Public Health Goal: Uranium in Drinking Water
Description
SUMMARY A Public Health Goal (PHG) has been developed for uranium in drinking water based on its radioactivity. All isotopes of uranium are radioactive, and the total radioactivity depends on the ratio of isotopes. The ionizing radiation from uranium is considered to be inherently carcinogenic. The PHG for uranium is based on the U.S. Environmental Protection Agency’s latest cancer risk calculations for uranium exposure (U.S. EPA, 1999), and recent data on ratio of uranium isotopes in California drinking water (Wong Uranium is a naturally occurring radioactive element that is ubiquitous in the earth’s crust. Uranium is found in ground and surface waters due to its natural occurrence in geological formations. The average uranium concentrations in surface, ground, and domestic water are 1, 3, and 2 pCi/L, respectively. The uranium intake from water is about equal to the total from other dietary components. Natural uranium contains 99.27 percent 238U, 0.72 percent 235U and 0.006 percent 234U by weight. The primary noncarcinogenic toxic effect of uranium is on the kidneys. Recently published studies in rats, rabbits, and humans show effects of chronic uranium exposure at low levels in drinking water. Effects seen in rats, at the lowest average dose of 0.06 mg U/kg-day, including histopathological lesions of the kidney tubules, glomeruli and interstitium are considered clearly adverse effects albeit not severe. Histopathological effects were also seen at the same exposure level in the liver including nuclear anisokaryosis and vesiculation. Effects on biochemical indicators of kidney function were seen in urine of humans exposed to low levels of uranium in drinking water for periods up to 33 years. These effects, such as increased urinary glucose, β2-microglobulin, and γ-glutamyl transferase, are indicative of potential kidney injury rather than toxicity per se. Uranium is an emitter of ionizing radiation, and ionizing radiation is carcinogenic, mutagenic and teratogenic. A level of 0.5 ppb (0.43 pCi/L) is considered protective for both carcinogenicity and kidney toxicity and is therefore established as the PHG for natural uranium in California drinking water. The U.S. Environmental Protection Agency (U.S. EPA) has established a maximum contaminant level (MCL) for natural uranium of 30 µg/L (ppb), based on a cost-benefit analysis (U.S. EPA, 2000). The U.S. EPA maximum contaminant level goal (MCLG) is zero. The State of California has an MCL for uranium of 20 pCi/L based on earlier studies of toxicity to the kidney in rabbits. et al., 1999), from which is calculated the uranium specific activity of 0.79 pCi/µg (radioactivity output per mass unit). The resulting PHG of 0.5 ppb (0.43 pCi/L) developed for natural uranium in drinking water is based on a de minimis 10-6 lifetime cancer risk for exposure to ionizing radiation. This PHG level is supported by a study showing changes in indicators of kidney function (increased β2-microglobulin and γ-glutamyl transferase levels in the urine) in a human population, associated with a no-observed-effect-level (NOEL) of 6 µg/day. OEHHA considers cancer risks below the de minimis one in a million theoretical risk to be negligible.
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