Is it true that bananas are radioactive, and if you eat too many it can kill you? If yes then how many bananas are good to eat at once.
When people compare the radiation received from other sources with the radiation from eating a banana, the point of the exercise is not to say that eating bananas is risky. It’s to say that the radiation level you’re exposed to from from things like nuclear power is safe, and you shouldn’t worry about it (or about eating bananas).
Yes, it's true that bananas are radioactive, however, the amount of radiation that you get from eating a banana is negligible. It's also true that eating too many bananas can kill you, but if you manage to eat enough bananas to get radiation poisoning, you're going to die from something else long before potassium exposure comes into the equation - like, say, the death penalty after murdering the next person to bring a banana within 50 miles of you.
Bananas are on the left, third from the top. To better communicate the scale, the top legal (i.e. totally negligible) radiation dose for US workers is equivalent to eating 500,000 bananas/year.
There is actually a unit of radiation exposure measurement called the Banana equivalent dose. Suffice to say, there are about a thousand other sources of radiation in your day to day life that you should worry about before how many bananas you eat starts to become a real concern. You can safely eat as many bananas as you please, at least, from a radioactivity standpoint.
In addition to the answers explaining that bananas do contain radioactive potassium, but in small amounts, it's also worth noting that your body maintains a fixed amount of potassium through metabolism.
So even if you somehow manage to consume 5 million bananas, your body won't actually be exposed to 5 million Banana equivalent doses of radiation.
While it's true that bananas are unusually radioactive (which actually means very slightly), you should keep in mind that all plants and animals are radioactive. For people, about half of our intrinsic radioactivity comes from the potassium in our bodies, and about half from the carbon-14 which we all carry around.
As for safe quantities, that's pretty simple: about 4 liters (9 pounds or so). But this has nothing to do with radioactivity - it's the approximate maximum capacity of the normal human stomach. Eating more than this runs the risk of tearing the stomach and dying.
The question of banana radioactivity has sparked the creation of the Banana Equivalent Dose of radiation. It's about 0.1 μSievert. Since it takes about 5 to 7 Sieverts to kill an adult human, the lethal dose (radiation) for bananas is something like 50 million bananas. And it's even harder to kill somebody that way than that rather silly number suggests. The body has a fairly comprehensive and efficient set of mechanisms for keeping things in balance, and excess potassium is typically excreted within 24 hours. Although, admittedly, the changes needed to support the ingestion of 50 million bananas would almost certainly cause other changes in the (formerly) human body, and any discussion of associated effects become pure speculation.
Yes, bananas are radioactive. They contain potassium, and a small part of this comes as a radioactive isotope 40K. The dose of an average banana is (rounded up) around 0.1 μSv. Other potassium-rich foods also naturally contain 40K, e.g. potatoes.
There is absolutely nothing to worry about. Our natural environment and cosmic radiation means our bodies are constantly exposed to radioactive material, but just in trace amounts.
If you were to somehow actually take all that radioactivity into your body you would be dead long before the radioactivity became an issue. The radioactive element is potassium--and potassium chloride is used as an execution drug.
In practice your body maintains potassium levels at the right amount (too little is also deadly), eating more bananas simply means more potassium in your urine.
Besides, I have a jar sitting here next to me. It's far more radioactive than a banana. It's meant as food, though:
Bananas do have a very small amount of radioactivity, but as showed by the chart linked in another answer, it's a really small amount, compared to the amount you are exposed to by being around everyday items/doing everyday activities.
You would need to eat several million bananas in a short time span to actually suffer ill effects/die, which we all know is basically impossible. The bananas would've probably killed you some other way.
Basically, don't worry.
Well, yes to both, up to a point. All things are radioactive - radioactive isotopes occur in minute concentrations in nature, so stones, trees, water, air, and indeed fruits are radioactive, and it can be measured too, but only because our instruments are very sensitive. Life has evolved in this environment for ~4 billion years, and it is clearly not enough to harm us. As for the second question - if you will forgive the tongue-in-cheek answer - anything will kill you if you eat too much; that could be concidered the definition of 'too much'. But I think you will have to eat an incredible number of bananas, and it won't be the radiation that kills you, but the overeating.