Are bananas radioactive, and what is a banana equivalent dose?

Yes, bananas are radioactive, but only to a trivial degree. A small fraction of the potassium in a banana is the radioactive isotope potassium-40. The banana equivalent dose (BED) is an informal unit that compares a radiation exposure to the dose from eating one average banana, usually put at about 0.1 microsieverts. [1] [2]

According to the US EPA, some foods contain trace amounts of naturally occurring radionuclides, which plants take up from soil, which settle on crops from the air, or which build up in animals. Bananas and Brazil nuts are the best-known examples. Bananas are a good dietary source of potassium, and a small fraction of all potassium is radioactive. Brazil nuts also contain a little radium taken up from the soil. The EPA says the amounts are too small to pose a radiation risk. [1]

McGill University's Office for Science and Society gives the potassium-40 share as about 0.012 percent of potassium atoms. It decays with a half-life of about 1.3 billion years, releasing beta radiation and gamma rays. Because the decay is so slow, only a few thousand atoms decay per second in a given sample. The same source notes that spinach, potatoes, oranges, granite countertops, the air and the human body are also radioactive. [3]

The BED is an educational comparison, not a formally adopted measurement. One BED is commonly given as 0.1 microsieverts (the EPA figure of 0.01 millirem is the same amount). A typical banana holds about half a gram of potassium and has an activity of roughly 15 becquerels. Using the EPA's dose coefficient, the calculation gives about 0.078 microsieverts, and the International Commission on Radiological Protection coefficient gives about 0.096, which is why the figure is usually rounded to 0.1. [2] [4]

For scale, the EPA says about 100 bananas would equal the average daily dose Americans receive from natural environmental radiation. McGill notes that a chest x-ray delivers about 10 millirem, which is on the order of a thousand bananas by this measure. A truckload of bananas can even set off radiation portal monitors at US ports. [1] [3] [2]

Defenders of the unit say it helps the public grasp relative radiation risks, and some institutions, such as TRIUMF, use it for that purpose. Several critics argue it is a flawed concept. The body keeps its potassium level roughly constant through homeostasis, so eating a banana does not add to the long-term amount of potassium-40 in the body. Excess potassium is excreted, and the extra exposure lasts only a few hours. The EPA coefficient assumes a longer residence time of about 30 days, so the estimate may overstate the dose. Both sides agree the exposure is very small. [2] [3] [5]

Natural radioactivity in food should not be confused with food irradiation, which passes radiation through food to kill bacteria and pests and does not make the food radioactive. [1]