"You are positively radioactive, dear!"

POTASSIUM, A RADIOACTIVE ELEMENT?

By R. E. Rowland
abrowland@holmstad.com

Yes, potassium is a radioactive element, identified by the chemical symbol K. Yet, this radioactive element is vital for our good health. It is an element that is essential for the body's growth and maintenance. Potassium is also necessary in order to maintain normal water transport between the cells and body fluids. It also plays an essential role in the response of nerves to stimulation and in the contraction of muscles.
Under normal circumstances it is by far the most abundant naturally occurring radioactive element within the human body. The average adult male contains about 140 g of K; the level varies with body weight and muscle mass. We ingest about 2.5 g per day of K from our food and excrete about the same amount.
The purpose of this document is not to scare anyone about the hazards of potassium; indeed, as indicated in the opening paragraph, it is an element vital to our well being. It is, however, a radioactive material, and in this era where the general public is concerned about radiation exposures and radioactive materials, it is appropriate to tell the truth about them. We live on and in a radioactive world. Our soil, in which our food stuff is grown, contains many radioactive elements. Where did they come from?
The answer is that they were present when our earth was formed. Any radioactive material originally present at the formation of the earth would have decayed and disappeared if its half life was short compared to the age of the earth. However, if its half life were long, close to or greater than the age of the earth, then such materials would not have disappeared but are still with us today.
There are several radioelements in this category, such as the well known elements uranium and thorium. Thorium (Th232) has a half life of 14,000,000,000 years, uranium has two long-lived radioisotopes; U238 has a half life of 4,500,000,000 years, and U235 has a half life of 710,000,000 years. These give rise to the radium and thorium atoms found in all humans, acquired from the food we eat. That food, of course, obtained these materials from the soil in which it grew or on which it grazed.
Potassium is also in this category. There are actually three potassium isotopes: K39, a stable isotope, is the most abundant, at 93.26 % of the total; K41 is next in abundance at 6.73 % and is also a stable isotope. The potassium isotope of interest is a radioactive isotope, K40. It is present in all potassium at a very low concentration, 0.0118 %. It has a very long half-life, 1,260,000,000 years. When it decays 89 % of the events give rise to the emission of a beta ray with maximum energy of 1.33 Mev. The other 11 % of the decays produce a gamma ray with an energy of 1.46 Mev. Find out more about why you glow in the dark here