Late last month, a soccer ball that had washed up on Middleton Island in the Gulf of Alaska was discovered by a technician at the radar station there. The ball was found to have come from a school in Japan, about 6000km away, which was struck by the tsunami of March 11, 2011.
As well as the enormous amount of debris swept into the Pacific Ocean, the tsunami caused extensive damage to the Fukushima nuclear power plant, which released significant amounts of radioactive material into the environment.
It is a tragedy that the cities of Fukushima, Hiroshima, Nagasaki and Chernobyl will forever 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 about 2g-4g a day, and this is generally obtained from such foods as potatoes, spinach and bananas. But it turns out a small percentage of the potassium atoms we ingest 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 half of any sample of 40K will disappear over this time, as it decays by emitting beta particles and gamma rays, both potentially harmful to humans.
A 75kg person contains about 150g of potassium. Of that, 0.018g 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 about 7000 40K atoms decay in your body per second. Each of these decays can potentially lead to DNA mutation, and there is 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 being.
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.
• Dr Blackman is an associate professor in the chemistry department at the University of Otago.
