CAS No. 7440-61-1
Uranium is a silver-white metal that is extremely dense and weakly radioactive. It usually occurs as an oxide and is extracted from ores containing less than 1% natural uranium. Natural uranium is a mixture of three isotopes: 238U (greater than 99%), 235U (about 0.72%), and 234U. Uranium has many commercial uses, including nuclear weapons, nuclear fuel, in some ceramics, and as an aid in electron microscopy and photography. Depleted uranium (DU) refers to uranium in which the proportions of 235U and 234U isotopes have been reduced compared with the proportion in natural uranium. Since the 1990’s, DU has been used by the military in armor-piercing ammunition and as a component of protective armor for tanks.
Variable concentrations of uranium occur naturally in drinking water sources. Thus, the primary exposure sources for nonoccupationally exposed persons are dietary (especially root vegetables) and drinking water. In workplaces that involve uranium mining, milling, or processing, human exposure occurs primarily by inhaling dust and other small particles. Exposure to DU may occur in military personnel from retention internal shrapnel that contains DU or exposure to dust generated from ammunition impact.
Soluble forms of uranium salts are poorly absorbed in the gastrointestinal tract. Depending upon the specific compound and solubility, 0.1%-6% of an ingested dose may be absorbed. Inhaled uranium-containing particles are retained in the lungs, where limited absorption occurs (less than 5%). In cases of retained DU shrapnel, the shrapnel acts as a source of chronic, low level exposure. After long term or repeated exposure, kidneys, liver, and bones can accumulate uranium with the largest amounts being stored in bones (Li et al., 2005). Uranium is eliminated in feces and urine; about 50% of the absorbed dose is eliminated in the urine within the first 24 hours. After exposure to soluble uranium salts, the initial half-life of uranium is about 15 days (Bhattacharyya et al., 1992), which represents distribution and excretion, with much slower elimination from bone. After inhalation, the half-life of insoluble uranium in the lungs is several years (Durakovic et al., 2003).
Human health effects from uranium at low environmental doses or at biomonitored levels from low environmental exposures are unknown. Radiation risks from exposure to natural uranium are very low. Health effects from uranium exposure result from chemical toxicity to the kidney, which can occur occasionally from high occupational exposure. Studies of persons with chronic exposure to soluble uranium salts in drinking water have not shown kidney injury associated with elevated urinary uranium levels (Kurttio et al., 2006; McDiarmid et al., 2006). IARC and NTP have no ratings for uranium human carcinogenicity.
Workplace air standards and guidelines for external exposure to soluble and insoluble uranium compounds have been established by OSHA and ACGIH, respectively. Drinking water and other environmental standards have been established by U.S. EPA. Information about external exposure (i.e., environmental levels) and health effects is available from ATSDR at https://www.atsdr.cdc.gov/toxprofiles/index.asp.
Levels of urinary uranium reflect recent and accumulated exposure. A previous nonrandom subsample from NHANES III (n = 499) (Ting et al., 1999) and other small populations have shown urinary concentrations that are similar to those in NHANES since 1999-2000 (CDC, 2012; Dang et al.,1992; Galletti, 2003; Karpas et al.,1996; Tolmachev et al., 2006). Older studies have demonstrated urinary uranium concentrations that are consistent with levels in the U.S. population, in that the levels were below their respective detection limits (Byrne et al., 1991; Hamilton et al., 1994; Komaromy-Hiller et al., 2000). In a study of 105 persons exposed to natural uranium in well water, urinary levels of uranium were as high as 9.55 µg/L (median 0.162 µg/L) (Orloff et al., 2004). Eighty-five percent of those levels were above the 95th percentile of the NHANES 1999-2000 population. In two studies of a Finnish population with high natural uranium concentrations in their drinking water, the median urinary concentration was 0.078 µg/L (ranging up to 5.65 µg/L), and no consistent effects on multiple endpoints of kidney function were found. (Kurttio et al., 2002, 2006).
The U.S. Nuclear Regulatory Commission (NRC) has set an action level of 15 µg/L urinary uranium to protect people who are occupationally exposed (U.S. NRC, 1978). Recent studies of veterans have been conducted to examine concerns about DU exposure during military conflicts. A cohort of 46 U.S. soldiers evaluated before, during, and after deployment had geometric mean urinary uranium concentrations that were less than the NHANES 1999-2000 and 2001-2002 geometric means at all three time periods, although slightly increased during and after deployment, (May et al., 2004). In 17 U.S. soldiers who had been injured and had embedded DU shrapnel for as long as eight years, the median urinary uranium concentration was 2.61 µg/g creatinine. In the same study, 28 soldiers who may have been exposed to DU by inhalation, ingestion, or wound contamination, but in whom no shrapnel was embedded, had a mean urinary uranium concentration of 0.066 µg/g creatinine (Gwiazda et al., 2004). In a much larger study of 446 Gulf War veterans who were concerned about past exposure to DU, the geometric mean urinary uranium concentration was 0.011 µg/L (McDiarmid et al., 2004). Follow up of 32 veterans with embedded shrapnel showed that increased urinary uranium levels persisted more than 12 years after the first exposure (McDiarmid et al, 2006). Six workers in a depleted uranium program showed concentrations of 0.110 to 45 µg/L (Ejnik et al., 2000). Urinary uranium measurements in 103 Canadian military personnel showed mean urinary levels slightly less than geometric means in the National Report on Human Exposure to Environmental Chemicals (CDC, 2012; Ough et al., 2002).
Finding a measurable amount of uranium in urine does not imply that the level of uranium causes an adverse health effect. Biomonitoring studies on levels of uranium provide physicians and public health officials with reference values so that they can determine whether people have been exposed to higher levels of uranium than are found in the general population. Biomonitoring data can also help scientists plan and conduct research on exposure and health effects.
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