Go Figure: What bananas tell us about radiation via BBC

There’s been concern about radiation after damage to Japan’s Fukushima nuclear plant in March and now a hot spot has been detected in Tokyo. But how do we think of radiation in ordinary terms, asks Michael Blastland in his regular column.

Freaky, isn’t it, radiation? Invisible, baffling, harmful (bombs) and helpful (X-rays) or both (nuclear power). And we go nuts if someone wants to put a load in our backyard even though it’s everywhere.

How much easier if our exposure to the hazards of radiation could all be reduced to bananas. Actually, it can, sort of. Welcome to the Banana Equivalent Dose or BED.


But I reckon the BED is useful for several reasons. First, it reminds us that radiation is commonplace. You can’t get much more ordinary than a banana.

Second, we know eating one banana won’t kill us. Not even nearly. Not without extreme violence. This affirms an age-old point about toxicity – that danger is in the dose. In other words most things, radiation included, are only dangerous in sufficient quantities. The distinction between toxic and safe is not really a distinction of kind, but of quantity. That goes for just about everything from water and vitamins to arsenic.

Third, think about eating 20 million bananas, equal to a dose causing severe, sometimes fatal, radiation poisoning. You’d probably die from something other than the radiation well before you were anywhere near 20 million. Do not attempt this at home. Even over an 80-year lifetime it’s nearly 700 a day. Brings to mind Cool Hand Luke’s 50 eggs in one hour.

So by putting all radiation exposure on one scale, the banana scale, we see clearly how huge a scale it is. At low doses the bananas come in bunches, then rise through the thousands to the millions, corresponding to micro-sieverts, milli-sieverts and sieverts, the SI unit.


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This article, while somewhat informative for the uninitiated, is rather misleading from a scientific perspective. The “banana equivalent dose” is frowned upon by radiation protection specialists like me. While it’s true that bananas contain potassium and, by extension, radioactive potassium-40, humans don’t simply absorb all of the radiation that the potassium-40 emits. The body keeps a more or less constant inventory of all the potassium it needs. When you ingest potassium, some of it is retained and the extra potassium is excreted. As a result, some of the “banana equivalent dose” is not retained in the body but passes right through. Because this amount also differs from person to person, it’s not a good method of comparison. Comparing it to a known quantity, such as a chest or dental x-ray, would be more scientifically accurate while allowing you to make the same point to your readers.

John Harvey PhD, Atlanta, Georgia, United States


Bananas are radioactive because they contain some Potassium-40. So do many things. But the reason this idea is absurd is that different radioisotopes exist which have different biological affinities. Potassium is uniformly distributed in the body and so can be compared with external radiation. Not so substances like Strontium-90 and Uranium 238 or Plutonium 239 which have high affinity to DNAS and so can deliver their energy where it is effective is causing mutation. Almost all of the potassium 40 radiation is wasted.

Prof Chris Busby, Aberystwyth

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