INGESTION OF CHROMIUM(VI) IN DRINKING-WATER BY HUMAN VOLUNTEERS - ABSORPTION, DISTRIBUTION, AND EXCRETION OF SINGLE AND REPEATED DOSES

This study examines the magnitude of hexavalent chromium [Cr(VI)] abso rption, distribution, and excretion following oral exposure to 5 and 1 0 mg Cr(VI)/L in drinking water administered as a single bolus dose (0 .5 L swallowed in 2 min) or for 3 d at a dosage of 1 L/d (3 doses of 0 .33 L each day, at 6-h intervals). Adult male volunteers ingested deio nized water containing various concentrations of potassium chromate, a nd sampler; of urine, plasma, and red blood cells (RBCs) were collecte d and analyzed for total chromium throughout the studies. In the bolus dose studies, a fairly consistent pattern of urinary chromium excreti on was observed, with an average half life of about 39 h. However, 4-d total urinary chromium excretion and peak concentrations in urine and blood varied considerably among the 5 volunteers. Studies of repeated exposure to smaller volumes ingested at a more gradual rate (i.e., 0. 33 L over 5-15 mini showed similar urinary chromium excretion patterns bur generally lower chromium uptake/excretion. Given that sustained e levations in RBC chromium levels provide a specific indication of chro mium absorption in the hexavalent state, these data suggest that virtu ally all (>99.7%) of the ingested Cr(VI) at 5 and 10 mg Cr(VI)/L was r educed to Cr(III) before entering the bloodstream. The interindividual differences in total chromium uptake and excretion are plausibly expl ained by ingestion of appreciable doses on an empty stomach, which lik ely results in the formation of well-absorbed Cr(III) organic complexe s in gastrointestinal tissues and possibly the blood. The lack of any clinical indications of toxicity in the volunteers and the patterns of blood uptake and urinary excretion of chromium are consistent with a pre dominant uptake of Cr(III) organic complexes [derived from Cr(VI)] that are excreted more slowly than inorganic forms of Cr(III). Theref ore, it appears that the endogenous reducing agents within the upper g astrointestinal tract and the blood provide sufficient reducing potent ial to prevent any substantial systemic uptake of Cr(VI) following dri nking-water exposures at 5-10 mg Cr(VI)/L. Based on these data, the ch emical environment in the gastrointestinal tract and the blood is effe ctive even under relative fasting conditions in reducing Cr(VI) to one or more forms of Cr(III).