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Everything in the world is composed of elements, (remember the periodic table?) which in turn are made of atoms. Some of these atoms are unstable and decay, or break apart. When this happens, they emit what we call “radiation” that can take the form of subatomic particles such as electrons, alpha particles and neutrons, or electromagnetic waves known as gamma rays. All of these carry energy and are capable of breaking down, or “ionizing,” molecules they encounter. It is damage to important biomolecules such as proteins or DNA that can trigger radiation sickness or cancer. As with chemical toxins, extent of exposure is critical and that is dependent on the nature of the radioactive element in question, the amount of the element, the distance we are from the radioactive material and the presence of shielding substances that may be between us and the source.
Potassium (K) is a widely encountered element with a very small fraction of its atoms, about 0.012%, being radioactive. These K-40 atoms spontaneously decay, releasing electrons (beta radiation) as well as gamma rays. Both of these are capable of doing damage to tissues. However, K-40 is not very radioactive, having a half-life of 1.3 billion years, meaning that only a few thousand atoms decay each second The question is how much damage can this do? What is the actual risk of eating a banana? That can be determined by the dose of the radiation that is absorbed by relevant human tissues as measured in “rem,” a unit that takes into account the amount of radiation absorbed and the medical effects of that radiation.
Rem measurement is complex, suffice it to say that a 10 millirem (mrem) dose increases an average adult’s risk of death by one in a million. (That is said to be an increase of 1 “micromort,” with the understanding that 1 mort means certain death.) A banana contains about 450 mg of potassium, and when eaten exposes the consumer to about 0.01 mrem due to its K-40 content. For comparison, a chest x-ray delivers 10 mrem. A quick calculation (10/.01) shows that it would take an ingestion of at least a thousand bananas to result in an exposure of 10 mrem, which would then increase the risk of death by 1 in a million. In other words, for death to ensue, a million times a thousand, or a billion, bananas would have to be consumed. And that would have to be at one sitting. Quite a challenge.
But what about the risk of eating bananas over a lifetime? Cumulative damage? That doesn’t happen since our bodies contain potassium as a natural component (about 120 grams), and the body maintains the amount of potassium at a constant level (homeostatic control). Some potassium is always taken in via the diet, and some is always excreted, meaning that there is no buildup of radioactive potassium. So, while bananas are indeed radioactive, the dose of radioactivity they deliver does not pose a risk. There is a greater, but still insignificant, exposure to radioactivity by sleeping next to someone. And if they are breathing heavy, exposure is increased due to C-14 in their exhaled carbon dioxide.
Bottom line: Enjoy that banana, the world’s most popular fruit. Actually, it’s a berry. Another story for another time. And the peel? Use it to polish your shoe.