Mercury occurs naturally in the environment.
According to FDA toxicologist Mike Bolger, Ph.D., approximately 2,700 to 6,000 tons of mercury are released annually into the atmosphere naturally by degassing from the Earth's crust and oceans. Another 2,000 to 3,000 tons are released annually into the atmosphere by human activities, primarily from burning household and industrial wastes, and especially from fossil fuels such as coal. Mercury vapor is easily transported in the atmosphere, deposited on land and water, and then, in part, released again to the atmosphere. Fish absorb methyl mercury from water as it passes over their gills and as they feed on aquatic organisms. Larger predator fish are exposed to higher levels of methyl mercury from their prey.