A recent study from the Environmental Working Group found that the drinking water in 89 percent of surveyed U.S. cities had detectable amounts of hexavalent chromium, a likely carcinogen.

But environmental health experts say the public shouldn't be alarmed because the amounts of chromium the study found are likely too low to cause cancer in most people, though past research has linked ingestion of hexavalent chromium with cancer in animals.

“Modern analytical chemistry can detect concentrations of carcinogens in water that are unimportant,” said Allan Smith, director of the Arsenic Health Effects Research Program at University of California, Berkeley.

The EWG study found that 31 of 35 surveyed cities — including Honolulu, Pittsburgh and Tallahassee, Fla. — had hexavalent chromium in their drinking water.

Past studies have shown that hexavalent chromium — different from trivalent chromium, which is an essential nutrient to our bodies — can cause cancer when inhaled. Hexavalent chromium is produced in the chrome plating and steel production industries.

There has been only limited research, though, on whether hexavalent chromium can cause cancer when ingested by humans, Smith said, though studies in China have suggested that ingestion of high amounts can lead to stomach cancer.

How much is too much?

Hexavalent chromium was made famous in 2000 by “Erin Brockovich,” a movie based on Brockovich's real-life crusade against Pacific Gas and Electric Company, which was accused of contaminating the drinking water of Hinkley, Calif.

While the presence of any carcinogen in drinking water isn't good, the amounts of hexavalent chromium that the EWG found in the water would likely have to be much higher to cause any cancerous effects, said Max Costa, professor and chairman of the Department of Environmental Medicine at the New York University School of Medicine.

The EWG study turned up chromium levels as high as nearly 13 parts per billion — more than 200 times higher than the state of California's recently proposed “public health goal” of limiting levels in drinking water to 0.06 parts per billion.

But that amount is still about 1,000 times lower than the amounts seen in the Erin Brockovich case, Costa said.

“I would say that you would be injured in some way if you drank above 100 parts per billion for 25 years,” Costa said, though he added this number is just an estimate.

Why hexavalent chromium is called a “likely” carcinogen

A study done by the National Toxicology Program in 2007 found that water with extremely high levels of hexavalent chromium, 10 times higher than the most contaminated human drinking water, caused tumor growth in rats.

Still, there has been limited research on the effects of ingested hexavalent chromium in humans. Some of what we know comes from studies done on people living in Liaoning Province in China, whose drinking water was found to be contaminated by the compound in the 1970s. The studies, including one published in 2008 in the journal Epidemiology, found that despite limited data, the Chinese people who drank the contaminated water had an increased risk of stomach cancer.

Currently, the Environmental Protection Agency doesn't limit the amount of hexavalent chromium in drinking water. But it does limit the total chromium (including the healthy, trivalent type) that can be present to 100 micrograms per liter of water, according to the National Institute of Environmental Health Sciences.

When inhaled , hexavalent chromium can cause gene mutations and other genetic damage in humans, Costa said.

“It does so many things in cells that you have to ask, ‘What does it not do?'” he said.

Right now, researchers are investigating cancer cases in an area of Greece where drinking water has been contaminated with chromium for at least 30 years, said Konstantinos Makris, an assistant professor in the Cyprus International Institute for Environmental and Public Health, in association with Harvard School of Public Health.

So far, they have found high death rates in the area from liver, kidney, lung and bladder cancers, Makris told MyHealthNewsDaily. However, more research is needed to link hexavalent chromium with these cancers, he said.

“Evidence is piling up, with regards to the carcinogenicity of [hexavalent chromium], refuting the notion that [it] is carcinogenic only via the inhalation route,” Makris said. “Studies as the one we conduct in the area of Greece illustrate that oral ingestion of [hexavalent chromium]-contaminated water may be” to blame for cancer in local residents.

Pass it on: Studies suggest that chromium is associated with cancer in humans when inhaled and possibly when ingested, but it's still not clear how much is needed to produce the detrimental health effects.

Follow MyHealthNewsDaily staff writer Amanda Chan on Twitter @AmandaLChan.