Did you know that bananas give off a little radiation? Bananas have a lot of potassium. A very small part of this potassium, about 1 in 10,000, is radioactive.

But eating bananas is safe. Your body keeps the same amount of potassium, so extra potassium leaves your body. In fact, your own body also gives off a little radiation every second. This is natural.

Bananas are rich in potassium, a mineral our muscles and nerves need. About 0.01 percent of natural potassium is potassium-40, a radioactive form that has existed since Earth was formed. Because of this, eating one banana is often said to give a dose of about 0.1 microsieverts. That is tiny compared with the roughly 2,400 microsieverts people receive from nature each year on average around the world.

In fact, even this number is too high, because our kidneys keep the amount of potassium in the body constant. Extra potassium is removed in urine, so radioactive material does not build up. An adult's body itself gives off radiation about 4,000 times every second.

Bananas are famously slightly radioactive. About one in every 10,000 potassium atoms is potassium-40, an unstable isotope with a half-life of roughly 1.3 billion years that emits beta and gamma radiation as it decays. Eating one banana is often said to deliver about 0.1 microsieverts, a figure so small that it has become a joking unit, the "banana equivalent dose."

A round-trip flight between Tokyo and New York, by comparison, equals more than a thousand bananas, while natural background radiation averages about 2.4 millisieverts per year worldwide, according to a 2008 United Nations report. However, the banana figure is itself an exaggeration. It assumes that the potassium-40 stays in the body, yet the kidneys maintain a stable potassium level through homeostasis, quickly excreting any excess.

Consequently, Japan's Ministry of the Environment notes that the internal dose from dietary potassium depends on body size rather than diet. In Japan, polonium-210 in seafood actually contributes more to food-related exposure. One caution remains: people with kidney disease who have been told to limit potassium should follow medical advice, for reasons unrelated to radiation.