Feces uranium

Studies show that the main sites of uranium deposition ate the renal cortex and the Hvet (8). Uranium is also stored in bones deposition in soft tissues is almost negligible. Utanium(VI) is deposited mostly in the kidneys and eliminated with the urine whereas, tetravalent uranium is preferentially deposited in the Hvet and eliminated in the feces. The elimination of uranium absorbed into the blood occurs via the kidneys in urine, and most, - 84%, of it is cleared within 4 to 24 hours (8). 

After inhalation exposure, the primary route of excretion is in the feces following ciliary clearance from the lungs to the gastrointestinal tract (Wrenn et al. 1981). Fecal excretion may account for as much as 97% of total excretion (Fisher et al. 1983). Higher levels of thorium-230 were excreted in the feces by active crushermen (uranium mill workers exposed to uranium ore dust in the crusher building) compared to retired workers or controls (Fisher et al. 1983). Levels of thorium-230 in the urine were comparable to those of retired workers, and the levels in both were significantly greater than controls. 

Studies in humans have shown that approximately 66% of an intravenous injection of uranium is eliminated from the plasma within 6 min, while 99% of the uranium is eliminated from the plasma 20 hours after injection (ATSDR, 1999 Harley et al, 1999 Luessenhop et al, 1958). Another study has shown that the kidneys excrete over 90% of intravenously injected soluble hexavalent uranium salt, with less than 1% excreted in the feces approximately 70% of the dose is excreted within the first 24 h (Bassett et al, 1948). 

The rate of deposition and clearance of uranium-containing particles from the lung depends upon its chemical form and particle size. As previously discussed in the adsorption section, most of the larger uranium particles are transported out of the respiratory system by mucocilhary action, or swallowed and eliminated in the feces. Smaller particles with higher solubilities are more rapidly absorbed into the systemic circulation but can then be excreted in the urine. 

When you eat foods and drink liquids containing uranium, most of it leaves within a few days in your feces and never enters your blood. A small portion will get into your blood and will leave your body through your urine within a few days. The rest can stay in your bones, kidneys, or other soft tissues. A small amount goes to your bones and may stay there for years. Most people have a very small amounts of uranium, about l/5,000th of the weight of an aspirin tablet, in their bodies, mainly in their bones. 

Estimates of absorption into the blood were derived from the excretion data of uranium mill workers (Wrenn et al. 1985). They estimated the daily mean absorption of inhaled uranium by mill workers at 24 pg U/day (0.34 pg U/kg for 70-kg reference man) based on measured excretion in feces and workplace ambient air concentrations. The absorption of uranium by these workers was estimated as 0.76% (range, 0.4-1.6%). Control subjects in a study of differential metabolism of °Th, and inhaled in uranium ore dust included 3 retired uranium mill workers (4—14 years since last employment as uranium ore crushermen), and 3 volunteers who lived in uranium milling communities but had no uranium work history. Two consecutive 24-hour urine and fecal collections were obtained and analyzed for and The apparent total intakes of uranium of these individuals ranged from 11 to 18 pg... 

The biological half-time of uranium dioxide in human lungs (occupational exposure) at German fuel fabrication facilities was estimated to be 109 days. Body burden measurements of uranium taken from 12 people who handled uranium oxides for 5-15 years were used for this determination. Twice a year for 6 years, a urinalysis was conducted on workers exposed to uranium. In vivo lung counting was performed on the last day before and the first day after a holiday period. Levels of uranium in feces were measured during the first 3 days and the last 3 days of a holiday period and the first 3 days after the restart of work. For some employees, the levels of uranium in feces was measured during 3 days one-half year after the holiday period (Schieferdecker et al. 1985). 

Animal studies have shown that most ingested uranium (99%) is not absorbed in rats, but is eliminated in the feces without being cycled through the bile. In rats, most of the absorbed uranium leaves the body within a few days in urine half is excreted in 2-6 days (Durbin and Wrenn 1975), and 98% within 7 days (Sullivan 1986). About 95% of the uranium in the kidneys of rats is excreted in urine within 1 week, and very little remains in any other organ (LaTouche et al. 1987 Sullivan 1980a, 1986). 

Ingested uranium is excreted mostly in the feces urinary excretion is generally low. The biological halftimes of soluble uranium compounds (uranium hexafluoride, uranyl fluoride, uranium tetrachloride, uranyl nitrate hexahydrate) are estimated in days or weeks those of the less soluble compounds (uranium tetrafluoride, uranium dioxide, triuranium octaoxide) are estimated in years. No information is currently available on the excretion of dermally absorbed uranium. Transdermally absorbed uranium is expected to behave identically to uranium compounds absorbed through the lungs and the gastrointestinal tract. 

Uranium content in soft tissue and bone could also be used as biomarkers of exposure to uranium since uranium also distributes to these tissues and other organs (Ballou et al. 1986 Diamond et al. 1989 Leach et al. 1973,1984 Morris et al. 1990 Morrow et al. 1972 Stokinger et al. 1953 Walinder 1989 Wrenn et al. 1987). Although soft tissues and bone are the most frequently analyzed biological media after urine and feces, these tissues are usually available for analysis only at autopsy. Therefore, this method is impractical and not used for routine screening purposes. 

An alternate method for estimating uranium intake is to measure the daily excretion of uranium in urine and feces. Using this method in a study of 12 subjects in Utah, it was estimated that the average dietary intake for the Salt Lake City population was 4.4 0.6 pg, an intake that is higher than that reported for New York City, Chicago, and San Francisco residents (1.3-1.4 pg) (Singh et al. 1990). 

In vitro uranium analyses are routinely performed in support of a personnel monitoring program, or in cases where the size of an operation does not justify the cost of whole body counter facilities. These analyses are usually done on urine samples, but other types of body materials may also be used (e.g., feces or blood). Urinalysis is effective for analysis of transportable or soluble uranium. A fraction of insoluble uranium also appears in the urine (DOE 1988). 

The excretion of uranium in fecal material results primarily from intakes by ingeshon, and includes uranium swallowed after inhalation. Usually, uranium will appear in feces within hours after intake thus providing a rapid means of determining whether an intake has occurred. Fecal analysis requires prechemistry preparation that includes ashing of the sample, cleaning by co-precipitation, and solvent extraction followed by electrodeposition. Alpha spectroscopy is then performed (Singh and Wrenn 1988). Urinalysis is typically favored over both fecal and blood analysis because it is generally more sensitive and less costly, and because fecal analysis provides no uptake or retention information and blood analyses is invasive. 

About 99 percent of the uranium ingested in food or water will leave a person s body in the feces, and the remainder will enter the blood. Most of this absorbed uranium will be removed by the kidneys and excreted in the urine within a few days. A small amount of the uranium in the bloodstream will deposit in a person s bones, where it will remain for years. 

Tests are available to mea.sure the amount of uranium in a urine or. stool sample. Hospitals do not perform these tests routinely. These tests are useful if a person is exposed to a large amount of uranium, because most uranium leaves the body in the feces within a tew days after ingestion. Uranium can be found in the urine for up to several months after exposure. However, the amount of uranium in the urine and feces does not always accurately show the level of uranium to which you may have been exposed. Since uranium is known to cause kidney damage,. special urine tests are often used to determine whether kidney damage has occurred. 

In Chapter 4, we discuss the effects of uranium on human life and well-being with a detailed survey of the methods and means of estimating internal exposure to uranium on the basis of bioassays (urine, feces, blood, hair, nails, and some nonstandard assays). We also present a detailed review of the analytical methods used to assess the amount of uranium in food products and drinking water that are the main pathways of exposure to uranium of the general population. 

In these models, the bloodstream is in contact (directly or indirectly) with each compartment and equilibrium exists between the uranium content of the compartment and the uranium couceutration in the blood. Elevated levels of uranium in the bloodstream would lead to deposition of uranium in the compartment. The exchange rate of this process is shown in the model of Eigure 4.7. If the uranium concentration in the blood is low, then uranium could be transported from the compartment (body organ) to the blood and eventually be ranoved from the body (excreted) either by the kidneys and bladder (urine), the intestine (feces), contained in keratin of the hair or nails, or even through exhaled breath or perspiration. 

Once equilibrium is attained between uranium in the blood and the other organs (skeleton and soft tissues), it is gradually excreted in the urine and feces. As mentioned earlier, excretion of ingested uranium is mainly (around 98%) through the feces, but removal of the uranium fraction that has entered the bloodstream is distributed between urine, feces, hair, nails, and perspiration (Figure 4.1). The rate of removal of uranium through urine depends in part on the pH of tubular urine. The uranyl hydrogen carbonate complex is stable under alkaline conditions and is excreted in the urine but low pH values would induce dissociation of this complex and the uranyl ion may then bind to cellular proteins in the tubular wall, which may then impair tubular function (Berlin 1986). 

The three main exposure pathways to uranium compounds are through the diet, inhalation, or injury, as described earlier. The uptake of uranium after ingestion is affected mainly by soluble uranium compounds (insoluble compounds are excreted through feces with little or no hazardous health effects) as discussed earlier. However, for inhaled compounds, the chemical and physical characteristics determine how long the uranium will be retained in the lungs and what fraction will enter the bloodstream during a given time period. The common practice is to divide these compounds into... 

The methods of assessing exposure to uranium are derived from the biokinetic models that were discussed earlier. Analytical procedures for the determination of the uranium content in excreta mainly urine and sometimes also feces, blood (serum or plasma),... 

Highlights Analytical procedures for bioassays of uranium in blood and feces have been developed and used in a limited number of studies. Compared to urinalysis, the collection and handling of blood and feces are complicated and entail special procedures. The preparation of fecal samples involves dry-ashing at an elevated temperature and then acid digestion. For alpha spectrometry, separation, purification, and deposition precede counting (that could last several days per sample). Other analytical techniques described in the literature include neutron activation analysis, but ICPMS is currently more widely used. 

The bioassays developed for assessment of the internal uranium contamination (body burden) are capable of regularly determining the uranium content in urine, feces, hair, nails, bones, and teeth samples. Biokinetic models use the results of these measurements to evaluate the body burden, but it should be kept in mind that these general models may not apply exactly to each individual, so there is quite a large uncertainty that is built-in to the assessment of internal exposure. We have preseuted the analytical methods and procedures used for the measurements and have tried to demonstrate the variability and complexity of the methods. 

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