Bananas are a good source of potassium, but some of that potassium comes in the form of the radioactive isotope potassium-40.  Thankfully the human body does a good job of self-regulating the amount of potassium it stores, so there isn't really a chance for a deadly buildup.  And just because bananas contain a radioactive substance doesn't mean they're unhealthy.  Different radioactive elements affect the body in different ways, so potassium-40 from bananas won't affect a person the same way as uranium-238 from a nuclear reactor.  Other foods with naturally-occurring radiation include Brazil nuts, carrots, potatoes, and lima beans. #food