Chromium takes two forms in nature: trivalent chromium, or Cr III, and hexavalent chromium, Cr VI. The two forms of chromium have dramatically different effects on health.

Trivalent chromium is an essential nutrient that plays vital roles in body functions such as lipid, protein, and fat metabolism. Many dietary supplements contain Cr III. This form of chromium does not dissolve in water, and passes through the body relatively unchanged. Even large doses of trivalent chromium exhibit very low toxicity and have not been shown to cause cancer.

In contrast, hexavalent chromium can be toxic, and has been shown to cause cancer if inhaled. The lethal inhaled dose is about 7 milligrams of Cr VI per kilogram of body weight. Inhaled hexavalent chromium damages the linings of nose, throat, and lungs. When swallowed, Cr VI can irritate the stomach and damage the liver and kidneys.

Until 2007, it was accepted that hexavalent chromium does not cause cancer when ingested because the body rapidly converts it to the benign trivalent form. The EPA has classified hexavalent chromium as a known carcinogen based upon studies of workers in chromium processing factories who developed lung cancer after inhaling unusually high concentrations of Cr VI for prolonged periods. A 150-pound person could safely ingest 100 mg (about three ounces) of trivalent chromium or 0.2 mg of hexavalent chromium per day without experiencing any adverse health effects, according to EPA studies. The EPA estimates that a person has a one-in-a-million chance of developing cancer if continuously exposed for a lifetime to air containing 0.00008 micrograms/cubic-meter hexavalent chromium, as particulates.

However, rats and mice developed benign and malignant tumors in mouth and small intestine after drinking water heavily contaminated with hexavalent chromium daily during a two-year study conducted by the National Institute of Environmental Health Sciences (NIEHS), a subgroup of the National Institutes of Health. The chromium compound used in the study was sodium dichromate dehydrate. The lowest dose of this compound added to the animals’ drinking water was 14.3 mg/l - ten times higher than what humans could consume from the most contaminated waters in California. The highest dose administered was more than 36 times greater than the lowest.

The EPA drinking water quality standard for all chromium compounds is 0.1 parts per million; that is, the combination of all chromium compounds may not exceed 1 part in 10 million. Additionally, the EPA requires facilities that discharge wastes into water supplies to immediately report discharges of 10 pounds or more of chromic acid (a primary source of hexavalent chromium) and 1,000 pounds of all other regulated chromium compounds within any 24-hour period. These facilities must also track and report annually normal discharges of regulated chromium compounds. A fine and other penalties amounting to $12 million was levied in May, 2007, against aerospace contractor Hamilton Sundstrand for violating this standard, and the firm was placed on probation for five years.

Chromium is not a significant water quality concern now, nor is it likely to become a widespread water pollutant in the future. Sensational headlines about "new" carcinogenic effects of chromium in drinking water should be taken in perspective; the concentrations of hexavalent chromium that caused tumors in mice are simply not even approached in drinking water anywhere today, and regulators are careful to keep it that way.