March 15, 2017

Using the Banana Equivalent Dose (BED) to measure hysteria in media reports on radiation

Filed under: Environment, Media, Science — Tags: , — Nicholas @ 03:00

It’s quite common to find media reports involving radiation that are heavy on the freak-out factor and light on the facts. Here’s an interesting and useful rule of thumb you can use … in the few cases that the reports actually provide any meaningful figures on radioactivity:

Long-time readers know that very useful measures of both radioactivity and radiation dose rates are the Banana Equivalent Dose (BED), and a similar measure I think I invented (because no one else ever bothered) called the Banana Equivalent Radioactivity (BER). (The units here are explained in my old article “Understanding Radiation.”)

Bananas are useful for these measures because bananas concentrate potassium, and a certain amount of that potassium is ⁴⁰K, which is naturally radioactive. The superscript “40” there is the atomic number, or the number of protons in the nucleus, of that particular potassium (symbol K) isotope. Because of that potassium content, bananas are mildly radioactive: a medium banana at around 150g emits about 1 micro-Sievert per hour (1 µSv/hr) and contains about 15 Becquerel (15 Bq) of radioactive material.

(Why bananas? There are a lot of plant-based foods that concentrate potassium. It is, however, an essential rule of humor that bananas are the funniest fruit.)

Our radioactive boars are considered unfit at 600 Bq per kilogram. So, a tiny bit of arithmetic [(1000 g/kg)/150 g/banana × 15 Bq/banana] gives us 100 Bq/kg for bananas. All right, so this boar meat has 6 times as much radioactivity as a banana. Personally, this wouldn’t worry me.

So let’s turn to the radioactivity detected off the Oregon coast. This is 0.3 Bq per cubic meter. Conveniently — the joys of metric — one cubic meter of water is one metric tonne is 1000 liters is 1000 kilograms, so the radiation content here is .0003 Bq/kg.

15/0.0003 is 50,000. So, bananas have 50,000 times more radiation than the seawater being reported.


  1. Isn’t the super script “40” the atomic weight, the number of protons plus the number of neutrons? After all, all atoms of the same element by definition have the same atomic number.

    Comment by Steve.muhlberger — March 15, 2017 @ 07:46

  2. There are two numbers to keep in mind when discussing elements: the atomic number and the atomic weight. Yes, all atoms of the same element have the same atomic number (the number of protons), but may differ in atomic weight (the sum of the atom’s proton and neutrons). The example most people will have heard of is Carbon-12 (also denoted 12C) and Carbon-14 (or 14C). Carbon-12 is the vastly more common form of the Carbon atom, containing 6 protons, 6 neutrons and 6 electrons. Carbon-14, on the other hand, differs from the “main” form of Carbon because it has two additional neutrons and is radioactive (which is very helpful for anthropologists and archaeologists).

    Comment by Nicholas — March 15, 2017 @ 09:12

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