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 phase-out under the United Nations treaty on persistent organic pollutants. PCBs have been linked to cancer and impaired fetal brain development.

The FDA stopped short of warning consumers to change their eating habits.