BOB Rowland Normal BOB Rowland 2 2004-04-07T23:11:00Z 2004-04-07T23:11:00Z 1 1 1 10.260
Our Radioactive Planet
A RADIOACTIVE ELEMENT?
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.
The long half-life and low abundance might seem to imply that this radioisotope, K40, is of little or no consequence, so it is usually ignored. In fact, the knowledge that all potassium contains this radioactive isotope does not seem to be widely known. Very few of the discussions about potassium mention its radioactivity, which seems to imply that either it is not known or that this trivial level can be safely ignored.
My cardiologist prescribed a potassium supplement for me when he put me on a diuretic; a common practice to counteract the loss of potassium caused by the increase in urinary volume. When I kidded him about prescribing a radioactive material he wouldn't believe that potassium was radioactive.
How much radioactivity are we talking about? The radium and thorium isotopes always present in the body are measured in units of a millionth or a billionth of a gram, but when it comes to potassium we are talking about grams of material. The 2.5 g of potassium ingested each day contains 80 Becquerel (Bq) (or 2100 pCi) of K40 which produce 80 radioactive decays each second. The 140 g of potassium in a normal male contains about 4400 Bq (or 120,000 pCi) of K40; that quantity produces a decay rate of about 4400 disintegrations per second. That means that 4400 radioactive K40 atoms decay and emit radiation in our bodies each second for as long as we live. Since potassium is found in the intracellular fluids, about 98 % of the potassium in the body is within cells. Thus at least 98 % of these disintegrations take place within body cells, and are potentially capable of altering the cell's DNA.
Every person on earth carries essentially the same amount of potassium in their body, and always has. Since body potassium is not considered to be a threat to mankind this must indicate that the body's repair processes are able to handle the potentially malignant changes produced by the radiation emitted by potassium. Further, the radiation emitted by K40, beta rays and gamma rays is no different than the beta rays and gamma rays emitted by other radioactive sources, which might suggest that any such radiation, at a similar intensity, would not produce visible damage.
I hope no one thinks that I am protesting the use of potassium; I eat a banana every morning, plus additional fruit all day. Further, I do take a potassium supplement every day, which is labeled 10 MEQ. This actually contains about the same amount of potassium as a banana, 390 mg, and emits a trivial 12 disintegrations per second.
Today there is a great deal of publicity about the use of potassium iodide pills to block the uptake of radioactive iodine by the thyroid. These pills have been given out to some residents living close to nuclear reactors, in case of a breach of the containment vessel, due to an act of terrorism. They are also widely advertised on the Internet. These pills usually contain 130 mg of potassium iodide (KI). They contain about 31 mg of potassium and produce about 1 radioactive event per second.
So take your "blocking" pills if ever needed, take your potassium supplements as directed, and eat plenty of fruits. Just grin and tell your friends that this potassium radioactivity is truly good for your health.
The rates of decay for the most active radioisotopes in the human body have been tabulated and are listed on an additional page.