Answer to Question #13249 Submitted to "Ask the Experts"
Category: Radiation Basics
The following question was answered by an expert in the appropriate field:
I am looking for advice for how to understand results of bioassay test for tritium in urine. Results are in Bq L-1. How does that relate to mSv y-1? What level is considered safe? What actions should be taken if results are over a safe level?
I shall attempt to address each part of your question. I shall base my answers on the methodologies that applied at the time that current US Nuclear Regulatory Commission (NRC) 10 CFR 20 regulations were implemented, those being primarily International Commission on Radiological Protection (ICRP) Publications 26 and 30.
The major concern with tritium exposure generally relates to its ingestion or inhalation as tritiated water for which the allowed intake is much less than for elemental tritium. In such instances, the tritiated water distributes uniformly among all the body water and the water, in turn, is assumed to distribute uniformly among all soft tissues of the body. The tritium delivers tissue dose through the weak beta radiation that is readily absorbed. All body water, including sputum, blood, urine, perspiration, and humidity in exhaled breath are assumed to contain the same concentration of tritium.
Typical bioassay measurements are of urine because of its relative convenience. This makes the intake estimation quite simple. For example, if a single measurement was made of the concentration in urine, this would allow estimation, at the time of sample collection, of the total body burden of tritium by multiplying the urine concentration by the expected total water volume of the affected individual; for reference man used for purposes of calculating appropriate limits, the body's water volume was assumed to be 42 L. The determined activity could then be corrected back to the time of intake (for a single intake) to determine the amount of tritium ingested or inhaled (for inhalation exposures, one third of the total tritium intake is assumed to occur through skin penetration). The effective half-life of tritium in the body is taken as 10 d, and the correction for a time interval of T days between intake and sample collection would be the factor e(ln2/10)T. In most instances of any significant exposures, more than one bioassay measurement would be made, and more detailed information may be gathered as to the specific behavior of tritium in the affected individual.
Once the intake estimate has been made, it is generally compared to the allowed legal intake limit. For NRC or agreement state regulations, this limit would be the Annual Limit on Intake (ALI), values for which are published in 10 CFR 20 Appendix B and given in international system (SI) units in ICRP Publication 30, Part 1, Limits for Intakes of Radionuclides by Workers. For tritiated water the published ALI value (for both ingestion and inhalation) is 3 x 109 Bq. The ALI, by definition, is the annual intake that will result in a 50-year committed effective dose of 50 mSv. Of course, since the effective half-life of tritiated water in the body is only 10 days, all of the committed dose would accrue within a few months. While some might argue about what constitutes a safe level, the Health Physics Society (HPS) and most professional health physicists accept this 50 mSv value as safe—i.e., a dose at which we would expect to see no adverse effects of the radiation and a level that implies a work environment at least as safe as that experienced by workers in other occupations/industries identified as safe.
Fortunately, tritiated water is one of the rather few radionuclide uptakes that can be fairly easily reduced with minimal risk to the affected individual. The usual recommended procedure is to increase the fluid intake by said individual; since the effective half-life of tritium in the body is governed by the turnover rate of water in the body, and since the body always works to attempt to maintain a homeostatic condition, if we increase one's water intake his/her body will respond by increasing the rate of water excretion. This results in a more rapid excretion of tritium from the body, ultimately yielding a lower committed effective dose. The technique is simple, safe, and effective.
I hope this addresses your major concerns.
George Chabot, PhD