Hexavalent chromium [
Cr(VI)] is recognized as a human
carcinogen via inhalation, based on elevated rates of
lung cancer among occupationally exposed workers in certain industries.
Cr(VI) is also genotoxic in bacterial and mammalian cell lines. In contrast, scientific panels in the United States and abroad have reviewed the weight of evidence (WOE) and decided that the available data are insufficient to conclude that
Cr(VI) is an oral
carcinogen. A criterion of 0.2 ppb was established by a California agency for
Cr(VI) in
drinking water to prevent
cancer, however, this criterion was withdrawn in November, 2001. This criterion was remarkably lower than the promulgated California and federal
drinking-water standards for total
chromium of 50 ppb and 100 ppb, respectively. Both of the promulgated standards are designed to be protective of humans who ingest
Cr(VI). This article describes a WOE analysis to examine the likelihood that
Cr(VI) in
drinking water poses a
cancer hazard at the current U.S.
drinking-water standard. The results indicate that: (1) From the historical epidemiological studies, there are a few reports of increased rates of
digestive system cancer among
Cr(VI)-exposed workers, although most are not statistically significant; (2) the preponderance of evidence from recent epidemiological studies of
Cr(VI)-exposed workers does not support an increased risk of
cancer outside of the respiratory system; (3) studies of four environmentally exposed populations are negative; (4) there is only one lifetime animal feeding study, and the findings from that study are considered to be flawed and inconclusive; and (5) recent kinetics and in vivo genotoxicity data demonstrate that
Cr(VI) is reduced to nontoxic Cr(III) in saliva, in the acidic conditions of the stomach, and in blood. In short, at concentrations at least as high as the current U.S. maximum contaminant level (100 ppb), and probably at least an order of magnitude higher,
Cr(VI) is reduced to Cr(III) prior to or upon systemic absorption. The weight of scientific evidence supports that
Cr(VI) is not carcinogenic in humans via the oral route of exposure at permissible
drinking-water concentrations.