Annual Radiation Dose: What Doesn't Add to It? - [Source]

The correct answer is: bananas.

Bananas contain a small amount of potassium-40, which is a radioactive isotope of potassium. However, the amount of potassium-40 in bananas is very small, and the average person would need to eat about 100 bananas per day in order to receive the same amount of radiation as they would from a single chest X-ray.