Answer to Question #12357 Submitted to "Ask the Experts"

Does the amount of radioactive atoms per square meter stay the same throughout the life span of a regular human or animal?

Unfortunately, from the wording of your question I am not sure to what medium the "per square meter" designation applies. Are you referring to the radioactive surface area concentration of land, or do you mean surface area concentration of radioactivity on a person's or other animal's skin or, possibly are you referring to radioactive contamination in the body of a person or other animal. If the latter is intended, a more proper designation of contamination would be amount of radioactivity (or radioactive atoms) per unit mass (e.g., Bq kg^-1) of specific contaminated tissue or amount of radioactivity per unit mass of the entire body. I shall make a few comments that might apply in each of these possible interpretations.

If you are referring to radioactive atoms spread over a particular land area, the major radionuclides that contribute to expected radioactivity are naturally occurring uranium and thorium and their radioactive progeny (decay products), especially isotopes of radium, ²²⁶Ra being commonly the most dominant. Short-lived progeny, such as ²¹⁴Pb and ²¹⁴Bi, are also important because they contribute notably to the external gamma radiation dose rate near the surface of the earth. Smaller amounts of other radionuclides, such as naturally occurring ⁴⁰K and manmade ¹³⁷Cs (from atmospheric bomb testing) are also present in the earth. The concentrations of all these radionuclides in soil and rock can vary quite dramatically with specific location on the earth. In a given area, the earthly concentrations of the naturally occurring radionuclides often remain relatively constant over seven or eight decades, the expected lifetime of human beings. Changes can occur under some circumstances, such as natural disasters, including flooding, soil erosion, earthquakes, and volcanic activity that can noticeably change the concentrations of radioactive species near the surface of the earth. The ¹³⁷Cs concentration decreases with time according to its 30-year half-life.

If your "square meter" refers to skin area, which I rather doubt, the expected concentrations of radionuclides would be expectedly subject to considerable variation. The most readily observed skin-contaminating radionuclides, barring accidental exposures to manmade radionuclides, are the short-lived progeny of radon gas, the radioactive daughter of radium. For example, ²²²Rn, the daughter of ²²⁶Ra, decays to ²¹⁸Po, which then decays to ²¹⁴Pb, and this decays to ²¹⁴Bi; these short-lived species, always present in the air, will deposit on the surface of the body, including on bare skin if it is exposed. While such depositions are typically extremely small, there have been numerous cases in which such contamination has been high enough to set off alarms on personnel radioactivity monitors in some facilities that monitored individuals entering the facilities. These higher concentrations have most often been associated with individuals who lived in homes in which the airborne radon levels were quite high.

The natural process of replenishment of the surface skin layers by normal regeneration occurs regularly and frequently so that any surface skin contamination is also impacted by this process, leading to more possible variability in skin contamination over time.

Finally, regarding possible internal contamination of individuals by radioactive materials, it is a fact of our existence on the earth that our ingestion of radioactive species, especially through food and water intake, and our inhalation of radioactive materials present in air are unavoidable pathways that contribute to such contamination. For example, if we assume no accidental intakes from events involving manmade radionuclides, the most notable radionuclide with respect to individual radiation dose is naturally occurring ⁴⁰K. This radionuclide is present in many foods, but concentrations are quite variable; any food that is high in stable potassium will also exhibit proportionately high ⁴⁰K levels. As you may well know, bananas are one of the foods cited often as being of high potassium content and contain commensurately high concentrations of ⁴⁰K.

Other naturally occurring radionuclides, such as ³H (tritium), ¹⁴C, and ²²⁶Ra are also present in all individuals. Because of variations in diet and possible local variations in concentrations in various media, some of these radionuclides may show considerable variability in concentrations in human or other animal tissues. A notable example is radium, which is present in very low concentrations in many food products, but is notably more concentrated in brazil nuts, a nut available for human consumption. People who consume such nuts in moderate to high amounts typically demonstrate considerably higher internal burdens of radium, most notably ²²⁶Ra. Radium is an alkaline earth element and, being like calcium, accumulates in the bone.

For individuals living in a given locale for most of their lives, we might expect that their typical intake rates of natural radionuclides by ingestion would not vary dramatically from year-to-year, although some variations might occur if individuals’ diets change significantly and/or if locations where food products are grown change over time. One of the most publicized natural radioactive contaminant that affects us all is ²²²Rn. Radon is a gas that decays to short lived progeny as noted earlier. Upon inhalation of radon and progeny, the progeny may deposit in the lungs and contribute to the dose that an individual receives. While outdoor radon levels in most areas are not high enough to present a concern, radon levels in dwellings and sometimes other buildings can vary, depending on the type of structure, the building materials, the concentration of uranium (and associated radium) in soil below the building, ingress pathways of radon gas from the soil to the building, the degree of building ventilation, and the extent of occupancy. This is one of the few cases of exposure to naturally occurring radionuclides in which we can exercise appreciable control. Any home that has high radon levels can be remediated through presently available methods.

The most significant manmade radionuclide that reaches human beings through ingestion is ¹³⁷Cs. For most of us this contributes very little to our internal burden of radionuclides. For some individuals, specifically those who eat animals that have concentrated higher levels of ¹³⁷Cs in their flesh (foraging animals such as deer, caribou, and the like), the burdens of ¹³⁷Cs may be noticeably higher.

I’m not sure I have addressed your specific concerns, but I hope this has been helpful.

George Chabot, PhD