The widespread use of chromium in industrial applications such as chemical production of pigments,
refractory brick production, tanning, metallurgy, electroplating, and combustion of fuels has lead to human
occupational exposure and to its increased introduction into the environment. Hexavalent chromium compounds
are established carcinogens but their mechanism of cell transformation is not known. Up to now, no microanalytical
technique was sensitive enough to allow the observation of chromium distribution, and oxidation state
identification, within isolated cells at carcinogenic concentrations. In this experiment, we used successfully the ID-21 X-ray microscope to map Cr(VI) and total Cr distributions in cells exposed in vitro to soluble, and insoluble,
Cr(VI) compounds. Exposure to soluble compounds, weak carcinogens, resulted in a homogeneous intracellular
distribution of Cr, confirming by in situ measurement that Cr is present in the cell nucleus. Cr(VI) was never
detected in cells which suggests a mechanism of rapid intracellular reducticn. On the other hand, exposure to
insoluble compounds, strong carcinogens, also resulted in a homogeneous distribution of reduced forms of Cr in
cells, and their nucleus. However, in this case, Cr(VI)-rich structures were observed into the cells suggesting that
carcinogenicity is enhanced when oxidation reactions due to Cr(VI) chronic exposure are associated to Cr-DNA alterations.
