Occurrence

• Chromium VI is naturally present in the environment (e.g. chromite ore component) and as a byproduct of industrial processes, including stainless steel production, dyes and pigments.
• Stainless steel exists in numerous formulations and commonly contains 10.5-18 percent chromium. The surface has a passive surface layer of chromium VI oxide that prevents iron oxidation. Chromium is leachable from the surface especially from initial use and in contact with acidic environments.
• Chromium VI is much more water soluble than Chromium III.
• Drinking water source water sometimes contains chromium in small amounts and it is possible that chromium III could be oxidized to Chromium VI during disinfection (e.g. chlorine, ozone), which is an oxidative process. However, this seems to be controlled by several factors and there are known situations in water supplies where little chromium oxidation occurs.

Chemistry

• Chromium exists predominantly as Cr VI or Cr III, but it can pass through Cr IV and Cr V intermediate forms during interconversion.
• After ingestion, Cr VI is readily converted to Cr III by the reducing conditions of human saliva and gastric juice and other fluids as has been demonstrated by several studies.

Health Effects

• Chromium III is considered an essential nutrient. The AI (Adequate Intake) for Chromium III is 25 micrograms per day, and an RDA has been debated. Dietary supplements commonly contain about 120 micrograms of Cr III per tablet of which about 2-10 percent is absorbed.

Toxicology

• The National Toxicology Program performed two year bioassays in rats and mice with sodium dichromate dihydrate and reported that there was clear evidence of carcinogenic activity in rats and mice. These bioassays utilized a maximum tolerated dose (MTD) and several lower doses including a zero dose control. Mice were dosed at 0 (control), 1.1, 2.6-3.9, 7-9, and 17-25 mg/kg/day. Rats were dosed at 0 (control), 0.6-0.7, 2.2-2.7, 6-7, and 17-20 mg/kg/day. The approximate equivalent human daily doses would be 0, 42-77, 154-273, 420-630, and 1190-1750 mg/day.
• Only the highest doses in the male and female rats and the two highest doses in male and female mice produced significantly increased combined adenomas and carcinomas. Animal survival of zero dose controls and all test doses were similar in both rats and mice. It has been suggested that the carcinogenicity at the higher doses but not lower doses resulted from exceeding the animal’s capacity to reduce the Cr VI to Cr III.

Risk Assessments

• EPA’s draft IRIS document and California’s PHG calculated the lifetime consumption of 0.06 µg/L of Cr VI in drinking water would hypothetically be equivalent to 1/1,000,000 lifetime incremental risk (California has since proposed a reduction to 0.02 µg/L). The margin of exposure for humans from the equivalent male mice dose at 7 mg/kg/day (the lowest dose that produced a significant increase in total cancers in the small intestine compared to controls) is about 1,440,000 times the 0.06 µg/L human drinking water dose (see above).
• The in vivo chemical reduction from Cr VI to Cr III was not utilized in the IRIS and California PHG risk computations, so it is likely that they overestimate the actual risk to humans from consumption at the low levels typically found in drinking water. It could also be argued that there could be a practical threshold below which there is essentially no risk.

Genotoxicity

• Some in vivo and several in vitro tests have indicated DNA damage. Intraperitoneal injection produced higher incidences of micronucleated erythrocytes in mice in a study by deFlora et al, 2006, but ingestion from drinking water did not. Other studies have produced mixed results. Very high doses in pregnant mice of either Cr VI or Cr III during gestation produced DNA deletions in pups.

Water treatment

• If Cr VI is present it may be most efficient to reduce it to Cr III, which is much less water soluble, prior to treatment.

Dr. Joseph Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He holds a Ph.D. in Physical Organic Chemistry and is a member of the World Health Organization’s Guidelines for Drinking-water Quality Committee, and expert advisory groups on drinking water quality, desalination, wastewater and water reuse.