WASHINGTON – A sharp rise in the consumption of farmed salmon may be posing a health threat to millions of Americans because of high levels of PCBs that have been found in limited samples of the popular fish, according to a study released Tuesday.

Diet- and health-conscious Americans have turned to salmon in growing numbers in recent years, and about 23 million people now eat the fish more than once a month. But a study by the Environmental Working Group found that seven of 10 farmed salmon recently purchased at grocery stores in Washington, D.C., San Francisco and Portland, Ore., contained concentrations of PCBs that were 16 times higher than those found in wild salmon fished from the ocean and roughly four times higher than those found in beef and other seafood.

A majority of the salmon consumed in the United States is produced on aquatic farms and is fed fishmeal that consists of mostly ground-up small fish that have absorbed PCBs. PCBs, or polychlorinated biphenyls, have been banned in the United States since the late 1970s and are among a dozen chemical contaminants targeted for worldwide phaseout under the United Nations treaty on persistent organic pollutants. PCBs, which were used as industrial insulators, are highly persistent in the environment and have been linked to cancer and impaired fetal brain development.

“When Congress banned PCBs in 1976, no one contemplated that 20-odd years later we would have invented a new industry that re-concentrates these toxins in our bodies,” said Jane Houlihan, vice president for research at the Environmental Work Group, an advocacy organization.

The group assessed the possible health threat by correlating data from several relatively small international studies of PCB levels in salmon with industry data on consumers’ eating habits.

The report elicited immediate condemnation from representatives of the farmed salmon industry, who said the group had used skimpy research to engage in “scare tactics.” The industry spokesmen disputed many of the study’s findings and stressed that PCB levels in farmed salmon are well below limits set by the Food and Drug Administration in 1984.

“Salmon farmers are concerned about meeting food safety standards – that is the basis of our operations,” said Alex Trent, acting executive director of Salmon of the Americas, an advocacy group representing farmed salmon producers in the United States, Canada and Chile. “What the EWG says in their report has nothing to do with farmed salmon exceeding current FDA standards for PCBs or any other currently accepted food safety issues related to farmed salmon.”

The environmental group said the FDA rules were out of date and should be brought in line with Environmental Protection Agency standards for PCB levels in salmon caught by recreational fishermen – standards they say are “500 times more protective” than the FDA standards.

An FDA official said Tuesday that his agency beginning in 2000 had “ramped up” its review of the prevalence of PCBs in salmon and other foods, and will consider a number of strategies once the investigation is complete, including revising its advice to consumers or revising its standards for the farm salmon industry.