It can come from many sources. It occurs naturally in the environment in rocks, soils, water and air. It can come from industrial pollution, especially the burning of coal and other fossil fuels and from burning household or industrial wastes. Mercury compounds settle into sediments of lakes, rivers and oceans. Fish primarily absorb methylmercury from the prey they eat.
Fish also absorb methylmercury from water as it passes over their gills.
Practicing rituals that include the use of mercury. Release of mercury from dental work and medical treatments. Health problems caused by mercury are most severe for a developing fetus and for young children. Babies whose mothers, during pregnancy, eat fish contaminated with large amounts of mercury are at greater risk for changes in their nervous systems, and possibly in their heart or blood vessels. Nervous system changes can affect their baby's ability to learn.