Monday, May 30, 2011

Incorrectly Correcting the Banana Equivalent Dose

Wikipedia is a living encyclopedia that changes with global events. The Banana Equivalent Dose entry has been corrected since the Fukushima incident, but was it really corrected?


The banana equivalent dose was introduced as a way to clarify the risk of radiation exposure that results from human activity, such as the use of nuclear power or medical procedures, by comparing it with the risk associated with natural doses. The BED calculation probably originated on a nuclear safety mailing list in 1995, where a value of 9.82×10-8 sieverts or about 0.1 ╬╝Sv was suggested.[1] However, that calculation has been criticized as misleading,[2] since excess potassium ingested (in the form of a banana) is quickly eliminated by the body.

In 2011, as the Fukushima nuclear disaster unfolded, the idea was popularised on xkcd[3] and slashdot.[4]

From this change it would seem that the BED is misleading, but the implication that the BED is misleading is more misleading than the original use of the BED.

The editor of the article implies that since the body constantly cycles potassium that the level of radioactive potassium 40 would remain constant and that other sources of radiation would not. While different types of radioactive isotopes do effect the body differently, the revision trivializes BED by not going into depth as to were it is effective and ineffective. Indicating that the editor may be emotionally or politically motivated.

One of the primary radioactive isotopes of concern following Fukushima is Cesium 137 contaminating food. Cesium 137 is chemically similar to potassium and like potassium also passes through the body. There are differences in the way Cs 137 and K40 react in the body. K40 has a half life of 1.3 billion years versus Cs137 with a half life of 30.7 years. Since Cs137 has a shorter half life, it tends to "pop" or have counts per second more often, so a smaller quantity of Cs137 produces the same quantity of radiation as a much larger quantity of K 40. However, the radiation concentration is based on "pops" so the health impact of Cs137 is virtually identical to potassium 40 at the same level of radiation. The editor's logic falls apart by assuming an atom for atom equivalent instead of a count per second equivalent. Based on count per count, the probability that a greater quantity of Cs137 will be absorbed than K40 is unlikely. The excess of each would be equally likely to pass through the body.

In the case of radioactive iodine, the BED dose needs to be qualified. Unlike potassium, iodine has a more limited role in the human body. Iodine is preferentially concentrated in the thyroid and low doses of radioactive iodine 131 are paradoxically more dangerous than high doses.

Like Iodine 131, where stable iodine reduces the absorption of radiation in the thyroid by filling iodine receptors, maintaining proper electrolyte levels reduces the absorption of Cesium 137 in the body.

With the exception of radioactive isotopes that have a greater tendency to accumulate in certain organs or glands, the impact per "pop" or count varies little in the human body. The Banana Equivalent Dose is an effective method of communicating radiation impact with limited qualifications.

It will be interesting to see if the Wikipedia Banana Equivalent Dose editor can revise his revision with a little more concrete wording and citations.

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