Bananas are measurably radioactive.

Bananas are rich in potassium, and a small, fixed fraction of all potassium in nature — about 0.012% — is the radioactive isotope potassium-40. That means every banana contains a tiny amount of naturally radioactive material, undergoing roughly a dozen or more atomic decays per second inside the fruit.

The dose is harmless. Eating one banana exposes you to on the order of 0.1 microsieverts of radiation, far too small to have any biological effect, and your body already carries potassium-40 from a lifetime of eating potassium-rich foods, keeping your internal levels in a steady equilibrium. Physicists still find the number useful as an intuition check: the informal “banana equivalent dose” is sometimes used to put small radiation exposures, like an airport scan or a dental X-ray, into a more relatable perspective.