Cod liver oil, obviously, is extracted from cod liver. That’s not a good starting point from the perspective of contamination of heavy metals. Cod is a predatory fish at the top of the food chain (meaning that a lot of toxins such as PCBs, and heavy metals have been accumulated from smaller fish). Krill oil and fish oil (which is mainly made from anchovies) are made from relatively safer sources because these species are lower in the food chain and are harvested from the waters around Antarctica and the southern Pacific Ocean.
Secondly, the liver is the organ with the highest concentrations of impurities because detoxification is one of the important things a liver does. Cod liver oil manufacturers must go to even greater length to purify the cod liver oil from PCBs and heavy metals than krill oil and fish oil manufucturers. It's unfortunate for consumers to know the exact extraction process that has been used for cod liver oil, fish oil or krill oil.
The omega 3 trade association GOED has set very strict standards for contaminants, and all GOED members volunteer to live by these standards, As a result, most fisg oils, inclduing coid liver oils, have been very clean. Follow this link to the GOED member list.
The third aspect to consider with potential the contamination is the quantity consumed. Acceptable contamination levels are typically set by GOED, EPA, FDA and the state of California in parts per million or parts per billion (PPM or PPB). This measure doesn't take into account the actual levels consumed.
Other articles in the WellWise krill oil versus cod liver oil series are: