Because of the cancer uncertainty, California has had a tumultuous history of setting water standards to protect people from chromium.
In 1999, after the Hinkley case, California set a water guideline, called a Public Health Goal, of 2.5 ppb. It was based on a 1968 study in Germany that found stomach tumors in animals that drank the substance. However, the U.S. EPA rejected that study as flawed and determined there was no evidence it was carcinogenic in water. California’s scientific advisors agreed, so the state rescinded its goal in 2001 and reverted to the 50 ppb standard, which was adopted in 1977 and based on the risks of skin irritation, not cancer.
The debate focused on whether hexavalent chromium is neutralized in the stomach by gastric acids that turn it into Chromium III, an essential nutrient.
California officials, seeking to resolve the controversy, asked the National Toxicology Program to conduct animal tests.
The study, published online in Environmental Health Perspectives in December, shows that although some of the substance is reduced in the stomach to Chromium III, it’s not enough to avoid toxic effects.
“Since they found tumors in the small intestine, that shows it was not eliminated in the stomach,” Alexeeff said.
Cancer in the small intestine is “relatively rare” in animals, even those exposed to other chemicals, the scientists reported. In addition, chromium caused mouth cancers, and infiltrated the cells of many organs, including livers and pancreatic lymph nodes.
Mice and rats were exposed to four different doses, and they contracted cancer at lower levels than in the 1968 study, according to Michelle Hooth, a toxicologist at the National Institute of Environmental Health Sciences who was the study’s lead scientist.
That suggests California’s new goal could be as stringent as the rescinded 2.5 ppb one.
Chromium is widely used in metal plating, stainless steel production, wood preservation and textile manufacturing. It has been detected in 30 percent of drinking water sources in California, at levels mostly under the existing 50 ppb state standard, according to the state health department.
Some of the rats and mice developed malignant intestinal tumors when fed doses as low as 57,000 ppb—100 times higher than the Hinkley water levels—for up to two years, Hooth said. The higher the dose, the more cancers found among the animals.
When setting a standard, scientists use high animal doses to extrapolate to a lower dose designed to protect people from a 70-year lifetime of exposure. Water standards are usually designed to keep the cancer risk to one case in every million or 100,000 people. Gwiazda, who has served on EPA and California scientific advisory panels, said extrapolating the animal findings for humans creates uncertainty because the rodents had to be fed higher doses.