Photo by Oktaviani Marvikasari.

Y’all know I’m a huge fan of fish oils for almost every aspect of health — from heart and mind to skin and joints, and that’s only for starters.

But a California environmental group found there’s a fly in the ointment, so to speak: They tested 10 (out of 100) common brands of fish oils and found them to be high in carcinogenic and toxic polychlorinated biphenyls, or PCBs. The chemicals were banned in the US 30 years ago, which just goes to show how toxins stick around in our environment once we let those genies out of the bottle. (This is a good thing to remember at a time when even the stalwart European Union is sanctioning new genetically engineered potatoes.)

California has a labeling law that, the environmental groups say, requires these fish-oil purveyors to list PCBs on the bottle, and they’ve just filed a lawsuit to this effect.

Of the 10 brands tested, eight showed problems. Top offenders were Now Foods Salmon Oil and Nature Made Cod Liver Oil. The lawsuit also named CVS Pharmacy, General Nutrition Corp,  Omega Protein, Rite Aid, Solgar and TwinLab Corp.

PCBs aren’t the only potential health concern. Heavy metals also can accumulate in the fishes highest in omega-3 fatty acids (the reason you’re taking fish oils in the first place). Good companies test for this and either choose their fish well or find ways to remove the chemicals.

So if you’re in the market for fish oils, look for brands that have been independently tested, or ask for recommendations from pros who carefully vet their products.

A version of this piece originally appeared on WellWire.com, where Dr. Izakson is a regular contributor.