The human body contains trace amounts of radionuclides ingested daily through water and food intake. These include trace quantities of uranium, thorium, radium, carbon-14, tritium, polonium and potassium-40.
Potassium-40 is by far the most abundant naturally occurring radioactive element in the body.An 11 stone man contains about 140 grammes of potassium, and potassium-40 is present in all potassium, though at a very low concentration: 0.0118 per cent.
When it decays, 89 per cent gives rise to the emission of a beta ray with a maximum energy of 1.33 Megaelectronvolts. The other 11 per cent produces a gamma ray with an energy of 1.46 Megaelectronvolts. Only gamma rays have the energy to exit the body.
The amount of radioactive potassium-40 in an 11 stone man is about 5,000 Becquerels, which represents 5,000 atoms undergoing radioactive decay each second.
A gamma ray is emitted in about one out of every ten disintegrations of potassium-40, implying that about 500 gamma rays are produced each second (remember only about 11 per cent of emissions are gamma rays).
Some will be attenuated in the body, and the dose rate from these gamma rays outside the individual's body will represent a very small fraction of the normal background dose.
One estimate suggests sleeping nightly with another person adds one millirem to your annual dose of 360 millirems of radiation per year.
Found in the Daily Mail's "Answer to Correspondents" column of Thursday, August 31, 2017 contributed by D. L. White of Birmingham.