Are Bananas Radioactive?

Why Are Bananas Radioactive?

Bananas are slightly radioactive because they contain potassium, and a tiny share of all potassium is radioactive. The EPA calls bananas and Brazil nuts the best-known examples of foods that contain radioactivity.^[1]

The radioactive share is an isotope called potassium-40, or K-40. It makes up about 0.0117 percent of natural potassium,^[6] which works out to roughly 1 atom in every 8,500. It decays very slowly, with a half-life of about 1.3 billion years. Most of its decays (89.3 percent) release a beta particle, and the rest (10.7 percent) happen by electron capture and release a gamma ray.^[4]

Your own body contains it too. The Health Physics Society estimates about 4,200 becquerels (Bq) of K-40 in a 70-kilogram person, where 1 Bq means one atom decaying per second.^[6]

How Much Potassium-40 Is in One Banana?

None of the sources we read gives a measured value for a banana, so here is a rough estimate of our own, with the assumptions showing.

• The FDA lists a medium banana (126 grams) at 450 milligrams of potassium.^[10]
• Natural potassium has an activity of 31.29 Bq per gram, the value used in a 2016 whole-body counting study.^[9]
• Multiply them: 0.45 grams times 31.29 Bq per gram is about 14 Bq.

That is arithmetic, not a banana we measured. A bigger or smaller banana would change the result.

For scale, the same study measured about 4,200 Bq in men aged 20 to 50 and about 3,000 Bq in women of the same age, all residents of Russia's Urals region.^[9] So one banana holds well under 1 percent of the K-40 already in an adult body at any moment.

What Is the Banana Equivalent Dose?

The EPA says consuming one banana would deliver a total dose of 0.01 millirem, or 0.1 microsieverts (µSv).^[1] That pairing also gives the conversion used in this article: 1 millirem (mrem) equals 10 µSv. A Health Physics Society answer gives the same "approximately 0.1 µSv," with no banana size or method.^[8]

The University of Groningen's radiation protection glossary calls the banana equivalent dose a "mock unit" of effective dose. It is defined as the committed effective dose from taking in the average amount of K-40 in one banana.^[5] In other words, it is a yardstick built from a calculation, not a detector reading taken after someone eats a snack.

Why Is the Banana Dose an Imperfect Comparison?

The unit is handy for showing how small some doses are, but it quietly assumes that a banana adds K-40 to your body and leaves it there. Your body does not work that way.

Oak Ridge Associated Universities (ORAU) says the body "maintains relatively tight homeostatic control over potassium levels," so potassium-rich foods will not raise the body's potassium content. It concludes that eating foods like bananas "does not increase your annual radiation dose."^[4] The Groningen glossary makes a similar point: eating bananas hardly changes the committed dose in practice, because metabolism removes excess potassium through urine almost immediately.^[5] George Chabot, answering a question for the Health Physics Society, also says the body excretes potassium it does not need.^[7]

That leaves two statements that sound like they clash. The EPA says a banana would "deliver" 0.1 µSv.^[1] ORAU says your annual dose does not go up.^[4] Our reading, which is an interpretation and not something either source states, is that they answer different questions. The first describes the K-40 in the banana taken on its own. The second describes your body's total over a year, which stays about the same.

So what does set the K-40 in your body? In the Urals study, amounts were higher in men than in women and declined in older age, and the authors linked body potassium mainly to muscle mass.^[9] ORAU puts the dose to a typical person from K-40 in the body at about 15 to 20 mrem per year.^[4]

How Does a Banana Compare With Everyday Radiation?

Here are published figures next to the banana yardstick. The microsievert column and the banana column are our own conversions from the published millirem figures, using the EPA's 0.01 mrem per banana.

The scale is logarithmic because a straight scale could not show these together: the banana would be invisible next to the annual totals. The banana dot is hollow as a reminder of the problem above. An x-ray or a flight is an added exposure, while a banana mostly trades K-40 you already had for K-40 you just ate.

The EPA offers its own comparison: you would need to eat about 100 bananas to get the exposure you receive each day in the United States from natural radiation.^[1] The NRC's 310 mrem per year works out to about 8.5 µSv per day, which is the same ballpark as 100 bananas at 0.1 µSv each.

Safety note: This article explains a unit. It is not a radiation safety judgment or health advice, and we ran no experiments or measurements of our own.

Myths vs Facts

What We Still Can't Tell You

We cannot give you a personal number. The NRC's annual figures are US averages and its chest x-ray figure is a typical value, so an individual exam or year can differ.^[2] The CDC flight figure covers one trip across the United States, not every route or altitude.^[3]

We also could not find, in the sources we read, a measurement of how much extra dose one banana adds during the short time before the excess potassium leaves the body. ORAU says none on an annual basis and the Groningen glossary says "hardly" any, but neither gives a number. The Urals study did not test bananas at all, and it measured one regional population with its own sampling limits, including few teenagers and no children under nine.^[9] Our 14 Bq estimate uses a nutrition value from a 2017 FDA page rather than a lab measurement of radioactivity. Finally, the question of how much potassium a person should eat is a separate nutrition topic that this article does not address.