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.
The nephrotoxic responses of mammalian species, including humans, to injected, inhaled, ingested, and topically applied uranium compounds have been thoroughly investigated. Because there appear to be no unequivocal reports of uranium‐induced radiation effects in humans, it is necessary to rely on experimental work with animals and on epidemiological investigations of human populations exposed to radium isotopes to infer human response to the alpha‐particle emissions of uranium. Metabolic models have been developed to relate the intake of uranium in water and food to its uptake in the human skeleton and soft tissues, of which the kidney is the critical organ. A US Environmental Protection Agency committee has recommended a limit for uranium in drinking water of 100 μg/L, which includes a substantial safety factor, to limit effects on the kidney.
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