In the world of over-the-counter dietary supplements, fish oil has attained an unrivaled status. Unlike fad drugs that flame out or quietly disappear, fish oil has steadily grown in popularity. Indeed, in a February poll, more respondents said they use fish oil than a standard multivitamin pill.

But a lawsuit filed on Tuesday says there's a dark side to the supplements that most consumers aren't aware of: many of them dangerously high levels of PCBs, a chemical linked to birth defects and several types of cancer.

The plaintiffs, led by the Mateel Environmental Justice Foundation and two environmentalists from New Jersey, tested 10 brands of fish oil supplements and found varying levels of PCBs in each; the highest level was 850 nanograms, and the lowest only 12.

But the 10 brands all had something in common: they all contained some level of PCBs, which puts them all in violation of California's Proposition 65. That law, enacted in 1986, requires manufacturers to warn consumers of exposure to any detectable amount of a listed chemical, including PCBs.

PCBs are a class of organic compound that were once common in coolant and certain electrical equipment. PCPs were banned by Congress in 1979 -- and by U.N. Treaty in 2001 -- due to their link to melanoma, liver cancer, and brain cancer, among others. Despite their three-decade hiatus from production, PCBs are still present in waterways and, unsurprisingly, in the fish that inhabit them.

Indeed, eating fish is the most common route of exposure to PCBs.

An EPA estimate from 2000 says that eating certain kinds of fish from the infamously PCB-contaminated Hudson River raises the risk of developing cancer to one in 2,500, an amount 1,000 times higher than the EPA's target level. And according to the California Office of Environmental Health Hazard Assessment, PCBs in fish can reach levels hundreds of thousands of times higher than the levels in water.

Whether the suit has any significant effect on fish oil sales remains to be seen. Omega-3, the supplement's purportedly beneficial ingredient, is said to reduce the risk of everything from heart disease to depression to Alzheimers.

Among the defendants are Omega Protein the world's biggest producer of fish oil supplements General Nutrition Corp, Now Health Group Inc, and CVS and Rite Aid, which sell the supplements.

The Council of Responsible Nutrition, which describes itself as the leading trade association representing dietary supplement manufacturers and ingredient suppliers, jumped on the suit as ill-informed and misguided.

PCBs are ubiquitous within the environment, which means that all fish -- whether fish found in oceans and rivers or fish oil supplements -- contain at least trace amounts of PCBs, said Andrew Shao, a Senior Vice President of CRN. Shao said that the plaintiffs are attempting to frame this as a public health concern, when in reality, fish oil has enjoyed decades of safe use.

Pharmavite, another fish oil supplement manufacturer named in the suit, said that the magnitude of the science supporting the benefits of consumption of fish oil far outweighs the results of this extremely limited investigation.