The greatest source of mercury in the biosphere is currently of human origin. Mercury is number three on the ATSDR 2017 Substance Priority List. Two-thirds of the mercury entering the environment comes from man-made sources including industrial plants, coal burning, and incinerators, and the additional one-third enters from natural sources. Many former chlor-alkali facilities are major point sources of mercury to aquatic ecosystems and are currently designated Superfund sites. Mercury is released into the air or directly into waterbodies and makes its way into lakes and estuaries, where some of it settles to the bottom. Mercury is considered a global pollutant capable of spreading far beyond its source area due to its stability in the atmosphere. Even the arctic, which has no known sources of mercury emissions, harbors mercury-contaminated fish, and recent studies indicate whales who feed in the arctic have high levels of mercury in their tissue. Lake, river, and estuary bottoms serve as the site for the transformation of inorganic mercury to organic methylmercury by bacteria living in the mud.
Methylmercury is the extremely toxic form of mercury that biomagnifies in aquatic food chains because unlike inorganic mercury in the atmosphere, methylmercury is a small organic compound that can be easily absorbed by living organisms. It is a potent neurotoxin and the easiest form for animals to store in their tissues. Methylmercury binds to proteins and easily crosses cell membranes, including the blood-brain barrier and the placenta. It harms the brain, affecting memory, understanding, and movement. Studies have shown that mercury exposure in humans can result in developmental delays in children, motor impairment, cardiovascular effects, and in acute cases, death. Its effects have been studied in fish, whales, seals, and seabirds. Affected wildlife, such as loons, develop behaviors that ultimately reduce their survival and reproduction, and put the population at risk. Studies conducted on human populations have estimated that between 200,000 and 400,000 children in the United States alone are born each year with pre-natal exposure to methylmercury sufficient to put them at risk of neurologic impairment.
Mercury in fish
People are exposed to mercury mainly through eating fish and shellfish and 95 percent or more of the mercury in fish is the more toxic methylmercury. According to EPA, people should not eat fish fillets containing more than .3 parts per million (ppm) of methylmercury. Fish caught in water with very low concentrations of mercury (less than 1 part per trillion) can nonetheless contain harmful levels of methylmercury. In some marine ecosystems, the concentration of methylmercury increases 10 million times as it makes its way up through the food web from microscopic algae to shark and tuna.
Mercury concentrations in fish from lakes and rivers throughout the United States now exceed the mercury levels that are cause for human and wildlife health concern. As of 2008, all 50 states and one territory have fish consumption advisories for mercury. In addition, all states on the Atlantic and Gulf of Mexico have coastal fish advisories. Although much of the scientific research on mercury in fish has focused on freshwater ecosystems, most Americans are exposed to mercury through seafood consumption. There are many questions to be answered about where the mercury in the fish that we eat comes from and what fish are safe to eat. Solutions to the complex problem of mercury pollution have been impeded by conflicting information on the sources, transport, and accumulation of mercury in the environment. Our program hopes to address these questions and therefore provide the scientific basis for solutions to this pressing environmental and human health issue.