Assessment of occupational exposure to uranium by indirect methods needs information on natural background variations. 
Urine monitoring is the preferred method to determine exposure to soluble compounds of uranium in workplaces.  The interpretation of uranium contents in workers bioassay samples requires knowledge on uranium excretion and its dependence on intake by diet.  Exceptionally high concentrations of natural uranium in private drinking water sources have been measured in the granite areas of Southern Finland.  Consequently, high concentrations of natural uranium have been observed in the urine and hair samples of people using water from their own drilled wells.  Natural uranium content in urine and hair samples of family members, who use uranium-rich household water, have been analyzed by using ICP-MS.  The uranium concentrations both in urine and hair samples of the study subjects were significantly higher than the world-wide average values.  In addition, gammaspectrometric methods have been tested for determining uranium in hair samples.  This method can be used only for samples with highly elevated uranium concentrations.