Mercury occurs naturally in our environment and we release it in different forms through industrial processes including energy production. Emissions from these facilities contribute to mercury in the atmosphere. These mercury emissions (elemental and inorganic) undergo some chemical reactions and eventually return to the earth in the form of inorganic mercury in precipitation and in particulate form, accumulating in the soil or in rivers, lakes, and streams. Once in water, Elemental and inorganic mercury can be transformed into organic mercury or methylmercury by microscopic organisms in the soil and water. This is a natural process. The methylmercury then accumulates in fish and shellfish and from there to birds and animals that eat fish. The most likely source of methylmercury exposure in humans is eating contaminated fish, which can result in long-term damage to the kidney, liver, and central nervous system. The image below shows how mercury enters and cycles through the ecosystem.

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	Image Courtesy of the Hubbard Brook Research Foundation. This image shows how mercury gets deposited into the air, cycles through the rain and transported through watersheds and converted to methylmercury.

Big Fish Eat Contaminated Smaller Fish – and We Eat the Big Fish
Organic mercury tends to increase up the food chain, particularly in lakes. The mud at the bottom of a lake may contain 100 or 1000 times the amount of mercury than is in the water. Worms and insects in the mud extract and concentrate the organic mercury. Also, phytoplankton absorb mercury very efficiently and end up with mercury concentrations that are often 10,000 times the concentrations in the water (this process is not completely understood yet). Small fish that eat these critters further concentrate the mercury in their bodies. This concentration process, known as bioaccumulation or biomagnification, continues as larger fish eat smaller fish until the top predator fish in the lake may have methylmercury levels in their tissues that are up to 1,000,000 times the methylmercury level in the water in which they live. The methylmercury can then transferred to our systems when we eat contaminated fish. The image below shows how mercury accumulates in lakes and gets into the food chain.

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	Image Courtesy of the Hubbard Brook Research Foundation. This image shows how mercury gets into the food chain.

How Mercury Affects Us
All forms of mercury can affect the human nervous system. Methylmercury and metallic mercury vapors are the most harmful forms for humans because they can affect the human brain. The most common exposure, however, is from eating contaminated fish and shellfish. The greatest health risk falls on children and developing fetuses where mercury can impair neurological development. Methylmercury poisoning in a fetus can simply be the result of the mother consuming contaminated fish and shellfish. The fetus' developing nervous system can be damaged causing severe disabilities.

The effects of methylmercury poisoning in adults can include impaired peripheral vision and speech, lack of coordination, and numerous other afflictions. Reducing mercury emissions in all forms has a beneficial effect on human health as well as the environment.

Fortunately, being aware of the issues is the first step to dealing with them and reducing exposures.