Bananas are radioactive—But they aren't a good way to explain radiation exposure

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Just look at that radioactive banana. There's nothing special about it or where it was grown. All bananas are radioactive, because all bananas contain the radioactive isotope Potassium-40. In fact, a lot of things you might not suspect of being radioactive are, including Brazil nuts, and your own body. And this fact is sometimes used to downplay the impact of exposure to radiation via medical treatments or accidental intake.

A post by nexusheli on the Submitterator turned me on to the idea of the Banana Equivalent Dose—a way of putting radiation exposure into context by comparing intake from, say, local milk just after the near-disaster at Three Mile Island, to intake from a normal, workaday banana. Wikipedia explains the point of this exercise:

The banana equivalent dose is the radiation exposure received by eating a single banana. Radiation leaks from nuclear plants are often measured in extraordinarily small units (the picocurie, a millionth of a millionth of a curie, is typical). By comparing the exposure from these events to a banana equivalent dose, a more realistic assessment of the actual risk can sometimes be obtained.

This isn't just about pro-nuclear propaganda. "Picocurie" is one of those words that really doesn't mean anything to lay people. Confusing units of measurement, when combined with the word "nuclear" can, understandably, freak people out. And, depending on the situation, there isn't always cause for said freak out. Having a way of explaining what picocurie means, in the context of everyday, normal, safe exposures, really is a useful tool for putting radioactivity into a context the public can understand.

But, the Banana Equivalent Dose probably isn't the best way to do that.

The problem is that this system implies that all radioisotopes are created equal—That there's no difference between 520 picocuries of Potassium-40 and a similar intake of, say, radioactive iodine. And that simply isn't true. I contacted Geoff Meggitt—a retired health physicist, and former editor of the Journal of Radiological Protection—to find out more.

Meggitt worked for the United Kingdom Atomic Energy Authority and its later commercial offshoots for 25 years. He says there's an enormous variation in the risks associated with swallowing the same amount of different radioactive materials—and even some difference between the same dose, of the same material, but in different chemical forms.

It all depends on two factors:
1)The physical characteristics of the radioactivity—i.e, What's its half-life? Is the radiation emitted alpha, beta or gamma?

2) The way the the radioactivity travels around and is taken up by the body—i.e., How much is absorbed by the blood stream? What tissues does this specific isotope tend to accumulate in?

The Potassium-40 in bananas is a particularly poor model isotope to use, Meggitt says, because the potassium content of our bodies seems to be under homeostatic control. When you eat a banana, your body's level of Potassium-40 doesn't increase. You just get rid of some excess Potassium-40. The net dose of a banana is zero.

And that's the difference between a useful educational tool and propaganda. (And I say this as somebody who is emphatically not against nuclear energy.) Bananas aren't really going to give anyone "a more realistic assessment of actual risk", they're just going to further distort the picture.

Geoff Meggitt has written a book about the history of radiation and protection. It's called Taming the Rays, and is available on Lulu.com.

Image: Some rights reserved by Jason Gulledge