Answer to Question #12869 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
To estimate the dose received because of ingestion or inhalation of a radioactive substance, "dose per unit intake" (DPUI) are used. Those DPUI usually give the dose for a 50-y period.
I would like to know if it is correct to divide this dose per 50 to estimate a yearly dose? I don't think so since biological half-life may be a factor. If not, how could one estimate the actual dose for the first x years?
The answer to your first question is, in general, "No, it is not correct to divide the 50-y committed dose or dose per unit intake by 50 to get the respective value for a year." The reason is that, as you have inferred, the activity present in a given tissue varies with time because of the fact that the removal (as well as the accumulation) of activity in a given tissue is typically not a linear function of time. As you likely know, if removal of a radionuclide from a single tissue is governed by a particular effective half-life, which depends on physical and biological half-life, the amount of the radionuclide in the tissue, following an initial uptake of the radionuclide by the tissue, will exhibit an exponential dependence on time.
The answer to your second question is not so simple because of the many factors that affect the uptakes, distributions, and elimination of radionuclides in the body following intake. For example, the chemical and physical characteristics of the material taken in can affect how it behaves in the body. Additionally, the time dependence of the intake is important—e.g., whether the intake is associated with a single exposure over a fairly short time interval, or whether it is the result of multiple acute exposures separated in time or, possibly, the result of a chronic, more-or-less continuous exposure over an extended time. The intake mode is also important. Whether the intake results from inhalation, ingestion, skin absorption, absorption from a contaminated wound, or some combination of such, intake pathways can have a dramatic effect on the time-dependent buildup of a given radionuclide in tissues. Also, the transportability of the radionuclide within the body can have a marked effect on its buildup and release in tissues of the body. For example, inhalation of radioactive particulates is associated with particular inhalation categories that relate to how rapidly such particulate radionuclides are transported out of the respiratory system to the systemic circulation. Finally, once a radionuclide enters systemic circulation, its behavior is generally described by a particular mathematical model that attempts to simulate the expected metabolic consequences, in terms of distribution among systemic tissues, and subsequent retention and excretion. As an example, for radionuclides that have relatively long physical half-lives and are taken into the body by inhalation in a difficultly transportable form, clearance from some compartments in the respiratory tract may be quite slow, and the mathematical models may demonstrate that some systemic tissues exhibit burdens of activity that vary in perhaps unexpected patterns in which tissue concentrations may increase, and not necessarily completely smoothly, over some period of time following intake and then decrease over the remaining time of interest, sometimes with effects of radionuclide recycling within the systemic circulation affecting retention and release.
How all these considerations apply can make it difficult to determine the dose or dose per unit intake for a specific number of years, other than the standard 50 years that have likely already been determined, following intake. The mathematical models necessary to describe all that is going on following intake can be quite complex, possibly involving not only uptake by a particular tissue and subsequent elimination from the body, but also recirculation within the systemic circulation such that some of the radioactivity released from a particular tissue may be released to body fluids and subsequently returned to the same tissue. If you are not equipped with the proper software to handle such cases, they may present a formidable challenge. For some limited situations, however, the results may be much more straightforward.
For example, if a radionuclide taken into the body in a particular form and via a particular pathway has a short retention time in all the tissues of the body, because of a short physical and/or biological half-life in all tissue compartments, all activity taken in may be removed from the body in a short time, and the total committed dose may accrue within a relatively short time period following the intake. A classic example of such an instance would involve the intake of tritiated water over a short period. In such an instance, although the physical half-life of tritium is 12.3 years, most of the tritium is eliminated with a short biological half-life of about 10 days. Thus, essentially all of the committed dose accrues within a couple of months following intake, and the first year committed dose is the same as the 50-year committed dose; the doses for each of the remaining 49 years are zero. There are numerous other cases in which, even though the biological retention times may be long in some tissues, the physical half-lives of the radionuclides may be relatively short so that all of the dose may be delivered within a short time. A common example in this regard is 131I, with a physical half-life of only about eight days. Retention in the thyroid gland is governed by a biological half-life of about 80 days, but the short physical half-life results in almost all the dose being delivered over the first month following intake.
The most common methodologies and physiological and mathematical information relating to appropriate models that are in general favor have been presented in documents prepared by the International Commission on Radiological Protection (ICRP). I shall not list the documents here, but you can find them through this link. The more current recommendations and information regarding determining of dose coefficients for occupational exposure to internal emitters are contained in documents 130, 134, and 137 (these are three parts of an eventual five-part series that will provide specific data for most radionuclides of potential interest), which will ultimately replace much of what was contained in publications 30 and 68. The pertinent anatomy, physiology, and the associated kinetic dependence of radionuclide transport within and out of the alimentary canal, associated with ingestion, and the respiratory tract, associated with inhalation of radionuclides, are presented in ICRP Publications 100 and 66, respectively. There are other reports that relate to exposures to members of the public, which you will also find listed.
I am sorry I cannot provide a simple answer to your question as to calculation of committed doses for intervals other than 50 years. I hope the discussion above is of some value to you.
George Chabot, PhD