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Trace Elements in Abiotic and Biotic Environments
burden is 90 μg, of which 66% is in the bones (ATSDR 2011), and the body burden
(70 kg) is 100 μg, of which up to 70 μg/kg is in bones (Emsley 2011).
Uranium can be taken into the body by food, drinking water, or breathing air.
People who work with materials and products that contain U may be exposed to
its elevated amounts. This includes workers who mine, mill, or process U, or make
items that contain U (e.g., phosphate fertilizers).
After ingestion, most U is excreted within a few days and never enters the bloodstream. The small fraction (0.2%–5%), which is absorbed into the bloodstream, is
deposited preferentially in the bones (about 22%) and kidneys (about 12%), with the
rest being distributed throughout the body (12%) and excreted. Most of what goes to
the kidneys leaves within a few days (in urine), whereas that deposited in the bones
can remain for a long time; the half-life of U in bones is 70–200 days. Uranium
deposited in other tissues leaves the body in 1–2 weeks. After inhalation, generally
only its small fraction penetrates to the lung’s alveolar region, where it can remain
for years, and from which it can also enter the bloodstream (Argonne 2007). The
main sites of long-term retention for inhaled insoluble compounds that are deposited in the deep respiratory tract are the lungs and pulmonary lymph nodes. In the
bloodstream, U is associated with red cells, and its clearance is relatively rapid.
Although renal toxicity is a major adverse effect of U, the metal has toxic effects
also on the cardiovascular system, liver, muscle, and nervous system (Taylor and
Taylor 1997).
The bioavailability of U after oral intake is relatively low. Depending on the doses
and solubility, U gastrointestinal absorption ranges from 0.1% to 6% of the ingested
dose. Oral bioavailability is limited, and only up to 1%–2% of soluble U and 0.2% of
insoluble U are absorbed. Toxicity of ingested U is related to the solubility of the U
compounds (EFSA 2009b).
Little information on the chronic health effects in humans due to the exposure to
environmental U is available (WHO 2011a). Several international agencies (IARC,
National Toxicology Program, Environmental Protection Agency) have not classified
natural U or DU, with respect to carcinogenicity. Natural U and DU have identical
chemical impact on the human body, and their health effects are due to chemical
properties and not to radiation (ATSDR 2011). The tolerable daily intake (TDI) for U
is estimated to be 60 μg/day (WHO 2011a). For most people, food (Table 58.2) and
drinking water (Table 58.1) are the main sources of U exposure. Water may contribute about 50% of the total U exposure (EFSA 2009b).
Uranium intake with food and water varies from about 1.0 to about 5 μg/day in
uncontaminated regions, and from 13 to above 18 μg/day in U mining areas (Taylor
and Taylor 1997). The average overall dietary exposure to U in European countries
was estimated to range from 0.05 to 0.09 μg/kg body weight (bw)/day. High dietary
exposure to U varies between 0.09 and 0.14 μg/kg bw/day. These dietary exposure
estimates are all well below the TDI of 0.6 μg/kg bw/day. Human daily intake of U
has been estimated to range from 0.9 to 1.5 μg/day (0.6–1.0 pCi/day) (ATSDR 2011).
However, there are also other data that its intake through food is between 1 and 4 μg/day.
Intake of U through drinking water is normally extremely low, but when it is present in drinking water sources, the majority of intake can be through drinking water
(WHO 2011a).