The U.S. EPA has launched a "rigorous and comprehensive" assessment of the effects of hexavalent chromium (also known as chromium-6) on human health. The announcement comes in light of a report released Tuesday that found that tap water from 31 of 35 U.S. cities tested contains chromium-6, popularized by the 2000 movie Erin Brockovich, according to the Environmental Working Group (EWG), a Washington, D.C.-based non-profit advocacy group.

The EPA's assessment will appear in its Integrated Risk Information System (IRIS) database by early summer 2011. The last IRIS review of chromium-6 was completed in 1998.

In September the EPA released a draft of its review of the compound and currently is gathering public comment through Dec. 29.

"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," the EPA said in a statement released Tuesday.

Treatment methods such as coagulation/filtration, ion exchange, reverse osmosis, and lime softening have been proven to reduce total chromium to below 0.1 milligrams per liter or 100 parts per billion (ppb), the maximum contaminant level goals (MCLG) for total chromium set by the U.S. EPA.

Yet the media blitz by EWG has thrust into the spotlight a compound that, as recently as March, had not been viewed as an immediate threat in drinking water. As part of its latest six-year review of drinking water standards in March 2010, EPA determined that the limit remained acceptable. Although the agency has determined that chromium-6 may cause cancer if inhaled, drinking water suppliers have not been required to test for the compound.

"Water utilities currently monitor for total chromium, of which hexavalent chromium is a sub-species," said David LaFrance, executive director for the American Water Works Association. "There is nearly universal compliance with the existing standard, which is good news for tap water consumers."

Odorless and tasteless, chromium is an inorganic metallic element that is found naturally in rocks, plants, and soil.

High concentrations of chromium-6 typically occur in industrial areas such as steel and pulp mills, since both chromium-3 and -6 are used for chrome plating, dyes and pigments, leather, and wood preservation. In those areas where its occurrence is greater, the compounds usually are released via leakage, poor storage, or improper disposal.

States may set more stringent drinking water maximum contaminant levels for chromium than EPA. In 2009 the State of California did just that, proposing setting a public health goal for hexavalent chromium in drinking water of 0.06 ppb - the first step toward establishing a statewide enforceable limit. But according to EWG, chromium-6 was found in concentrations exceeding California's proposed maximum in 25 of the cities tested, and more than 200 times higher in Norman, Okla., which has a concentration of 12.90 ppb.

LaFrance urged water suppliers to help put the study into perspective when communicating with their consumers. "The key question to answer is whether the substance presents health concerns at the level it is detected. That's why the federal regulatory process requires EPA to examine potential health impacts of the substance, paths of exposure and occurrence data. A thorough evaluation of all this data increases the likelihood that new regulations will offer meaningful risk reduction," he added.