Congressional concern about mercury in the environment has greatly increased in recent years due to emerging scientific evidence that exposure to low levels of mercury may harm the developing nervous systems of young children. At higher levels of exposure, mercury is known to be a potent neurotoxin. People in the United States are exposed to mercury primarily by eating large, predatory fish. Risks of health problems for people who consume mercury in fish have caused wide public concern and prompted the U.S. Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA) to issue consumer alerts, warning women of child-bearing age and young children to avoid certain fish altogether and to limit the number of meals for other fish. Numerous legislative proposals in the 109 th Congress aim to reduce levels of mercury in the environmentâin consumer products, in solid waste, in utility and other emission sources, and in surface water. Most of these proposals focus on sources of mercury emissions to air, because atmospheric mercury deposition accounts for most of the mercury in U.S. freshwater lakes and streams. At least five proposals target emissions from coal-fired electric utilities, because they are thought to be the last remaining major uncontrolled source of mercury emissions. These various proposals and a final regulation promulgated by the U.S. Environmental Protection Agency (EPA) on March 15, 2005, differ in how much and how soon emission reduction would be required, as well as in the extent to which reductions would be distributed geographically across the United States. Analysis of the competing policy proposals for reducing mercury emissions raises questions about the urgency of a need for emission controls, the likelihood that they will reduce mercury contamination of fish, and the possibility that overall reductions might be achieved at the expense of local "hot spots" of mercury contamination. To answer such questions requires an understanding of the sources, fate, and toxicity of mercury in the environmentâan understanding that is growing quickly as the results of numerous scientific studies are being reported. This CRS report provides background information about mercury, and summarizes recent scientific findings. It discusses the sources (i.e., natural versus industrial, historic versus modern) and chemical forms of mercury in the environment; how mercury moves through the environment and concentrates in fish (i.e., the fate of mercury); and the risks to human health and wildlife of mercury exposure through fish consumption. Each of these major sections of the report aims to summarize scientific evidence relevant to specific arguments and questions that have emerged in the policy context. For example, the section on mercury in the environment addresses the question "Are utility emissions deposited locally or regionally, or do they rise to merge with the global atmospheric mercury pool?" For information about specific regulatory proposals to reduce environmental mercury, see CRS Report RL32868, Mercury Emissions from Electric Power Plants: An Analysis of EPA ' s Cap-and-Trade Regulations ; CRS Issue Brief IB10137, Clean Air Act Issues in the 109 th Congress , both by [author name scrubbed]; or CRS Report RL31908, Mercury in Products and Waste: Legislative and Regulatory Activities to Control Mercury , by [author name scrubbed] (pdf). Mercury is a natural element, a silver-colored, shiny, liquid metal that is found in a variety of chemical forms in rocks, soil, water, air, plants, and animals. Sometimes mercury occurs in its elemental, relatively pure form, as a liquid or vapor, but more commonly mercury is found combined with other elements in various compounds, which may be inorganic (e.g., the mineral cinnabar, a combination of mercury and sulfur) or organic (e.g., methylmercury). Natural forces move mercury through the environment, from air to soil to water, and back again. Volcanoes and deep sea vents release tons of mercury to the atmosphere and oceans. Mercury in the air falls to earth with dust, rain, and snow. Mercury evaporates from the oceans, leaves of plants, and other surfaces back into the air. Depending on geologic and meteorologic conditions, the relative amounts of mercury in the atmosphere, surface water, or soil may vary from one year, decade, century, or millennium to another. During the past 500 years or so, human activities have released mercury from its relatively stable and water-insoluble form (cinnabar) in rocks and soil through mining, fossil fuel combustion, and other activities, and so have increased the portion of mercury that is actively cycling through the atmosphere, surface waters, plants, and animals as it changes chemical and physical form. Released mercury may enter the air, persist in the atmosphere and travel great distances or be deposited locally, dissolve in water droplets, settle back onto the land or water, re-enter the air (i.e., be re-emitted), be buried in lake or ocean sediments, or be taken into plants and animals. The generally accepted estimate is that roughly three to five times as much mercury is mobilized today as was mobile before industrialization. However, the author of one recent study argues that the mercury deposited from the atmosphere today is at least 10 times the amount of mercury that was being deposited 500 years ago. In 1995, about 1,913 metric tons (roughly 2,104 U.S. tons) of mercury were newly emitted globally as a result of stationary combustion, metal production, cement production, and waste disposal. Roughly another 514 metric tons (565 U.S. tons) were emitted from other human sources, including chlor-alkali plants, gold production, and mercury uses. Thus, 2,427 metric tons (2,670 U.S. tons) of mercury were released due to human activities in 1995, according to recent estimates. These and other mercury emissions from human activities (past and present) account for at least 50% and perhaps as much as 75% of current, annual, global mercury emissions from all sources (including natural sources), but a large, unknown portion of those mercury emissions is due to past rather than current human activities, according to EPA estimates. The most recent estimates of global, natural mercury emissions range between roughly 1,600 and 3,200 metric tons (1,960 and 3,520 U.S. tons) per year. People have released mercury to the environment primarily through mining and smelting of minerals, burning of fossil fuels (e.g., coal, oil, and diesel fuel), use and disposal of mercury, certain industrial processes (e.g., chlorine production and cement production), and burning of municipal and medical wastes. In some parts of the world such activities are increasing, but in the United States, annual mercury emissions are decreasing. Most of the largest and most direct sources of U.S. mercury releases to water and air have been eliminated. Among the remaining U.S. industrial sources, coal-fired electric utilities are the most important, accounting for about 40% of current U.S. mercury releases. Three estimates of U.S. national emissions are presented in Table 1 . The first two estimates were made by EPA for the National Emissions Inventory. CRS added 12 tons of emissions from gold mines to the EPA emission inventory that was conducted for 1995, at the suggestion of EPA. EPA was unaware of the emissions from that source at the time the inventory was conducted. The "other" category encompasses emissions from various unidentified industries, including most iron and steel mills. EPA advised CRS to note that there are some sources not accounted for in the 1999 EPA inventory, such as iron and steel production using mercury-contaminated scrap, which probably accounts for 7-10 tons of emissions per year. These emissions are not included in the "other" category. EPA also does not include mobile source emissions in its inventory, although these might be significant, because the agency is still developing an estimate. Since the time that EPA completed its 1999 inventory, the medical waste incinerator rules promulgated under the Clean Air Act have been fully implemented, which may have further reduced emissions from that source, and gold mining emissions have decreased due to a voluntary project. Chlorine production emissions also may have declined since the 1999 inventory, because some facilities closed, but one additional facility was identified and included in emission estimates by Seigneur et al., which appear in the third column. These latter estimates were calculated by researchers with Atmospheric & Environmental Research, Inc., and published in 2004, but represent emissions in the year 1998. It is not clear why the Seigneur estimates for 1998 emissions from waste incineration are so much larger than EPA estimates for emissions from that category in 1999. Seigneur included emissions from landfills and electric arc furnaces in the "other" category. The Electric Power Research Institute (EPRI) provided the estimates used in that article for utility emissions. Both the EPRI calculations and the EPA estimate for 1999 utility emissions were based on measurements of mercury content in coal and stack emissions that were collected for the year 1999, in response to an information collection request issued by EPA. Chemical form generally determines the ease with which mercury moves through the air, water, and soil and over distances. For example, elemental mercury emissions may remain airborne for more than a year, traveling around the world as part of the so-called "global pool" of atmospheric mercury. About 95% of atmospheric mercury is elemental. Particulate and reactive gaseous mercury (both organic and inorganic) are found in the atmosphere in smaller amounts, because they travel shorter distances from the point of emission and are more quickly deposited. Reactive gaseous mercury typically is deposited within about 100 kilometers of the point of emission. Coal-fired electric utility emissions vary depending on the technology and coal used at each plant, but are roughly 50% elemental mercury, according to EPA. However, the chemical form of mercury emissions can and does change in the atmosphere, making it difficult to predict the fate of particular emissions, including utility emissions.