30th June 1808. Have a Radio-Active Banana.

The Radio-Activity measure of 0.1 micro Sievert = one ‘Banana Equivalent Dose’ (BED), is a reminder that harmless amounts occur in the  familiar, common place banana.

The banana, a natural source of Potassium is an informal measure (BED) of radio-activity-just as a large area is calculated as so many football pitches-putting things in some understandable context.

The banana is a natural, low-level, Radio-Isotope, though in bulk, enough to trigger sensors at ports. We eat bananas but not highly Radio-Active Iodine.(1)

It was Today in 1808 that Humphrey Davy reported to the Royal Society his findings on isolating, by what we now call electrolysis, the elements Potassium(K) along with Magnesium (Mg), Calcium (Ca) and Strontium (Sr), all of which have radio-active isotopes.

Bananas have naturally occurring Potassium-40 isotope.

Bananas have naturally occurring Potassium-40 isotope.

Potassium is the 2nd least dense metal (to Lithium) and has 24 isotopes, one of which K -40 (40 K) has a very long half-life of decay, thus an harmless source of radio-activity in animals and humans.(2)

Our attitude to risk is emotional, cultural and runs deep in fear and nothing is more frightening than invisible rays, mysterious, baffling, and harmful (bombs), helpful (X-rays) or both (nuclear power). But when we think in terms of a banana it reminds us that Ionizing Radiation is all around us.


Radio-activity is due to unstable Protons and Neutrons in the nucleus of the atom. It results in decay involving the discharge of alpha, beta and gamma particles eventually into more stable elements.

The key factor is the half-life of this decay, Uranium taking billions of years to decay into stable Lead-206. Some have an extremely short half-life and decay of less than a second. The shorter the half-life the more radio-active and dangerous.


(1) Radio-Activity is Ionizing-Radiation, measured by Geiger Counters and in large amounts harmful. The standard biological measure is a Sievert- a big dose.

(2)  Strontium (Sr) is the 15th most abundant element. One effect of nuclear radiation is contamination with the strontium isotope (90Sr).

However Strontium, like other elements, has many beneficial applications in chemicals such as Strontium Chloride (Sr Cl 2) used in toothpaste for sensitive teeth, and said to have improving effects on osteoporosis as its composition, similar to calcium, is absorbed similarly by the bones.

Ref: wikipedia.org/history_of_elements.

Ref: rsc.org/learn-chem.

Ref: bbc.co.uk/news/mag. Exposure reduced to bananas. Welcome to BED. Michael Blastland. 13.10.2011.


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About colindunkerley

My name is Colin Dunkerley who having spent two years in the Royal Army Pay Corps ploughed many a barren industrial furrow until drawn to the 'chalk-face' as a teacher, now retired. I have spent the last 15 years researching all aspects of life in Britain since Roman times.

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