Though fish absorb methylmercury very readily, they excrete it very slowly. Thus, the older a fish is, the higher its methylmercury concentration tends to be. Because small fish and other aquatic organisms bioconcentrate methylmercury, they in effect provide concentrated doses of methylmercury to fish that prey on them. The increase in chemical concentration through higher trophic levels is called biomagnification. Biomagnification can result in a significant methylmercury exposure to humans, bald eagles, and other top level predators.
Humans do excrete methylmercury, though only very slowly.
As with fish, if humans take in methylmercury faster than they get rid of it, the amount remaining in their body will increase. Thus, with this type of chemical, limited short-term exposure is generally less important than cumulative exposure over time. The total amount of a chemical in the body is called the body burden of that chemical. In evaluating potential health impacts, it is generally the body burden which is of primary concern. A question we are sometimes asked is why our mercury advisories target women of reproductive age as a sensitive group.