A report published Monday by the Environmental Working Group concluded that Tallahassee and Miami are among 31 U.S. cities it found to have the carcinogenic chemical chromium-6 present in its municipal tap water. The Washington, D.C.-based nonprofit said it conducted the study in part to encourage the EPA to establish standards for the toxic chemical in drinking water, which is currently unregulated.

From the report:

Despite mounting evidence of the contaminant’s toxic effects, including a U.S. Environmental Protection Agency (EPA) draft toxicological review that classifies it as “likely to be carcinogenic to humans” when consumed in drinking water, the agency has not set a legal limit for chromium-6 in tap water and does not require water utilities to test for it. Hexavalent chromium is commonly discharged from steel and pulp mills as well as metal-plating and leather-tanning facilities. It can also pollute water through erosion of soil and rock.

According to the report, Tallahassee had the sixth-highest levels of hexavalent chromium (more commonly known as chromium-6), at 1.25 parts per billion, out of 35 cities for which it collected samples. Miami ranked 29th on the list.

In 1996, environmental activist and legal clerk Erin Brockovich helped secure a $333 million settlement from Pacific Gas and Electric for residents of Hinkley, Calif., who had been suffering from an unusually high rate of cancer. The company had knowingly polluted municipal tap water with the chromium-6, and the case became the basis for the 2000 Julia Roberts film Erin Brockovich.

While the EPA has established standards for “total chromium,” the EWG report notes that grouping the toxic chromium-6 with chromium-3, a harmless nutrient, skews the results.

Chromium has multiple forms, and the two most common have dramatically different consequences for human health. Trivalent chromium (chromium-3) is a nutrient essential to sugar and lipid metabolism, but hexavalent chromium (chromium-6) is a dangerous toxin. Since 1990, international health authorities have identified it as a known human carcinogen when inhaled (IARC 1990), and a growing body of evidence has linked hexavalent chromium in drinking water to stomach and gastrointestinal cancers.

In 1992, the EPA set the legal limit in tap water for total chromium — a mixture of hexavalent and trivalent chromium — at 100 ppb to protect against skin reactions known as “allergic dermatitis” (EPA 2010b). However, a safety standard that lumps levels of a toxic carcinogen with a nutrient necessary for health is like grouping arsenic and vitamin C.

The EPA told CNN in a statement:

“In order to protect people’s health, EPA has had drinking water standards for total chromium, which includes chromium-6,” the agency said in a statement to CNN. “When this scientific assessment is finalized in 2011, EPA will carefully review the conclusions and consider all relevant information, including the Environmental Working Group’s study, to determine if a new standard needs to be set.”

“I was expecting to find hexavalent chromium in some of the cities we checked, but I didn’t expect it to be so widespread,” said Rebecca Sutton, a senior scientist with the Environmental Working Group and the lead author of the study.