Answer to Question #12404 Submitted to "Ask the Experts"

According to an article in Radiation Protection Dosimetry, after 500 days from the initial measurement, the caesium clearance slowed. I wonder if this applies to others such as strontium and plutonium. Will radioactive particles (including caesium, strontium, and plutonium) entering the human body due to a nuclear accident eventually be cleared off from the body thoroughly through metabolism? If so, how long does this process take? I have friends who grew up in Ukraine and Japan, and should I be concerned about the possible remaining radioactive particles inside them which might still be emitting radiation or might travel from their bodies into mine?

I should say up front that I don't believe you should be concerned about the possible dose you might have received from association with your friends. I shall attempt to provide some discussion regarding retention and elimination of radioactive materials from the body to provide some background.

In general, radioactive particles taken into the body by inhalation or ingestion do get cleared from the body, although the clearance times may vary considerably and some radionuclides may remain in the body over many years. Particles inhaled into the lungs may be cleared from the respiratory tract by being moved upward into the mouth and either being eliminated by expectoration or swallowed. Some particles that are swallowed may pass directly through the gastrointestinal (GI) tract and be eliminated. Whether in the lungs or in the GI tract, however, some of the particles and the radioactivity may be dissolved in body fluids, get into the blood stream or lymph system, and be transported to other tissues in the body where they may be retained for various lengths of time.

The case that you cite is noted as unusual because the removal of the cesium from the body appeared to follow expected removal rates over the earlier times, but clearance at longer times appeared to be slower than what was expected. Residence times may range from a few minutes to tens of years. Clearance from the body can vary dramatically. This time depends on a number of specific characteristics of the radioactive material taken into the body, including: the radioactive half-life; the material's physical form (gas, liquid, or solid), the solubilities of the radionuclides in body fluids, the size and solubilities of any particles taken in which contain radionuclides, and which tissues in the body take up the radionuclides.

People have performed extensive work over many years to derive appropriate mathematical models that incorporate the various biological processes that affect retention and removal from the tissues and organs of the body. Such models are important because they can be used to project or retrospectively calculate doses to exposed individuals. Both human and animal data have been used to gather the information needed. The human data have come from accidental exposures to some radionuclides and in some cases intentional exposures. Animal studies have been intentional and highly controlled so that extensive investigations have been performed. Some studies have extended over many years. Additionally, considerable human autopsy data have been available and used to gather information about the distributions of various stable elements in the body. Used in conjunction with dietary intake information, it is possible to generate information about the kinetic behavior of these elements in the body. These stable element results have been used to extend results to behavior of radioisotopes of these elements in the body.

Regardless of how carefully studies have been done, there are always uncertainties associated with radionuclide behavior in specific individuals. Much of this has to do with the considerable biological and sometimes behavioral variability observed among individuals. Intakes may involve differences in physical and/or chemical properties between the material actually taken in and the materials that have been studied in establishing the models. It is not surprising then, that we find cases from time-to-time in which a radionuclide of concern does not follow completely the behavior predicted by the model(s) being used.

Regarding your concern about receiving exposure from friends who may have taken certain radioactive materials into their bodies, I do not think you need to be worried. Some radionuclides, such as the strontium and plutonium that you mention may have some components with long residence times in the body, but they do not emit any significant penetrating radiations that will escape from the body of the person with the intake. The cesium, ¹³⁷Cs being the isotope of concern, may have exhibited a component with a longer residence time than expected and does emit penetrating gamma radiation, but this component would represent a small fraction of the original intake and would be extremely unlikely to produce any measurable dose to people in the vicinity of the affected individual. In fact, the magnitudes of intakes that would have been likely by your friends, assuming they were not directly involved in a serious accidental exposure or in any attempts at mitigation activities following a severe incident, would almost certainly not have been a health concern for anyone near the exposed individuals at any time following the initiation of intakes by them.

Indeed, the only cases I am aware of in which it may be necessary to take precautions regarding secondary exposures (to people other than the recipient of the radioactivity) involving people from the general population are those in which radioactivity has been administered in a medical setting, sometimes for diagnostic purposes but especially in instances where the radioactivity has been administered for therapeutic purposes. The most notable instance in this regard is the administration of enough radioactive iodine, typically ¹³¹I, to destroy the function of the thyroid gland, a procedure often used for treatment of thyroid cancer and sometimes hyperthyroidism. In these cases the amounts of radioactivity administered are many thousands of times greater than what any lay person not directly involved, either as a victim or as a mitigator, in a major nuclear incident would likely have taken into his or her body following such event.

I hope this addresses your concerns.

George Chabot, PhD