Go Figure: What bananas tell us about radiation

Bananas

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.

Bananas are a natural source of radioactive isotopes. True, there's not much in one banana. But enough, according to Nuclear Threat Initiative - a security-minded think tank - for a few bananas to trigger radiation sensors used at US ports to detect smuggled nuclear material.

The standard measure of the biological effect of radiation is the sievert. One sievert is a heck of a big dose, but one tenth of a millionth of a sievert, or 0.1 micro sieverts, is roughly the dose from eating one banana.

So we can use one banana as our basic unit and convert other radiation exposures to so many bananas. The data for the table comes from here. I don't claim to have checked it.

Number of bananas equivalent Selected exposures to radiation

500 million

Ten minutes next to Chernobyl reactor core after explosion and meltdown

80 million

Fatal dose even with treatment

20 million

Severe radiation poisoning, fatal in some cases

500,000

Maximum legal yearly dose for a US radiation worker

70,000

Chest CT scan

40,000

Ten years of normal background dose, 85% of which is from natural sources

4000

Mammogram

1000

Approximate total dose received at Fukushima Town Hall in two weeks following accident

400

Flight from London to New York

300

Yearly release target for a nuclear power plant

200

Chest X-ray

50

Dental X-ray

1

Eating a banana

0.5

Sleeping with someone

But why bother converting this to bananas? Partly because it's hoped BED is friendlier than sieverts and grays and rads and rems, and all the other paraphernalia. I'd agree. Though not everyone likes the BED because of problems counting changing level of exposure from the radiation in a banana as it passes through the body.

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.

Officials in Japan check for signs of radiation on children Checks are done for radiation levels in Japan

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.

Usually, graphs with this kind of problem use a logarithmic scale, as with decibels, but logarithmic scales trouble some readers. Bananas don't have that problem. For another visualisation that tries to get over the breadth of scale, take a look at this David McCandless effort here.

By talking bananas, Go Figure doesn't mean to trivialize the health risk of radiation. Radiation - strictly speaking we're talking ionizing radiation here, the type that can damage human cells - is often far from trivial. The US National Cancer Institute has estimated, for example, that the millions of CT scans in the US in 2007 alone will eventually cause 29,000 cancers.

But the way we measure things can change how we think about them. Take the radiation from a chest CT scan. If I say that it compares to being a mile and a bit from the epicentre of the Hiroshima atom bomb, you might just see the risk of that scan differently than if I compare it to eating 70,000 bananas.

Believe it or not, both comparisons are just about valid. The exposure caused by the bomb falls away quite quickly with distance. Decide for yourself which sounds worse.

So does looking at radiation exposure through a banana change anything for you? If not, it might be because of the freakiness with which we began.

Attitudes to risk are complicated, emotional and cultural, and they run deep. Fear, for instance, isn't easily quantifiable. It's not obvious how to put the psychology of risk on a graph. Numbers only take us so far, even when converted to your everyday friendly banana.

Send us your comments using the form below. A selection will be published.

That's bad science - you assert that "one banana won't kill us - not even slightly". However, whats to say that that dose of radiation that you get from the banana is *the* dose that changes the dna in one of your cells that eventually leads to cancer? No level of radiation is a "safe dose".

Lepista, N Yorks

Loved this way of looking at radiation. Very helpful for any lay people. Pity it didn't include radiation from luminous watches and clocks or that from background radiation from rocks, etc.

Dr Bernard Juby, France

Interesting discussion, should always weigh up benefit when talking about risk. This applies both to the individual and to the population. Standing near the Hiroshima epicentre clearly carried no individual health benefit, unlike a CT scan for which the benefit varies with the indication for carrying it out. Working near the Chernobyl meltdown core similarly conferred no health benefit to the individual, however, it did confer a population benefit in terms of averting even more catastrophic radiation release.

Dr Giles Roditi, Killearn, Scotland

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

Having taught science for many years I always tried to get the perspective on radiation. Many people fly and that increases their dose, BUT THEY STILL FLY. I used brazil nuts as an example of a safe way to 'eat radiation'. I dont think that 'non-scientists' ever understand the differences between safe and dangerous radiation Curing cancer is somehow different from radiation that escapes during accidents.

Marion Salmon, Halland, England

A very sound and timely piece. Radiation is something it can be difficult to have a rational perspective on, so something this clear and easy to take in is a great antidote to scare stories.

Derek Macleod, Edinburgh

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

On two occasions I asked about the levels of radiation to which mammograms would expose me. On one occasion I was told it was the equivalent of walking around London for a day. Although I haven't done the conversion via bananas, that doesn't sound quite right. On the most recent occasion I was told it was the equivalent to taking a flight. Maybe there's a need for more accurate information, whether via a banana conversion table or not.

Susan Koksal, London

The news and media are responsible for fear-mongering about radiation. Whenever there is a partial meltdown or a small leak in a nuclear power plant the press have a little fit and go completely mental over some radiation. Cosmic radiation, Radon gases, radiation from the ground and food makes up 80% of all radiation we encounter in our daily lives however radiation from weapons testing doesn't even scrape 0.1%. Thanks to the news, people are more ignorant than ever about what is harmful and what they think harms them.

Tom, Liverpool

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