Would you eat a radioactive banana? Well obviously not. We wouldn’t eat anything that we knew to be radioactive! But as my colleague Peter Woolliams pointed out to me, ALL bananas are naturally radioactive because they are a good source of potassium and all potassium contains a naturally radioactive isotope potassium-40. Peter alerted me to a new (informal) metrological standard – the Banana Equivalent Dose – the amount of radioactive dose that one receives from a standard 150 gram banana – this amounts to ingesting around 520 picoCuries of radioactivity. And eating one banana a day for a year would result in a dose of around 36 micro-sieverts – which is a significant fraction (1.5 per cent or one seventieth part) of our expected dose of 2400 micro-sieverts per year.
Although this may seem like trivia, it is actually an important metrological development. The purpose of measurement is to allow us to quantitatively compare one thing with another. Not just to say that one thing is bigger or smaller than another thing, but to say how much bigger or smaller. For this purpose it is useful to have units of measurement which familiar to us: for example it is easier to measure the size of a television screen in centimetres than in kilometres – though strictly either comparison would be a valid measurement.
This might not seem significant until one considers the fear and anxiety that accompanies any statement about radioactivity. To state that milk contains 20 picocuries/litre of radioactivity might well cause alarm – it might cause one to pause before pouring such milk on one’s cornflakes. To find out that a glass of such milk would contain one seventy-fifth of the amount of amount of radiation in a banana might cause one to feel differently about breakfast. And that is before one even thinks about the tasty electrons surrounding a nourishing proton and neutron core. Mmmm… I never eat anything else 🙂