Mercury is emitted into the air from several sources: volcanic eruptions and other natural emissions, burning coal in power plants, gold refining, cement production, and waste incineration. Mercury in the air travels all over the planet high above the earth. Some of it settles in oceans and rivers where it becomes “methyl mercury” which builds up in fish over time.
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Fish that are older, larger and that eat other fish have higher levels of mercury in their bodies than others. This is because mercury is passed up the food chain, from algae eaten by small fish to larger fish that feed on smaller fish. Because it’s not excreted well, mercury levels in the fish continue to increase over the fish’s life as they eat more fish and grow in size. That’s why fish like shark contains 10 times more mercury than herring.
Other pollutants also build up in fish, like polychlorinated biphenyls (PCBs) and some pesticides, which are found in the fatty parts of fish and fish oil. Different types of seafood have very different levels of pollutants, depending on their species. The Fish List posted on this website shows which fish are high in pollutants and should be avoided. It also lists fish that are healthy to eat. To find out the health problems that can be cause by eating too much high-mercury fish, go to the Health Effects of Mercury tab on this website.