Banana Equivalent Deception

There are many kinds of radiation.  One kind of radiation is visible light; it usually does not damage living tissue, because it is not generally ionizing.  The damaging radiation is ionizing, and this includes ultraviolet (UV) light, among other things. The fact that UV light is ionizing is the reason smart people do not expose themselves to unscreened sunlight for long.

The truth is, we need UV light for health, even though it is ionizing. We need to be exposed to it for a few minutes per day to make vitamin D. On the other hand, excessive exposure is not good, and can lead to cancer or death, so we limit the exposure, for balance.

There are many other kinds of ionizing radiation, and all of it has the potential to have unwanted health effects.  Most of the radiation most people are exposed to is what is called naturally occurring radiation. Some is UV. Other sources of natural radiation include Radon gas some people have in their basements or well waster, cosmic rays, and carbon-14.

The most important source of natural radiation for most people is quite probably potassium-40 (40K), which is just about everywhere in small quantities. Like UV light, we need potassium, even though about one atom in 80,000 is radioactive. There is no practical way to separate radioactive and radiologically inert potassium. So we live with it and hope for the best.

There is also man-made radiation, including radiation from such medical sources as X-rays and medical isotopes. Other sources are fallout from atomic bomb testing and meltdowns at nuclear power plants.

When we add the amount of naturally occurring radiation to the man-made radiation people are continually exposed to, result is what we call Background Radiation. Most of the background radiation we are exposed to is either naturally occurring or used for such medical purposes as X-rays.

Pro-nuclear people take advantage of the presence of radioactive potassium to belittle the concerns many people have about man-made radioactivity. To do this, they refer to the Banana Equivalent Dose (BED). It is the supposed dose of radiation a person receives from eating a single banana. Since a banana has a lot of potassium in it, it has the advantage of providing people with much-needed potassium, but it is also much more radioactive than most other foods.

The health effects of exposure, according to the BED theory, if you eat a banana, you are ingesting radiation amounting to about 0.1 μSv. For comparison, we might look at this table:

Dose, μSv Bananas
Banana Equivalent Dose 0.1 1
Natural background radiation, global avg. 0.27/hour 2.7/hour
Average dose for people within 10 miles (16 km) of the Three Mile Island Accident 80 800
Average two view mammogram 500 5000

There is a bit of a problem here, and it has led the BED to be criticized by the US Environmental Protection Agency, among others. The problem is that the human body regulates the amount of potassium in it through a process called homeostasis. If you eat a banana, you increase the amount of potassium in your body. Unless you were deficient in potassium, your body immediately starts to excrete potassium to return to a balanced amount, and so it returns to the amount you started with in a few hours. Since the percentage of 40K in the potassium is constant, you will excrete almost precisely the amount you ate in the banana. Since the half-life of potassium is very long, the amount of radiation you have been exposed to is really tiny.

The sievert is measured relative to the number of decays per second, weighted for the types of emissions, because different emissions do different amounts of damage. It is not weighted according to time.  Potassium in your body is kept constant, and so the effect of adding to it is of very short duration. Radioactive cesium, by contrast, displaces potassium in the body, is many times as radioactive as radioactive potassium, and stays in the body longer.  The logic of the BED fails in the comparison, as it does to some degree in all comparisons.

Radioactive cesium is one of the problematical materials associated with a nuclear meltdown. The BED can be used to trivialize the damage that might be done by radioactive cesium. It has no value to clarify because it cannot be used to compare damage that might be done.

The Banana Equivalent Dose is not science, and does not convey useful information.  If it is not a product of ignorance, then it is intentional deception.

 

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