Millions of Americans eat salmon these days because of the well-documented health benefits this fish provides. Salmon is loaded with omega-3 fatty acids and other nutrients that are especially good for the heart. But can we get too much of a good thing? A new study has raised concern -- and confusion -- about how much salmon is safe to eat.

Researchers from Indiana University reported in the journal Science (Vol. 303, No. 5655: 226-229) that farmed salmon have higher levels of potentially cancer-causing chemicals than wild salmon. Most Americans eat farmed salmon because it is generally more available and less expensive than wild salmon.

"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," said lead researcher Ronald Hites, PhD, of the university's School of Public and Environmental Affairs.

Although virtually everyone agrees that the contaminants are undesirable, other experts say it is not clear how much of a risk they pose to humans, or how much salmon in particular contributes to the amount of these toxins that might be in the human body.

"Unfortunately, the contaminants measured in salmon are also present in other food products … including dairy products and meats," said Elizabeth Ward, PhD, director of surveillance research for the American Cancer Society.

Also, consumers have little way to know how contaminated any given piece of fish might be, Ward noted, or what the realistic consequences might be of eating it. While some of the compounds measured by the Indiana team have shown toxic effects in animals, it's unclear how much of them it would take to harm a human.

Hites and his colleagues sampled about 2 metric tons of salmon, including wild Pacific salmon and farmed Atlantic salmon from North America, South America, and Europe. The farmed salmon had significantly higher concentrations of PCBs (polychlorinated biphenyls) and dioxins, chemicals that have been linked to cancer in some studies. Farmed fish from Scotland and Denmark's Faroe Islands had the highest levels of toxins; farmed fish from Chile had the lowest, although these were still higher than what was found in wild salmon.

Hites said the food given to farmed fish is probably to blame for the higher contaminant levels. Farmed salmon are fed a concentrated mix of fish meal and fish oil, while wild salmon eat other fish. Because PCBs and dioxins collect in animal fat, all salmon have the potential to accumulate these toxins, Hites said. But the levels in the concentrated feed are likely higher than what wild salmon would consume naturally.

The levels of toxins Hites found in the farmed salmon were still well below what the US Food and Drug Administration (FDA), which regulates food, considers hazardous. But using more stringent guidelines from the Environmental Protection Agency (EPA), Hites and his team recommended that people eat only 1 or 2 servings of farmed salmon a month. (Wild salmon can be eaten as often as 8 times per month, they said.)

"The EPA guidelines are developed very specifically for cancer risk," Hites said. "The FDA guidelines are based on a whole variety of other factors. They're mandated to take into account other diseases and availability in the food supply, so EPA limits are always more stringent."

Hites said he hoped his research would spur the two agencies to bring their recommendations closer together.

The study drew immediate criticism from salmon farmers.

Industry group Salmon of the Americas said it was "misleading" to base consumption recommendations on the EPA levels, and accused the researchers of ignoring "the overwhelming evidence about the health benefits of farmed salmon." The group also said it has reduced PCBs in its fish since the study was conducted by substituting vegetable matter for a portion of the fish oil and fish meal in the salmon feed.

Those types of changes are important, Ward said. "By raising consumer awareness, the new study will hopefully motivate regulators and the fish farming industry to change the food supply and reduce contamination," she added.

But what does all this mean for the average consumer? Is it still safe to eat salmon?

"Based on current data, it seems premature to stop eating salmon as part of a healthy, varied diet," said Ward. "At this point, there's no solid scientific guidance that compares risks with benefits. Most concerns about levels of specific contaminants in specific foods can be addressed by following ACS dietary guidelines."

ACS recommends people eat a variety of foods, with an emphasis on plant sources. The guidelines also advise substituting fish, poultry, lean meat, and beans for fatty red meat.

The FDA recommends trimming visible fat on meat, removing the skin from fish and poultry, and cooking in ways that reduce fat (like broiling or grilling) to reduce the risk of eating contaminants.

Hites said the decision on how much salmon to eat is one people may have to make for themselves, depending on their "life stage."

"I'm an old guy, and I eat salmon once every couple of weeks," he said.