] especially brazil nuts, which may have up to 444 Bq/kg (12 nCi/kg) — four times the radioactivity of bananas.[14][15]
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.[16] However, the same criticisms raised against the BED may apply to this estimate as well.