Is it true that bananas are radioactive? – answered by a PhD.

Yes, bananas are radioactive, but so are you.

Yes, you will certainly die from radiation poisoning if you are able to eat 10.000.000 bananas at once. You may also witness chronic symptoms if you eat 274 bananas a day for seven years.

Here’s how I calculated these numbers.

First, some definitions: to keep it simple, I’m going to use the banana equivalent dose (or BED), a rather large estimate of radiation from eating a single banana.
In addition, roughly the two scenarios in terms of death through radiation: chronic and acute exposure, with the chronic form estimates based on data from the Kyshtym disaster:

  1. BED of one banana, at most, one microsievert or 1 μSv (1)
  2. To die from a single exposure (within days): 10 Gy= 10Sv (2)
  3. To start having symptoms from chronic exposure: 0.1 Gy/yr for at least 7 years

How many bananas do you have to eat at once to die?

10Sv/1μSv= 10.000.000: Ten million bananas

How many bananas do you have to eat to start having symptoms?

274 bananas a day for 7 years.

“Alle Dinge sind Gift und nichts ist ohne Gift, allein die Dosis macht es, dass ein Ding kein Gift ist.” [Everything is poison and nothing is without poison, it is only the dose that makes some things not a poison]

— Paracelsus

Sources:
1- Original calculation here: Banana Equivalent Dose
2- Acute Radiation Syndrome: A Fact Sheet for Physicians
3- Gusev, Igor A.; Gusʹkova, Angelina Konstantinovna; Mettler, Fred Albert (2001-03-28). Medical Management of Radiation Accidents. CRC Press. pp. 15–29. ISBN 978-0-8493-7004-5.