Bananas are slightly radioactive, and so are you

A banana gives off a little radiation from the potassium in it. Here is what that dose amounts to, and why eating more of them doesn't add up the way you'd think.

Bananas are radioactive. It sounds like a warning, but it is really a lesson in scale, and the same is true of much of what you eat, and of you.

The source is potassium, which bananas are known for. A typical banana holds about half a gram of it. Most potassium atoms are stable, but a small share, about 0.0117 percent, are potassium-40, a radioactive form that was already here when Earth formed. Its half-life is about 1.25 billion years, so it fades extremely slowly, and some of it is still in every rock, plant and animal.

In a typical banana, that works out to roughly 15 atoms decaying every second. Physicists write this as 15 becquerels.

Putting the dose in scale

The effect of radiation on the body is measured in sieverts. Eating one banana is usually estimated at about 0.1 microsieverts, a ten-millionth of a sievert. The figure became an informal teaching unit, the banana equivalent dose, because it makes tiny numbers easier to picture. It was never a formal unit, and the published estimates differ a little.

On that scale, the natural background radiation everyone receives in an average day is about 100 bananas' worth. A chest CT scan is about 70,000. The potassium-40 already in your own body gives you around 0.2 millisieverts a year, about 2,000 bananas, and it is the largest source of radiation inside you.

There is also a reason the banana number overstates things. The body keeps its potassium level steady. When you eat more, the extra is soon passed out, mainly in urine, so a banana raises the potassium-40 you carry only for a few hours. A dozen bananas do not stack up into a dozen doses the way the arithmetic suggests.

Bananas are not special here. Any food rich in potassium carries potassium-40 in the same small proportion. Bananas just travel in bulk: a truckload can set off the radiation monitors at ports that are there to catch smuggled nuclear material.

An adult body holds well over 100 grams of potassium, so a few thousand of these decays happen inside each of us every second. Being slightly radioactive is simply part of being made of ordinary matter.