Mercury is a toxic natural element that enters the food chain from both natural and human sources. Although concentrations of mercury in air are usually low and of little direct concern, mercury in water, having entered directly from soil or with precipitation, can be converted by microorganisms into its most toxic, organic form. This compound, methyl mercury, accumulates in fish tissue and becomes more concentrated as it moves up the food chain.

Human exposure to mercury occurs primarily through eating contaminated fish. In sufficient doses, methyl mercury can damage the nervous system, especially in developing fetuses and young children. The magnitude of exposure depends on the amount of mercury in the fish consumed and the amount of fish consumed.

In U.S. regulatory decision-making around mercury, it was noted that there is a “plausible” link between releases from human sources to the atmosphere and methyl mercury in fish. It is considered “plausible” because science has established neither the relative contributions of human versus natural sources, nor the pathways mercury follows from source to receptor.