They bought and tested farmed salmon filets from 10 grocery stores in Washington, San Francisco, and Portland, Oregon, and found seven were contaminated with high levels of polychlorinated biphenyls.

"These first-ever tests of farmed salmon from U.S. grocery stores show that farmed salmon are likely the most PCB-contaminated protein source in the U.S. food supply," the group, a non-profit organization that investigates environmental matters, said in a statement.

"EWG's analysis of seafood industry fish consumption data shows that one quarter of all adult Americans (52 million people) eat salmon, and about 23 million of them eat salmon more often than once a month," the group said in a statement.

"Based on these data we estimate that 800,000 people face an excess lifetime cancer risk ... from eating farmed salmon."

They called on the U.S. Food and Drug Administration to do its own study and issue health warnings as needed.

PCBs come from hydraulic fluids and oils, electrical capacitors and transformers.

They are endocrine disrupters - chemicals that act like hormones. They and related chemicals known as dioxins can cause cancer, infertility and perhaps other sexual changes.

A BANNED CHEMICAL

In the United States, PCBs have been banned for use in all but completely enclosed areas since 1979.

But they persist in the environment and in animal fat. Predators such as salmon eat other fish, which carry the chemicals in their tissues, and it can build up.

The EWG said its limited study found the farmed salmon had 16 times the PCBs found in wild salmon, four times the levels in beef, and 3.4 times the levels found in other seafood.

They said their findings are supported by other studies done in Canada, Ireland and Britain.

"In the case of farmed salmon, you have high-density fish pens off the coast of British Columbia, for example, where you have an environment that is relatively pristine but these fish are fed fishmeal from all over the world," EWG Vice President for Research Jane Houlihan said in a telephone interview.

Often this fishmeal is heavily contaminated with PCBs, Houlihan said.

"On top of that, the fish farming industry produces fish with up to twice the fat of wild salmon," she added.

The fat is the source of the omega-3 fatty acids that are supposed to be healthful - but also provides a place for the PCBs to build up.

Terry Traxell, director of the FDA's office of plant and dairy foods, told the Washington Post newspaper his office was reviewing PCBs in salmon and other foods.

Wild Oats Markets, Inc., which specializes in selling organic and "natural" foods, said it had a new source of salmon from Ireland that, while farmed, was lower in PCBs.

"Testing revealed the feed used for the Clare Island farmed salmon tested to .568 parts per trillion for PCB levels," the company said in a statement.