Ever since 1896, when Henri Becquerel noticed that certain minerals blurred photograph plates sealed in a paper, radiation has fascinated and frightened the public. Radiation can be either natural or man-made.
Aside from cosmic rays, natural radiation comes from the fact that the Earth is basically one huge low-level fission reactor. Our planet contains loads of radioactive elements, like uranium, radium, and thorium. As to man-made radiation, for most people, 96% is from medical X-rays – a chest X-ray delivers 10 mrem, a full-body CT scan 1,000 mrem, and a dental X-ray 1.5 mrem.
A banana contains about 450 mg of potassium and a very small fraction of that naturally-occurring potassium is in fact radioactive. The radioactive variety of potassium is potassium-40 (K-40, 0.012% of total potassium).
The radiation from bananas measures out as 3,520 picocuries per kilo – that's high enough to set off the more sensitive type of radiation alarms. If you ate one banana per day, you'd receive a dose of 2.6 mrem per year.