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.




Half Life


Isotope Mass
in the Body

Element Mass
in the Body

Activity within
the Body

Potassium 40

1.26 x 109




Carbon 14


1.6 x 10-8



Rubidium 87

4.9 x 1010




Lead 210


5.4 x 10-10



Tritium (3H)


2 x 10-14



Uranium 238

4.46 x 109

1 x 10-4

1 x 10-4

3 - 5

Radium 228


4.6 x 10-14

3.6 x 10-11


Radium 226


3.6 x 10-11

3.6 x 10-11


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.


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


R. E. Rowland