Mercury (Hg) is a naturally occurring element in the Earth’s crust, and is present in trace quantities in coal and other geologic materials. Mercury undergoes a variety of chemical and physical changes as it cycles through the environment, but the same amount of mercury has existed on Earth since its formation.
Most mercury is released into the atmosphere from surface and undersea volcanic eruptions, as well as from deep-sea vents, hot springs, ocean evaporation and the erosion of soil and rock. According to researchers with the Smithsonian Institution, more than 5,000 surface and underwater volcanoes erupt 50 to 60 times a month. Consequently, volcanic “degassing” is considered to be the single largest source of atmospheric mercury. This fact has important implications for determining how best to address mercury exposure in Michigan.
Mercury is the only pure metal liquid at room temperature. The most recognizable form is the heavy silver liquid that beads at the slightest touch. This “elemental” form of mercury vaporizes quickly and, therefore, is the most common form of mercury in the atmosphere. Because gaseous elemental mercury can persist for weeks or months, it may be transported on wind currents across long distances. This fact also must be considered in crafting an effective mercury-control regimen.
The mercury found in soil and water is typically the product of a process called “oxidation.” That is to say, when elemental mercury encounters oxidants such as ozone, bromine and chlorine in the atmosphere, it loses electrons, becomes more water soluble, and is carried to Earth by rain and snow. This process is termed “wet deposition.”
Mercury enters the food chain primarily through fish and other seafood. As explained above, oxidized mercury is deposited into lakes and oceans through precipitation. Some of this oxidized mercury is absorbed into sediment on the lake or ocean floor. A portion of the oxidized mercury is ingested by bacteria in the water, some of which, in turn, generate methane. This type of bacteria “methylates” the mercury, producing methylmercury, which is considered to be the most toxic form of mercury. Methylmercury accumulates in the tissue of fish from feeding on the higher end of the food chain. High doses of methylmercury over extended periods of time can be highly toxic.