Updated 4:00 p.m. ET



The tap water of more than 30 U.S. cities contains hexavalent chromium, a probable carcinogen made famous by the film Erin Brockovich, an environmental group reports Monday.

The chemical was found in 31 of 35 cities tested, according to the study by the Washington-based Environmental Working Group. Its levels were highest in Norman, Okla.; Honolulu, Hawaii; Riverside, Calif; Madison, Wis.; and San Jose, Calif.

The U.S. Environmental Protection Agency is considering whether to set a limit for hexavalent chromium in tap water after the National Institutes of Health deemed it a "probable carcinogen" in 2008. The chemical has been linked in animals to leukemia and other cancers as well as liver and kidney damage.

Currently, EPA restricts the amount of "total chromium" in drinking water, which contains both hexavalent and trivalent forms, to 100 parts per billion. Some argue this standard doesn't differentiate between its bad and good forms.

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"It's like lumping arsenic with Vitamin D," says study author Rebecca Sutton, noting trivalent chromium is sometimes used as a nutritional supplement. The hexavalent version, also known as chromium-6, is commonly discharged from steel and pulp mills as well as metal-plating and leather-tanning facilities.

Last year, California took the first step nationwide to limit the chemical in drinking water by proposing a "public health goal" of 0.06 parts per billion. Of the 35 cities tested, EWG found 25 had levels exceeding California's proposed goal.

"It's a potential health threat in certain areas," says Max Costa, who chairs the department of environmental medicine at New York University's School of Medicine. He testified as an expert witness for the plaintiffs in a case brought by Erin Brockovich, who accused Pacific Gas & Electric of leaking hexavalent chromium into the groundwater of Hinkley, Calif., for decades and sickening its residents. The case, settled in 1996, was the subject of a 2000 movie starring Julia Roberts.

Costa says the levels in the Brockovich case, 10 parts per million, were far higher than the amounts cited in the EWG study.

"Everything depends on dose," he says, adding: "You don't want to add to your carcinogenic load....I always filter my water."

EWG commissioned laboratory testing of tap water from 35 big and small cities where total chromium had been found in its 2009 analysis of water utility tests from 48,000 communities in 42 states.

"It has been well-known for years that low levels of hexavalent chromium exist naturally in groundwater in certain geological formations," Ann Mason of the American Chemistry Council (ACC) said in a statement.

Her group, representing the chemical industry, cites a recent survey by the California Cancer Registry that did not find a disproportionately high number of cancers in Hinkley, Calif., from 1996 to 2008. Epidemiologist John Morgan concluded that the 196 cancers identified among residents of the Census tract including Hinkley is lower than the 224 cancers that would have been expected given its demographic characteristics.

Mason said the ACC supports uniform, national standard for hexavalent chromium, based on "sound science."

But Sutton says "the industry is fighting this tooth and nail." She cites other research on the chemical's health hazards, including a study by the U.S. National Toxicology Program that found hexavalent chromium in drinking water caused intestinal cancer in male rats and female mice.

She says removing the chemical from tap water can be expensive, but reverse osmosis — a water treatment method used by more modern utilities — is effective.