Sources, Fate and Transport of Mercury in the Environment
Mercury is released into the environment from a variety of sources, both natural and anthropogenic. Natural sources, primarily in the form of elemental mercury, include volcanic emissions, degassing from soils, and volatilization from the ocean. Before the industrial revolution, mercury was naturally emitted into the environment at levels believed to pose no risk to human health. It is impossible to separate out current levels of mercury in the environment as either anthropogenic or natural, but several experts have estimated that humans have doubled or tripled the amount of mercury released into the environment (1).
Anthropogenic emissions of mercury are from use of fossil fuels (especially coal), and other extracted, treated, or recycled mineral materials as well as from mercury used intentionally in products or processes. Mercury has been used in thousands of products and industrial processes including chlorine and caustic soda manufacture; use in laboratories; paint manufactured before 1991; electronic uses such lighting (e.g. fluorescent lamps), wiring devices and switches and batteries; thermometers, thermostats, barometers, and other related instruments; and dental supplies (e.g., dental amalgam fillings) and medical equipment.
Anthropogenic mercury releases are mainly from industrial processes and combustion sources. EPA estimates that combustion point sources account for 85% of anthropogenic mercury emissions. Four specific combustion source categories make up the majority of emissions: municipal and medical waste incineration (25% each); utility boilers (21%); and commercial/industrial boilers (12%)(2).
Mercury is released into the atmosphere from anthropogenic emissions as either vapor (elemental or oxidized mercury) or as particles (oxidized compounds). Natural emissions are mainly in elemental mercury form. Mercury may reside in the atmosphere for about one year, allowing global circulation systems to transport elemental mercury emissions from source of emission to anywhere on earth before transformation and deposition take place(3). Mercury is transferred from the atmosphere to the earths surface via wet or dry deposition.
The majority of mercury in surface soil is in the form of oxidized mercury complexes/compounds; however, a small fraction is methylmercury and elemental mercury. Mercury complexes deposited in soils can be transformed back into gaseous mercury by light and humic substances and re-enter the atmosphere. Studies have consistently shown that plant uptake is negligible and consequently, animals foraging on plants accumulate little mercury (4).
In addition to direct deposition, mercury can also reach water from soil run-off, although the amount partitioning to run-off is expected to be small since mercury binds to soil; run-off is probably in the form of suspended sediments. Once in water, mercury can either enter the food chain, settle into sediment, or volatilize back into the atmosphere. Entrance into the food chain begins with bacteria in water which can take up mercury in its inorganic form and metabolize it to methylmercury. The methylmercury-containing bacteria may be consumed by the next level in the food chain, or they may excrete the methylmercury into the water where it can adsorb to plankton, which are also consumed by the next level in the food chain. Even small environmental concentrations of mercury in water can readily accumulate to potentially harmful concentrations in fish and fish-eating people; the ratio of concentration of methyl mercury in fish tissue to that in water is usually between 10,000 and 100,000(5). Fish higher in the food chain, such as sharks and swordfish, have much higher mercury concentrations than fish lower on the food chain. According to EPA, forty-one states have advisories for mercury in one or more water bodies, and eleven states have issued statewide mercury advisories.
Ultimate Fate of Mercury in the Environment
Mercury is continuously mobilized, deposited and re-mobilized in the environment. The only sinks for removal from the biosphere are deep-seas sediments or well-controlled landfills. If the release of mercury into the environment is reduced, resultant decreases in mercury concentrations in the environment would occur slowly, most likely over many decades or centuries. Although the U.S. and many other industrialized countries have reduced mercury use and release in recent times, these reductions are not reflected yet in the environment and no water body has ever had a fish consumption advisory removed.
Fate of Mercury and Relevance to Human Health
As described above, mercury concentrations in dissolved water and air are relatively low and of little direct concern to human health. However, when mercury enters water, the biological processes detailed above transform it into a more toxic form of mercury, methylmercury, which builds up in fish and animals that eat fish. Therefore, the greatest exposure of toxicological concern to humans is consumption of contaminated fish.