But other scientists say that's an extremely small amount. One part per billion is equivalent to about a drop in 250 gallon drums of water, or three seconds in a century. Even if a city such as Norman has the highest concentration of chromium-6 of all the cities tested, that doesn't mean it places the residents at a higher risk for developing cancer than in other cities.
Toxicology experts say inhaling chromium-6 can cause cancer, but there isn't much data on the dangers of drinking it.
"The evidence is fairly good that it's carcinogenic in people in occupational settings who inhale it and get a good dose," said Dr. Shan Yin, assistant medical director of the Cincinnati Drug and Poison Information Center.
"No one has really established what is a carcinogenic level for drinking water," said Alfred Aleguas, managing director at the Northern Ohio Poison Control Center in Cleveland. "We need to establish what is a limit we have to be concerned about."
Aleguas also said that the levels of exposure in Hinkley were much higher -- 580 ppb -- than the 31 ppb the Environmental Working Group found in Norman, Okla., the city with the highest concentration of chromium-6 in the group's report.
Most unintentional chromium exposure comes from industrial processes, such as leather tanning and metal plating. It's also a naturally occurring substance.
Chromium-3 is a nontoxic form of chromium that is vital to the body's glucose metabolism. But while there's still debate over how much chromium-6 is too much, the EPA says it is currently assessing the effects of chromium-6 on human health. The review should be completed later this year, and the agency will determine whether to set a new standard for chromium-6.