Mercury (Hg on the periodic table) is a naturally occurring chemical element that is found in air, water, and soil. It exists in several forms: elemental or metallic mercury, inorganic mercury compounds, and organic mercury compounds. Mercury circulates through the environment in different chemical forms and different physical states. In its inorganic form, it exists in three oxidation states: elemental mercury (Hg⁰), monovalent mercury (Hg⁺¹), or divalent mercury (Hg⁺²).

Inorganic mercury compounds take the form of mercury salts (powder or crystal form) and are usually white, except for cinnabar, which is red. Inorganic mercury is mined as cinnabar ore, which contains mercuric sulfide. By heating the cinnabar ore to temperatures above 1,000 °F (538 °C), the mercury in the ore is vaporized to its metallic form, or elemental mercury. The vapors are then cooled to form liquid elemental mercury.

In its elemental form, mercury is a very dense, shiny, silvery metal that is liquid at room temperature. Elemental mercury conducts electricity and expands and contracts in response to changes in temperature or pressure. These unique properties of mercury have made it useful in a variety of products (e.g., thermometers, barometers, switches, and lamps).

Organic mercury compounds are formed when inorganic mercury combines with carbon. Methylmercury (CH₃Hg⁺) is the most common form of mercury in the environment, and it is the form that is found in fish. Methylmercury is formed by bacteria in soil or water.

Mercury in the Environment:
Once mobilized in the environment, mercury can cycle through land, air, and water, undergoing a number of complex chemical and physical transformations. While it circulates in the environment and changes its form, mercury is persistent and not biodegradable. Typically, mercury is emitted to the atmosphere as a gas or as particulate matter; once released it may return to the earth's surface by either dry (e.g., gravitational settling) or wet (e.g., precipitation) deposition. Erosion, rainfall, and leaching transports the mercury from land surfaces into streams, lakes, and oceans where it accumulates in sediments. Once mercury is present in a biological system, it can "bioaccumulate" (increase its concentration) as it moves up the food chain. Mercury, once released, is thus hard to control.

Because mercury is persistent in the environment, historical contamination is a significant concern. Past discharges of mercury from tanneries, paint factories, and other industries, as well as from natural sources contribute to the levels of mercury in the environment today. For example, mercury can accumulate in sewer pipes, leading to long-term releases of mercury to municipal wastewater, even after the original source has been eliminated.

Much of the mercury entering the environment today is being deposited from the air. Although the atmospheric concentrations of mercury are generally low, small amounts of mercury continually settle on the earth and can have significant impacts on the land and water.

Sources: NESCAUM (Northeast States for Coordinated Air Use Management) et. al., "Northeast States and Eastern Canadian Provinces: Mercury Study: A Framework for Action," February 1998. Massachusetts Department of Environmental Protection, "Mercury in Massachusetts: An Evaluation of Sources, Impacts, Emissions and Controls," June 1996. http://www.mass.gov/dep/toxics/stypes/hgexsum.htm

Releases to the Atmosphere:
A combination of local, regional, and distant anthropogenic sources, natural sources, and re-emitted mercury (of either human or natural origin from existing reservoirs of previously mobilized mercury) may contribute to mercury deposition at any given location. The cycling nature of mercury presents many uncertainties in models of mercury deposition and concentrations.

Photo Source: Massachusetts Executive Office of Energy and Environmental Affairs Department of Environmental Protection

The major human contributions of mercury to the environment occur when:

• Fossil fuels are burned (gas, coal, oil),
• Wastes that contain mercury are burned or landfilled,
• Products are manufactured or recycled,
• Products release mercury during use, such as in laboratories and dental offices,
• Products are broken (e.g., fluorescent bulbs and mercury fever thermometers), and
• Gold and mercury mining operations.

Anthropogenic mercury emissions are caused both by the deliberate use of mercury (e.g., when mercury is used for gold recovery or chemical production, or when mercury-containing products are produced and disposed of) and by the use of raw materials that are naturally contaminated with mercury, such as coal, metal ores, and limestone. In fact, the largest anthropogenic mercury emitters are likely coal combustion, artisanal mining, and metal smelting, which contribute to mercury pollution on a global scale.

When solid waste is burned in a municipal solid waste or medical waste incinerator, mercury that is present is volatilized and may be released to the atmosphere, depending on the air pollution control device(s). Some mercury found in landfills, land-applied sludge, and ash may also be released to the atmosphere, although good data on mercury emissions from these sources are presently lacking in most of the U.S.

Natural activities that can release mercury into the environment include:

• Volcano eruptions
• Weathering of rocks
• Forest fires
• Emissions of previously deposited mercury by biologic processes (e.g., vaporization of mercury from water bodies after biologic conversion to methylmercury)

The United Nations Environmental Programme (UNEP) is currently finalizing a report, Global Atmospheric Mercury Assessment: Sources, Emissions, and Transport, which includes the most current data available on global mercury emissions: http://www.chem.unep.ch/mercury/Atmospheric_Emissions/UNEP%20SUMMARY%20REPORT%20-%20CORRECTED%20May09%20%20final%20for%20WEB%202008.pdf.

Last Updated: 11/14/08