Bananas are naturally radioactive, owing to their high levels of potassium. When you eat one, it temporarily raises the amount of potassium in your body, and you get a little bit of dose. In fact, you can get some radiation just sitting next to a crate of bananas. The amount of dose you get from a backscatter (those full body machines at the airport) x-ray scan is about the same as eating a banana. The risk of death is on par with smoking 1/700th of a cigarette, or spending a third of a second in a canoe.