As per US-EPA (US-DOE, 2007) the maximum concentration level (MCL) in drinking water is 0.03mg/L or 27pCi/L for natural uranium. Level of 67pCi/L of natural uranium in drinking water is also suggested by some authors. It is assumed that 02 to 5% of the intake is absorbed in to the blood stream of which 22% is deposited on bone, 12% in kidneys and rest excreted. The uranium deposited in kidneys is excreted within few days. The mean U concentration in drinking water detected in some SU cities is 2.55 micro-g/L
WHO threshold value to prevent sub-clinical renal effects is 0.002mg/L.
The chemical toxicity value for soluble salts of uranium is 0.003mg /kg-day. This is the highest dose that can be taken every day, over a life time without causing adverse health effects. This is generated based on animal studies and normalized to humans using an uncertainty factor of 1000. Lowest observed adverse health effect is at an intake level of soluble uranium of 3mg/kg-day. Daily intake of uranium by adults (60kg) through air, water and food is 2.2 micro-g and total body burden of uranium in humans is 40 micro-g. At equilibrium, the excretion rate through urine is 4.4 micro-g per day.
In alkaline conditions in the kidney, uranium hydrogen carbonate complex is most stable and hence uranium is excreted out from the kidney. In acidic conditions the uranium (as uranyl ion) gets deposited in the tubular wall in the kidney. The most common renal injury caused by uranium in experimental animals is damage to the proximal convoluted tubules in kidney. Nephritis is the primary induced effect of uranium in humans.
The threshold uranium concentration for chemical toxicity is 1 micro-g/g of kidney tissue. For protection of the public, further safety factor of 10 is applied and values in the range of 0.1 to 0.3 micro-g/g of the tissue are reported as threshold limits for public by some authors.
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