THE RADIOACTIVITY OF THE NORMAL ADULT BODY

In spite of the frequently stated phrases that "all radiation is harmful" and that "there is no safe dose of radiation", we humans contain, survive, and thrive with rather remarkable quantities of radioactive materials in our bodies. This is not unexpected, for we do live on a somewhat radioactive planet.

Consider a warning given to us by a respected anti-nuclear activist' Dr. Helen Caldicott, who stated "It takes only one radioactive atom, one cell and one gene to initiate a cancer".

Does this imply that every time a radioactive atom decays in the body a cancer results? Or does it mean that once in a while such an event may cause a cancer? The statement is meaningless without an estimate of the risk per event.

Let's consider the source of the nearly 8000 radioactive events that take place in our bodies every second.

RADIOACTIVE ELEMENTS IN THE HUMAN BODY

Radioactive Isotope	Half Life (years)	Isotope Mass in the Body (grams)	Element Mass in the Body (grams)	Activity within the Body (Disintegrations/sec)
Potassium 40	1.26 x 10^9	0.0165	140	4,340
Carbon 14	5,730	1.6 x 10^-8	16,000	3,080
Rubidium 87	4.9 x 10^10	0.19	0.7	600
Lead 210	22.3	5.4 x 10^-10	0.12	15
Tritium (^3H)	12.43	2 x 10^-14	7,000	7
Uranium 238	4.46 x 10^9	1 x 10^-4	1 x 10^-4	3 - 5
Radium 228	5.76	4.6 x 10^-14	3.6 x 10^-11	5
Radium 226	1,620	3.6 x 10^-11	3.6 x 10^-11	3

In the table above are listed those radioelements that produce most of the radioactive decays that take place within the adult body. Three of these, Uranium-238 (²³⁸U), Potassium-40 (⁴⁰K) and Rubidium-87 (⁸⁷Rb) are termed primordial radioisotopes, for they were present when the earth was formed. The fact that they are still present in our environment is due to the fact that their half lives are comparable to the age of the earth, and thus they have not yet decayed into stable elements.

Three of the above listed isotopes, Lead-210 (²¹⁰Pb), and the radium isotopes ²²⁶Ra and ²²⁸Ra are present today because they have primordial parents; ²³²Th, with a half life of 1.41 x 10¹⁰ years, is the parent of ²²⁸Ra, while both ²²⁶Ra and ²¹⁰Pb are daughters of ²³⁸U. The remaining two isotopes, Tritium (³H) and Carbon-14 (¹⁴C), are both continuously being created by cosmic rays in the earth's upper atmosphere. Today, much of the Tritium in the atmosphere is manmade in nuclear reactors, but prior to the nuclear era the only source of ³H was cosmic ray bombardment of carbon.

There are many other radioisotopes in the body in addition to those listed above. Most of those omitted contribute very few decays per second, and are thus trivial compared to those in the table. These include familiar isotopes that are found in the fallout from nuclear weapons, such as Cesium-137 and Strontium-90. Other primordial radioelements may be present, such as ²³²Th, and radionuclides used in diagnostic procedures by the medical profession may also be present in some persons. Radon, in the form ²²²Rn, is always present in the air we breath.

No attempt has been made to express the dose that might be delivered to the body by the isotopes incorporated within the body. The are two reasons for this. First, dose is defined as the energy delivered to a sensitive organ or tissue, divided by the mass of that tissue or organ. It is this author's belief that we do not know the target we should use to calculate dose for any of these radioelements. Consider the radium in the body. We do know that radium mimics calcium within the body, and thus most of it is found in bone. Historically, such doses have been expressed as the total energy emitted by the radium and its daughters divided by the mass of bone in the body. Since many of the radium daughters emit alpha particles, which can produce a lot of damage, this alpha energy is multiplied by a factor to account for this increased damage. In actuality, most of the alpha particles never reach a sensitive tissue, such as a cell, and thus most of this energy is wasted. Indeed, most of the times an alpha particle does hit a cell, it kills the cell, but dead cells don't give rise to malignancies.

The second of my reasons for not calculating dose has to do with my belief that the cell is actually a proper target for dose calculation, but we do not know how to account for repair processes which we know do exist within the body. I like to consider ⁴⁰K as an example. Potassium within the body is primarily within the intracellular fluids; 98% of the intracellular fluid is within cells. Thus 98% of ⁴⁰K decays occur within cells. In this location a radioactive decay has an excellent chance of doing damage to that cell's DNA. Since every person on earth experiences about 4400 potential cell damages from ⁴⁰K every second of their life, how is it so many of us are still here to ask such a question?

How do these radioactive atoms get into our bodies? Read on if you are interested.

Potassium-40 is the most radioactive of the normal body radioelements, and enters the body within all the food we eat. Potassium is an abundant element, is an essential constituent for plant growth, is found in most soils, and is thus incorporated in growing plants. It is also an essential part of the human diet. The level of potassium in the body is maintained by a homeostatic process. The average adult consumes about 2.5 grams of potassium each day. There are three different potassium isotopes: ³⁹K, a stable isotope, is the most abundant, at 93.26 % of the total; ⁴¹K is next in abundance at 6.73 % and is also a stable isotope. The potassium isotope of interest is ⁴⁰K. It is primordial radioisotope present in all potassium at a very low concentration, 0.0118 %.

Carbon-14, produces almost as many disintegrations per second in the body as does ⁴⁰K. The 16,000 grams of carbon in our bodies is exceeded by only one other element, the 43,000 grams of oxygen. Since one out of every 10¹⁸th carbon atoms is ¹⁴C, that results in a large number of radioactive ¹⁴C atoms in every person on earth. This radioactive tracer in all carbon is the result of the continual bombardment of the earth's atmosphere by cosmic radiation. These cosmic rays contain neutrons, which produce ¹⁴C in the upper atmosphere by hitting nitrogen atoms and transmuting them to ¹⁴C. This process has been going on for as long as the earth has had an atmosphere, with the result that about one out of every 10¹⁸th carbon atoms is now ¹⁴C. All living things, trees, plants, animals or fish, as they grow and consume food, continue to add and replace carbon in their structures. However, after death, no new carbon atoms are added, so their ¹⁴C content slowly decreases as the ¹⁴C decays and thus allows their age to be determined by ^14C dating. Similarly, ⁴⁰K and ⁸⁷Rb can also be used for dating, but only over much longer time periods.

Rubidium-87 is the next most abundant radioisotope in the human body, yet rubidium has no known biological function in the body. It is remarkably abundant in the earth's crust, about equal to the abundance of carbon, yet it is not an element that one hears much about. Rubidium probably mimics potassium in its patterns of absorption by and distribution within the body, but, unlike potassium, it is not under homeostatic control. The body mass of this element was obtained from ICRP 23 The Standard Man.

Lead-210 enters the human body primarily through the diet, but some fraction is inhaled from the air (from the decay of ²²²Rn), and some enters as a consequence of smoking cigarettes. Polonium-210 (not considered separately in the table above) also may enter the body by these routes, but a much larger fraction of its intake is from the smoking of cigarettes. Thus in smokers ²¹⁰Po can contribute a significant fraction of the total radioactivity in the body. It was not considered in the table for it is not a "normal" constituent of the body, and because the body content is variable, depending on both the number of cigarettes smoked per day and the length of time the individual has smoked. The decay rate given in the table above for ²¹⁰Pb includes its two radioactive daughters, ²¹⁰Bi and ²¹⁰Po.

Tritium, like ¹⁴C, is also produced in the atmosphere. The bombardment of nitrogen in the atmosphere by cosmic-ray neutrons produces ³H, (¹⁴N + neutron = ¹²C + ³H), which combines with oxygen to form tritiated water . However, since the introduction of nuclear reactors, which produce small amounts of ³H, more tritium has entered the atmosphere from these sources than from the natural process. Thus the tritium in humans today is somewhat greater than would have existed in mankind prior to the nuclear age.

Uranium-238 is also found in all humans, entering the body as a contaminate with various foods. This heavy element is the first radioisotope in a long chain of radioisotopes which includes ²²⁶Ra. However, the ²²⁶Ra in the table above does not originate from the ²³⁸U in the body, for after two prompt daughter products of ²³⁸U the decay process is effectively halted while the third daughter, ²³⁴U grows in, for it has a long half life, 2.4 x 10⁵ years. The calculated decay rate, 4.6 disintegrations per second, includes only the two prompt daughters which may stay in the body after the decaying atom is transformed from uranium into Thorium-234 and then into Protactinium-234.

The two radium atoms listed in the table enter the body incorporated in all the foods we eat, and in some areas, in the water we drink. Radium is found rather uniformly distributed in all soils, and thus in the products grown on these soils. From our studies of radium in the human body it has been observed that a representative 70kg man contains about 32 pCi of ²²⁶Ra in the skeleton and 4 pCi in the soft tissues. The values for ²²⁸Ra are not as well known; a value of 18 pCi in the skeleton has been assumed for this isotope.

Each ²²⁸Ra decay is followed by eight radioactive daughters which in turn decay before a stable isotope of lead (²⁰⁸Pb) is created, so the decay rate given above includes these daughter products. However, the fourth isotope following ²²⁸Ra is the noble gas, radon (²²⁰Rn, half life 55 seconds). Some of this gas may escape from the body before it decays, so a value of 90% for the retention of radon is assumed. Under these assumptions the decay rate for ²²⁸Ra and its daughters within the body is 5.7 disintegrations per second.

Following the decay of ²²⁶Ra the first daughter created is the noble gas, radon (²²²Rn, half life 3.8 days). Only 30% of this longer-lived radon stays in the body, so only this fraction of the daughter isotopes following ²²⁶Ra are included in the calculation of total decay rate. Further, after five daughter products decay, the isotope ²¹⁰Pb is created. This isotope of lead has a half life of 22 years, so the calculation of decays from ²²⁶Ra in the body is considered to be halted at ²¹⁰Pb.

BACKGROUND MATERIALS

I invite interested readers to investigate the concepts presented here. For this purpose listed below are references needed to verify my calculations. An excellent start is provided by several of the publications of the National Council on Radiation Protection and Measurement (NCRP) and also some by the National Academy Press.

NCRP 77 (1984). Exposure from the Uranium Series with Emphasis on Radon and its Daughters. NCRP Publication Bethesda, MD.

NCRP 94 (1987).. Exposure of the population in the United States and Canada from natural background radiation. NCRP Publications, Bethesda., MD.

BEIR IV (1988). Health Risks Of Radon and other Internally Deposited Alpha
Emitters: National Academy Press, Washington D. C.

See also:

The Elements, by J. Emsley

Comments?

R. E. Rowland
abrowland@holmstad.com