The literature on metabolism of U and Ra for man relevant to deriving drinking water standards has been reviewed and summarized. Radium is well understood, but significant gaps remain in our knowledge about U metabolism. Limits should be based on an equilibrium model where a constant relationship between intake and organ burden is established, using the best and most likely metabolic parameters. For the skeleton we conclude that the best estimate of skeletal burden expressed in days equivalent intake are 25 days for 226Ra, 10 days for 228Ra, and 0.3 days for 224Ra. For long-lived isotopes of U, we chose 11 days, with a range between 1 and 35 days. The committee believes that intake of natural U in water should be limited by considerations of toxicity to the kidney, and we believe that the metabolic model of Spoor and Hursh with a modified gastrointestinal (GI) absorption (1.4%) should be used to infer kidney content. Our review and analysis of the world literature leads us to believe the average human GI absorption of U is most likely 1-2% and is probably reasonably independent of age or the mass of U ingested. Using a safety factor of 50-150, the committee recommends a limit of U in water of 100 micrograms/l in order to limit toxic effects in the kidney. One hundred micrograms/liter is equivalent to 67 pCi/l of long-lived alpha-emitting natural U isotopes. Further research into the distribution of U in the human body is desirable, especially at natural levels in kidney and skeleton, the time-dependent pharmacokinetics of U in animals, the GI absorption of U in man from water and food, toxicological and U distribution studies in animals under conditions of chronic oral U intake, and metabolic model error propagation.
A new mathematical method employing weighted least-squares has been developed for the analysis of air samples for radon daughters. This method allows calculation of the concentrations in air of the daughter products of both 220Rn and z22Rn based upon the radioactive decay of a sample. It employs observations of the total counts of alpha particles from the sample during various periods of time, rather than estimates of the counting rate at any particular time. There is no upper limit to the number of observations that may be used in the computations. Errors are estimated based upon the statistical limitations of the data.The method has been tested on radon daughter samples taken in uranium mines under conditions both of equilibrium and extreme disequilibrium and compared to the commonly employed Tsivoglou method. Filters were counted in the mines using a combination of ZnS phosphor, photomultiplier tube, and scalar. The concentrations of radium A, B, and C were calculated with both methods. The results of this comparison indicate major advantages of the new technique in research applications, including greater accuracy and clearly defined estimates of errors.
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