Banana equivalent dose is an informal measurement of ionizing 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-sized banana. Bananas contain naturally occurringradioactive isotopes, particularly potassium-40, one of several naturally-occurring isotopes of potassium. One BED is often correlated to 10 sievert ; however, in practice, this dose is not cumulative, as the principal radioactive component is excreted to maintain metabolic equilibrium. The BED is only meant to inform the public about the existence of very low levels of natural radioactivity within a natural food and is not a formally adopted dose measurement.
History
The origins of the concept are uncertain, but one early mention can be found on the RadSafenuclear safetymailing list in 1995, where Gary Mansfield of the Lawrence Livermore National Laboratory mentions that he has found the "banana equivalent dose" to be "very useful in attempting to explain infinitesimal doses to members of the public". A value of 9.82×10−8 sieverts or about was suggested for a banana.
Usage
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. The radiation exposure from consuming a banana is approximately 1% of the average daily exposure to radiation, which is 100 banana equivalent doses. The maximum permitted radiation leakage for a nuclear power plant is equivalent to 2,500 BED per year, while a chest CT scan delivers 70,000 BED. A lethal dose of radiation is approximately 35,000,000 BED. A person living from the Three Mile Islandnuclear reactor received an average of 800 BED of exposure to radiation during the 1979 Three Mile Island accident.
Dose calculation
Criticism
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. The committed dose in the human body due to bananas is not cumulative because the amount of potassium in the human body is fairly constant due to homeostasis, 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, 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 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 of 2.5 grams per kilogram, 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.
Radiation from other household consumables
Other foods rich in potassium include potatoes, kidney beans, sunflower seeds, and nuts. Brazil nuts in particular may also contain significant amounts of radium, which have been measured at up to 444 Bq/kg. Tobacco contains traces of thorium, polonium and uranium.
