Are Bananas Radioactive?

Written by

Melissa Petruzzello

Fact-checked by

Britannica Editors

History

While it might seem like an oxymoron, bananas are a healthy—and radioactive—snack. As it turns out, those two facts are closely connected, as it is the fruit’s abundance of potassium that makes it radioactive.

Potassium-39The most plentiful isotope of potassium is potassium-39, with a stable nucleus of 19 protons and 20 neutrons.

Potassium is an essential nutrient that helps nerves, muscles, and the heart function properly. Most potassium atoms are a stable form known as potassium-39, containing 19 protons and 20 neutrons. A tiny fraction—about 0.01 percent—is made up of atoms of a radioactive isotope known as potassium-40 (⁴⁰K), which has one extra neutron. That additional neutron makes the nucleus unstable, so it occasionally decays into another element, usually calcium-40 or argon-40, emitting radiation in the process.

Banana Equivalent Dose (BED)

Dental X-ray: ~50 BEDs
Chest X-ray: 200–1000 BEDs
Flight from London to New York: ~400 BEDs
Chest CT scan: ~70,000 BEDs

Because bananas are a good source of potassium, every banana also contains a tiny amount of radioactive potassium-40. That might sound alarming, but the dose of radiation is extremely small—roughly 0.1 microsieverts of ionizing radiation. Scientists sometimes informally use the “banana equivalent dose” to help illustrate other radiation exposures. Furthermore, unlike X-rays or other exposures to radiation, the radiation dose from consuming a banana is not considered cumulative, because your body tightly regulates the amount of potassium it maintains. Excess potassium, including the minuscule amounts of potassium-40, is excreted.

It is also worth noting that bananas are far from unique. Potatoes, beans, spinach, Brazil nuts, and many other foods naturally contain small amounts of radioactive isotopes. Even your own body is naturally radioactive because it contains potassium-40 and carbon-14, another naturally occurring isotope. Very low levels of radiation are just a normal and uneventful part of everyday life. But now you have a fun fact to share.

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