Hexavalent Chromium
Authors: Proctor Deborah M. 
Published: September 2008

Abstract

Valence speciation and use of toxicology and exposure data that is specific to hexavalent chromium [Cr(VI)] are critical for accurate risk assessment of Cr(VI) exposures in all scenarios. Because trivalent chromium [Cr(III)] is naturally occurring and predominates in most environmental media, measures of total chromium alone cannot be used to characterize risk. Also, the toxicity and environmental fate of the two valences differ importantly. While Cr(III) is of very low toxicity and bioavailability, Cr(VI) is a known human carcinogen, associated with an increased risk of lung cancer among workers exposed to high concentrations in certain industries. High concentration Cr(VI) exposure has also been shown to cause respiratory tract irritation and inflammation. Theoretical lung cancer risks for lifetime exposure at the current Occupational Safety and Health Administration limit of 5 µg m
−3
is estimated to be 10–45 per thousand, and the theoretical risk associated with lifetime environmental exposure at 1 ng m
−3
is estimated to be approximately 1 in 100 000, although no study of environmentally exposed populations has found an increased cancer risk. These risk estimates are likely overestimated at the lower dose range because they were derived by linear low dose extrapolation.

Cr(VI) has also been shown to cause allergic contact dermatitis (ACD) from dermal contact with Cr(VI) in occupational settings, notably work with wet cement, and from contact with tanned leather. A recent repeated open application test (ROAT) to establish a minimum elicitation threshold among 10% of pre‐Cr(VI) sensitized individuals (MET
10%
) can be used to set a skin reference dose (skin‐RfD) protective of sensitization and elicitation of an allergic response in 90% of sensitized individuals. The skin‐RfD protective of ACD from the ROAT study, for application in environmental and occupational risk assessment, is 364 ng cm
−2
, which is consistent with a Cr(VI) soil concentration for dermal contact of 1820 mg kg
−1
.

Cr(VI) health‐based soil screening levels protective of soil ingestion among children for a residential scenario, leaching of Cr(VI) from soil to groundwater, inhalation exposure due to wind erosion and soil suspension from physical disturbance (such as truck traffic) range from approximately 20–500 mg kg
−1
. However, when adequate data are available, site‐specific standards should be set, taking into account actual site conditions.

The US drinking water maximum contaminant level (MCL) is the only chromium standard in the United States that is not valence specific; the limit is 100 µg l
−1
as total chromium. While the current total chromium limit is considered to be protective of noncancer effects assuming total chromium is Cr(VI), it is likely that this standard will be updated with a valence specific standard for Cr(VI) using the results of the 2007 National Toxicology Program (NTP) rodent bioassay. This study observed oral cavity cancers in rats and small intestinal tumors in mice from chronic high concentration exposures (at least 200 times the current MCL). Research to evaluate the mode of action for these tumors, interspecies variability, and extrapolation from rodents to humans, and low dose extrapolation of the dose‐response relationship will be necessary for use of the NTP data for human risk assessment of ingested Cr(VI).