Many health experts urge people to eat fish such as salmon because it contains healthy fats, especially the omega-3 fatty acids that can lower the risk of heart disease and perhaps have other health benefits, too.

But the researchers, as well as environmental groups, said the findings in Science indicate that people should choose their fish carefully. They should also demand that salmon be clearly labeled to indicate whether it is farmed or wild so they can make informed choices about which fish to eat.

The team at Indiana University, University at Albany, Cornell University and elsewhere analyzed toxic contaminants in 700 farmed and wild salmon taken from markets in 16 cities in Europe and North America.

"We think it's important for people who eat salmon to know that farmed salmon have higher levels of toxins than wild salmon from the open ocean," environmental affairs professor Ronald Hites of Albany, who led the study, said in a statement.

They looked for 13 different chemicals known to build up in the flesh of fish, including polychlorinated biphenyls or PCBs, dioxins, toxaphene, dieldrin, hexachlorobenzene, lindane, heptachlor epoxide, cis-nonachlor, trans-nonachlor, gamma-chlordane, alpha-chlordane, Mirex, endrin and DDT.

Some are pesticides, others are industrial by-products, and many are known or suspected cancer-causing agents.

EAT ONCE A MONTH OR LESS

Farmed salmon taken from markets in Frankfurt, Edinburgh, Paris, London, Oslo, Boston, San Francisco, and Toronto had the highest levels, and the researchers said consumers should eat no more than one-half to one meal of salmon per month. A meal was eight ounces (one-quarter of a kg) of uncooked meat.

Farmed salmon from supermarkets in Los Angeles, Washington, D.C., Seattle, Chicago, New York and Vancouver had toxins high enough to suggest that people eat no more than two salmon meals a month, based on Environmental Protection Agency standards.  

In contrast, it would be safe to eat up to eight meals a month of wild salmon, they said. Other groups note that walnuts, flaxseeds and other non-fish sources are rich in omega-3s.

Many chemicals can build up in the body, staying for years or even a lifetime. But the body also processes some out, so experts can figure out a safe or acceptable level of intake.

The study fits in with other research on chemicals in salmon. Two studies published in the journal Chemosphere last year found elevated levels of PCBs, certain pesticides, and flame retardants in farmed salmon.

And last year the Environmental Working Group said it found elevated PCB levels in farmed salmon filets taken from 10 U.S. grocery stores.

"This unquestionably large, new study strongly confirms earlier research, and it leaves little room for the farmed fish industry to argue away the problems of polluted farmed seafood," the Environmental Working Group's Jane Houlihan said.

But Charles Santerre, a food and nutrition expert at Indiana's Purdue University, said the study in fact showed that farmed salmon is safe.

"The study demonstrates that farmed salmon is very low in contaminants and meets or exceeds standards established by the Food and Drug Administration and the World Health Organization," Santerre said in a statement.