Every banana is slightly radioactive.
Bananas are rich in potassium, and about 0.012 % of it is the radioactive isotope potassium-40. One banana delivers roughly 0.1 microsievert of radiation dose. A person's natural annual dose is around 2 to 3 millisieverts, the equivalent of about 25,000 bananas. Your body keeps its potassium level constant, so extra bananas don't raise your exposure over time.
Useful: The banana equivalent dose is a handy yardstick for putting tiny radiation numbers in the news into perspective.
Source: en.wikipedia.org