Mercury (Hg)

Mercury is a trace metal. It is occurs naturally in several forms. Mercury combines with other elements, such as chlorine, sulfur, or oxygen, to form inorganic mercury compounds or “salts,” which are usually white powders or crystals. Mercury also combines with carbon to make organic mercury compounds. The most common one, methylmercury, is produced mainly by microscopic organisms in the water and soil. More mercury in the environment can increase the amounts of methylmercury that these small organisms make. Metallic mercury is used to produce chlorine gas and caustic soda, and is also used in thermometers, dental fillings, and batteries. Mercury salts are sometimes used in skin lightening creams and as antiseptic creams and ointments. Metallic mercury and inorganic mercury compounds enters the environment from mining ore deposits, burning coal and waste, manufacturing plants, natural deposits, disposal of wastes, and volcanic activity.

Elemental mercury, inorganic mercury, and methylmercury are the three most important forms of mercury in natural aquatic environments. Most mercury is released into the environment as inorganic mercury, which is primarily bound to particulates and organic substances and may not be available for direct uptake by aquatic organisms. The process of methylation of inorganic mercury to methylmercury, which is highly bioavailable, is thus an important key to the fate of mercury in the environment. The mercuric ion (Hg[II]) combines with both inorganic and organic ligands, and can be methylated. Methylation in aquatic habitats is primarily a biological process. Mono- and dimethylmercury are formed by microorganisms in both sediment and water through the methylation of inorganic mercuric ions (Hg[II]). Dimethylmercury, which is highly volatile, is generally not persistent in aquatic environments.

According to the United States Geological Survey (USGS), in the United States, the primary source of mercury is from the recovery of mercury from obsolete or wornout items, including batteries, chlor-alkali wastewater sludges, dental amalgams, electrical apparatus, fluorescent light tubes, and measuring instruments. The USGS estimated this amount at 400 tons in 1998. In addition, a very small quantity of mercury was produced in 1998 as a byproduct from approximately 10 gold mining operations in California, Nevada and Utah. Also in 1998, the USGS estimated that approximately 35% of the mercury consumed in the United States was used in the manufacture of chlorine and caustic soda, 35% for applications such as measuring and control instruments and dental amalgams, and 30% for electrical and electronic applications.
Mercury can be released to the atmosphere by human (anthropogenic) and natural activities. The 1997 “Mercury Study: Report to Congress” by the U.S. Environmental Protection Agency (EPA) reports that between 50 and 75 percent of the total annual mercury input to the atmosphere from all sources is the result of human activities.

Health Effects:

• The nervous system is very sensitive to all forms of mercury. Methylmercury and metallic mercury vapors are more harmful than other forms, because more mercury in these forms reaches the brain. Exposure to high levels of metallic, inorganic, or organic mercury can permanently damage the brain, kidneys, and developing fetus.
  

• Although inorganic mercury is the dominant form of mercury in the environment and is easily taken up, it is also depurated relatively quickly. 
• Methylmercury accumulates quickly, depurates very slowly (it is not excreted), and therefore biomagnifies in higher trophic species.
• Methylmercury builds up in the tissues of fish and in the liver and feathers of birds.
  
• The interaction of mercury and other trace elements (e.g., cadmium, copper, selenium, and zinc) can be both antagonistic and synergistic, primarily depending on exposure concentrations and form of mercury.
• The ERL and ERM for this metal are less accurate than for other metals in predicting adverse effects, highlighting the need for site-specific effects data to determine concentrations of mercury in sediment that pose a threat to aquatic biota.
• Mercury is the metal responsible for Minamata Disease and Mad Hatter's Disease
• One drop of dimethylmercury on the skin can kill a human (it is not found naturally due to its extreme volatility).