Some fish oil capsules sold as as health supplements contain trace amounts of PCBs, according to a lawsuit filed Tuesday in California by an environmental group.

The Mateel Environmental Justice Foundation introduced the lawsuit, along with two other plaintiffs, alleging fish oil companies have broken California law by not putting PCB levels on their labels.

California is the only state that requires labelling to warn consumers a product may contain trace amounts of polychlorinated biphenyl compounds, a toxic, man-made chemical that has been banned from production in the U.S. since 1979.

The Mateel group commissioned tests on 10 fish oils and found varying amounts of PCBs in all of them. The amounts ranged from 12 nanograms a day to more than 852 nanograms a day, based on the daily recommended dose written on the labels.

The allegations in the lawsuit have not been proven in court.

The fish oils came from shark, cod and salmon. There are more than 100 fish oil brands, and further testing is being done. Outside California, there are no labelling requirements for PCBs in the U.S.

"The people buying these fish oil products are not being told that the PCBs are there," lawyer and plaintiff David Roe said during a conference call to announce the lawsuit. Roe was involved in the fight in California for stricter requirements on label information.

"People buy fish oil products to improve their health, not to put it at risk, so it's particularly important that they know about this," he said.

Beyond the labelling concern, the foundation hopes the lawsuit will force companies to reduce PCB levels in their products. PCBs are known carcinogens and have been shown to cause birth defects.

Despite being banned from production in the U.S. for 30 years, PCBs are extremely persistent in the environment and continue to show up in fish taken from bodies of water where the compounds exist.

"Ironically, fish oil products are often touted for their Omega-3 health benefits, and our message today to consumers of fish oil is 'Buyer beware,'" Peter Kelley, a spokesman for the plaintiffs, said in an interview with CBC News.

"These substances are so toxic they're tested to the billionth of a gram. There is no known safe level of these things. Because these supplements are recommended for pregnant women, it's particularly important."

The lawsuit names six international manufacturers of fish oil and two drugstore chains: CVS Pharmacy, Inc.; General Nutrition Corp.; Now Health Group, Inc.; Omega Protein Inc.; Pharmavite LLC (Nature Made brand); Rite Aid Corp.; Solgar Inc.; and TwinLab Corp.

It isn't known if the fish oils they tested are sold in Canada, but Kelley said it is likely, since several of the companies ship globally.

Fish oil manufacturers have processes to remove PCBs, but they are not completely reliable, the testing commissioned by the plaintiffs indicates. Until there is proper labelling, a consumer can't know which products have the lowest levels of PCBs, said another plaintiff, California environmentalist Chris Manthey.

"It isn't good for the good guys in the industry because there's no incentive for the ones who are cleaner to put the levels on their product," Manthey said. "It's part of the reason there's been such confusion."