Answer to Question #957 Submitted to "Ask the Experts"

Q

Thank you for your answer to my question #948—What is the biological half-life of ⁸⁵Kr? Here is some more information that may help with the answer: Our employees could be exposed to the gas itself and breathe it in. Being that it is a noble gas, it does not participate in any biological processes. But it could enter the bloodstream as a dissolved gas, travel throughout the body, and then be eliminated via exhalation. This may not be considered a "traditional" biological half-life situation, but how long could it stay in the bloodstream as a dissolved gas? I have data that ¹³³Xe (as a gas with inhalation as the route of entry and exhalation as elimination) has a "biological half-life" of five minutes. Being that xenon would behave similarily to krypton, would it be safe to assume that ⁸⁵Kr's "biological half-life" is also five minutes?

A

I think the most authoritative report on ¹³³Xe metabolism is the MIRD Committee Report of 1980 (Journal of Nuclear Medicine 21:459-465). After five minutes of breathing and rebreathing a very concentrated ¹³³Xe air/gas mixture (the purpose being to have enough xenon in the lungs to get an image with a nuclear medicine camera), the authors of the report found that about 78%, 8%, 7%, 5%, and 2% were retained in the body with biological half-times of 0.3, 3, 24, 160, and 630 minutes, respectively. They suggest that the first two compartments represent lung and the last three other body tissues. I think that if one is breathing a far more dilute mixture, as would be found in the workplace, there would be far less "crossover" of gas into the blood, as this process is driven by the difference between the lung-gas and blood-gas concentrations. So I think that these latter three fractions would be less in the case you are considering. I have not seen a similar analysis for krypton gases; it might be reasonable to think that the behavior is similar to that of xenon. But the key point to consider here is where the important radiation dose is coming from in such a case, and I believe that incorporation of gas into the body tissues is not the main point of concern here. In the International Commission on Radiological Protection (ICRP) Publication 30 (Pergamon Press 1979), the authors show that for the noble gases the radiation dose to a person's body from external radiation (krypton floating around in the air in the room) is more than 130 times higher than the dose from any gas contained in the person's lungs, and more than 200 times higher than any gas which may have crossed over into the body tissues. So they provide only dose factors that give the external radiation dose per unit time (Sv h^-1) per unit radioactivity in the air (Bq m^-3). Similar factors are given by the International Atomic Energy Agency in their "Safety Series 115" report and are recommended for use in radiation protection in these circumstances. I can't reproduce all of these numbers here; if you don't have easy access to these reports, you can contact me (contact our Web site for my contact information) and I'll fax you a few pages of data. But I agree with them that this is the main source of radiation exposure to consider in this case. I worked on one case many years ago involving a worker in a nuclear power plant who was suddenly exposed to a high concentration of ¹³³Xe gas. There was measurable activity in his body and we followed the activity in his body over time using whole-body counting techniques, and the observed activity fit the MIRD model very well. We also looked at the dose from the radioactive gas in the room environment and the radiation dose from the gas in the room was, as the ICRP suggests, many, many times higher than the dose from activity in the body tissues. So I think that this is really the only source that needs to be considered for worker safety.

Michael G. Stabin, PhD, CHP
Vanderbilt University