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The isotopic ratio 234U/238U in drinking water and in hair, toenail, and urine samples from 45 individuals who consumed 0.2-2775 μg d-1 of uranium in their drinking water was determined using a multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS). The 234U/238U atom ratio in the water samples varied from 51 × 10-6 to 252 × 10-6 whereas in secular equilibrium (i.e., unity activity ratio) the ratio is 54.9 × 10-6. The correlation of the 234U/238U ratio between hair and nail samples was 0.98, and between hair and nails and urine the ratio was 0.91 and 0.89, respectively. The correlation of the ratio between water and the hair or nails was 0.97 but only 0.72 for water and urine, possibly due to spectral interferences. These results conclusively demonstrated that the uranium found in the bioassays can be traced to the drinking water, thus providing a direct link to the source of exposure. Hair may serve as an excellent indicator of occupational or environmental exposure to uranium and provide information regarding its source. Bioassay of hair is attractive as it is an effective bio-concentrator, samples can be easily stored, the concentration reflects an integrated value, and, finally, the measurement of the 234U/238U isotopic ratio in digested hair samples by MC-ICPMS is feasible and highly informative. Hair bioassay can also be used to assess exposure to depleted uranium long after the subjects have left the area suspected of contamination. [ABSTRACT FROM AUTHOR]