The Banana Debunk

I intend to do the Banana debunk here.

Here is one from Washington's Blog

But the EPA explains:

The human body is born with potassium-40 [the type of radiation found in bananas] in its tissues and it is the most common radionuclide in human tissues and in food. We evolved in the presence of potassium-40 and our bodies have well-developed repair mechanisms to respond to its effects. The concentration of potassium-40 in the human body is constant and not affected by concentrations in the environment.

Wikipedia notes:

The amount of potassium (and therefore of ⁴⁰K) in the human body is fairly constant because of homeostatsis, 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, namely the time it takes for the normal potassium contents of the body to be restored by the kidneys.

BoingBoing reports:
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.
***
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

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Background information. Thinking about your body-potassium. So, roughly, you’d best have enough potassium in you to fill a block about an inch & two-thirds on edge, because it does all manner of wondrous things for us, including allowing our neurons to think. Now, a tiny, tiny (~one per ten-thousand) fraction of those potassium atoms are quite peculiar: not only are they stable enough to always be with us (only half vanish in a tenth the age of the universe), but quite-rarely, they triple-route decay (by either beta, gamma, or positron emission). They are radioactive! So, to visualize this portion, we have gone from a bit more than a golf-ball of comforting and quite vital potassium, to about as much terrifying p-40, as would fill a space roughly sized so you could think of it as half a rice-grain. Much of the disquietude which feeds our fears of radioactivity stems from the all but un-image-able tiny-ness of the atomic realm. Thus, the facts thus far mentioned, might lead one to surmise, that not much could be happening in that rice grain. The famous historian of the A-bomb, Richard Rhodes, analogized that each fissioning atom releases about the energy of an exploding popcorn kernel, but radioactivity is only a percent or two as powerful. How-some-ever, if you still had half of your p-40 left, or a quarter-rice grain, even should you live for a billion years, you might think not many kernels are a’popping, say, per hour. The fact is, 4,400 of your p-40s blow up each second! Potassium is a metal, and should we be able to hold our imaginary broken rice grain in our hand—its heat would surely be sensible to the touch. There is no easy way to sort out the rice isotopes from your potassium golf ball, thank Christ, but if there were, and if one could place it in a specific location in our bodies, it would rather magically transform itself from harmless “us”, into something similar to the ultra terrifying menace of inhaled plutonium. Well, not quite similar, because plutonium is an alpha emitter, but, the mechanism which makes microscopic plutonium dust-specs lethally carcinogenic, is their capacity to repeatedly bombard and damage the same adjacent cells in the human lung. This “magical” dimension to ionizing radiation is rather subtle, however. So, for example, one could lie on a bed of metallic plutonium, warmed by its decay without harm, because the alphas cannot penetrate a piece of paper, or your outer layers of un-living skin. Now, lets think about the 4,400 decays each second in a 154 pound, typical human, and try to relate this to Dr. Martini’s depiction of the Fukushima radiation enhancement in the Pacific. Simple divisions gives us a potassium shine of 29 disintegrations per second per pound. This is near to the decay rate of each gram of full potassium (31), and about double the dose in a banana, which is fifteen pops-per-second. THIS is our becquerel: One fifteenth of a banana’s p-40 decay rate. One atom-burst per second.