It is best known as the contaminant exposed by campaigner Erin Brockovich, whose battle against a polluter was dramatized in the May 2000 movie of the same name.

"I am relieved and pleased and sorry because there are a lot of people who have ingested chromium 6," said Brockovich, who still works in Los Angeles as a legal consultant on environmental issues.

"It is high time but it is no surprise to me," she told Reuters. "This is a chemical that there have been ongoing arguments about, and now a third party has concluded that it can cause cancer by ingestion."

Environmentalists, who have been fighting for decades for tighter limits on how much chromium can be present in drinking water, said the findings offered a basis for such restrictions.

High doses of chromium 6 given to rats and mice in drinking water caused malignant tumors, the two-year study by the NIH's National Toxicology Program or NTP found.

"In the rats we saw oral cavity tumors," said Michelle Hooth, who worked on the report. "In the mice we saw tumors in the small intestine."

Hooth said the animals were given much higher doses of chromium than people would ever encounter in drinking water, which is the usual practice in testing chemicals for cancer-causing potential.

INDUSTRIAL USES

Hexavalent chromium compounds are often used in electroplating, leather tanning and textile manufacturing and have been found in some drinking water sources, the NTP said.

Brockovich started investigations in 1991 into exposure to chromium 6 in drinking water in the town of Hinkley, California. In 1996, she and lawyer Ed Masry won a landmark US$333 million settlement with Pacific Gas and Electricity over claims of toxic exposure.

Brockovich said on Wednesday she had settled another lawsuit with PG&E involving the contaminant last year. But she said there were potentially dozens more toxic sites around the country.

The lowest doses given to rats and mice in the study were 10 times higher than what humans could consume from the most highly contaminated water sources identified at Hinkley, the researchers said.

From 1987 to 1993, according to the Toxics Release Inventory, chromium compound releases to land and water in various US states totaled nearly 200 million pounds.

"The chromium industry has been trying to convince regulators for years that hexavalent chromium is actually quite safe when consumed via drinking water, even though it has long been known to be carcinogenic when inhaled," said Renee Sharp, a senior analyst at the Environmental Working Group.

"NTP's findings will finally allow state and federal regulators to set drinking water standards based on up-to-date sound science, rather than having to rely on old, inadequate, and/or biased studies often funded by chromium polluters," added Sharp, whose group has lobbied for tighter regulation of chromium and other chemicals.