I was reading about an article regarding certain omega-3 fish oil supplements that were contaminated with PCBs (polychlorinated biphenyls). There was a study done on 10 randomly selected products from a health food store. The levels of PCBs found ranged from very low to amounts high enough to caus concern. Now there is a lawsuit filed against CVS, GNC, Now Health, Omega Protein, Pharmavite, Rite Aid, Solgar, and Twin Lab due to their high concentrations of contaminants. Remember that these products were randomly selected for testing, so there could be other companies that are contaminated.
There is an "acceptable" upper limit for PCBs in fish oil: .09 ppm. Unfortunately, since we do live in a toxic world, it is nearly impossible to remove all the contaminants. I have always said to make sure you know where your supplements are sourced from and how they are processed. The two brands of fish oil I have recommended are Metagenics and Nordic Naturals. They are both made in small batches and have high turnover, which means they aren't sitting on a shelf somewhere for a long time. Also, Metagenics will not ship on a Thursday or Friday to make sure the oils aren't sitting somewhere over the weekend in the heat! That's quality control!