It is a tragedy that the cities of Fukushima, Hiroshima, Nagasaki and Chernobyl will for ever be associated with the word “radioactivity”. It is, in my opinion, fair to say that a significant number of people think of radioactivity as resulting solely from the actions of human beings, by way of nuclear power stations or nuclear weapons, and that it didn’t exist prior to the 20th century. So it may come as some surprise to you that your body, my body, and, indeed, the bodies of everybody on planet Earth, are teeming with radioactive atoms, the majority of which derive from a natural source – the element potassium.

Potassium (elemental symbol K) is an essential element for life. Humans require around two to four grams a day, and this is generally obtained from such foods as potatoes, spinach and bananas. But it turns out that, of all the potassium atoms we ingest, a small percentage are radioactive. Natural potassium consists of three isotopes, ³⁹K, ⁴⁰K and ⁴¹K. All three contain 19 positively-charged protons in their nucleus, but differ in the number of neutrons – 20, 21 and 22, respectively. The ⁴⁰K isotope is radioactive, and comprises about 0.012% of all the atoms of potassium on Earth. It has a half-life of just over one billion years, meaning that one half of any sample of ⁴⁰K will disappear over this time, and it decays by emitting beta particles and gamma rays, both of which are potentially harmful to humans.

An ‘average’ 75 kg person contains about 150 g of potassium. Of that 150 g, 0.018 g is due to the radioactive ⁴⁰K isotope. This might not sound much, but when this mass is converted to an actual number of atoms, we find that it corresponds to about 270,000,000,000,000,000,000 atoms of radioactive potassium in the body. That’s a lot. Given the billion year half-life of this isotope, you might perhaps expect that not many of these atoms would decay over our lifetime, but again, you may be surprised to find that around 7000 ⁴⁰K atoms decay per second. Each of these decays can potentially lead to DNA mutation, and there’s absolutely nothing we can do about it! Obviously it is impossible for us to gauge the health effects of these radioactive decays, as it’s rather difficult to prepare a potassium-free human.

Like it or not, natural radioactivity, whether it be in the form of ⁴⁰K, the most abundant radioactive isotope in the body, ¹⁴C, which we ingest primarily through breathing in _¹⁴CO₂ from the air, or literally hundreds of other radioactive isotopes, is ubiquitous, and will always be with us – well, at least for the next few billion years, anyway.