Answer to Question #948 Submitted to "Ask the Experts"

Q

What is the biological half-life of ⁸⁵Kr?

A

You are asking an unusual question, and I might need more information before I know how to answer it completely. In internal dose assessment, we often speak of radioactive material incorporated into the human body as having a "biological half-life," meaning the time that it takes for half of the material to be eliminated only by biological processes (that is, not including any radioactive decay that may have occurred also). For inhaled noble gases like ⁸⁵Kr, however, we usually do not apply this kind of approach, but we model the movement of this material in and out of the lungs more simply, assuming fixed breathing rates and lung volumes. I have seen that in the case of another noble gas (¹³³Xe) some crossing over of the inhaled gas from the lung to the bloodstream can occur, with a very minute amount trapped in body tissues with an observable "biological half-life," but I have never seen this studied for ⁸⁵Kr. For this nuclide, we generally assume that all significant radiation exposure comes from outside of the body, if one is standing in an atmosphere with krypton gas present. There are (or were at one time) unique applications in nuclear medicine in which ¹³³Xe gas dissolved in saline was injected into patients, to study gas exchange properties of the body. In such a case, one could more directly discuss the idea of a biological half-life. But in most cases involving ⁸⁵Kr, I think that the concept of biological half-life would not be applied, and therefore, unless you are dealing with an unusual situation, I don't have a direct answer for you. Michael G. Stabin, Ph.D., CHP Assistant Professor of Radiology and Radiological Sciences Department of Radiology and Radiological Sciences Vanderbilt University