Bananas are rich in potassium, and like other potassium rich fruits and vegetables, they are slightly radioactive. In fact, so are you! In any sample of naturally occurring potassium, some of it will be potassium-40, which is radioactive. Since the general public goes ballistic anytime the word radiation is mentioned, nuclear scientists talk about low level exposure in BEDs (banana equivalent dose). Eating an average banana exposes you to about 520 picocuries of radiation. While some other fruits and vegetables have a higher potassium content, and therefore are more radioactive, bananas sound funnier, and so make a stronger point with the public when talking about small amounts of radiation.