	Introduction

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. 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. 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. 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. 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). Sources of Mercury in the Environment

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. Natural forces move mercury through the environment, from air to soil to water, and back again. 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. Methylmercury Formation and Accumulation

The most biologically significant transformation of mercury occurs in soil or sediments of lakes or streams, where bacteria (primarily sulfate-reducing bacteria) are capable of converting inorganic mercury to methylmercury. In human adults, absorbed methylmercury is dispersed throughout the body in blood and enters the brain, where it may cause structural damage. This condition is predictive of more severe toxicity. Methylmercury readily crosses the placenta of pregnant women. It concluded that scientific studies have demonstrated the sensitivity of the human fetus to pre-natal methylmercury exposure, and that the risk to women who eat large amounts of fish and seafood during pregnancy is "likely to be sufficient to result in an increase in the number of children who have to struggle to keep up in school." U.S. In making choices about fish consumption, factors other than, or in addition to, methylmercury concentration should be considered. In particular, the health benefits of eating fish high in omega fatty acids are important, especially for cardiovascular health and fetal development. However, it is not clear whether typical levels of environmental contamination are stressful for wildlife. Electric utilities are the only uncontrolled major stationary source of U.S. mercury emissions.
Concern about mercury in the environment has increased in recent years due to emerging evidencehat exposure to low levels of mercury may harm the developing rvous systems of unborn children. At least five bills in the 109 th Congress aim to reduce mercury emissions from coal-fired electric utilities. The 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, and in whether reductions would be achieved through controls at each plant or through a nationwide cap and trade system. The latter approach could allow individual plants to continue emitting current levels of mercury, potentially worsening conditions at nearby "hot spots." Analysis of competing proposals raises questions about the sources, fate, and toxicity of mercury in the environment. This CRS report provides background information about mercury and summarizes recent scientific findings. For information about regulatory proposals to reduce environmental emissions of mercury, see CRS Report RL32868, Mercury Emissions from Electric Power Plants: An Analysis of EPA ' s Cap-and-Trade Regulations , by [author name scrubbed].
Mercury is a natural element found in rocks, soil, water, air, plants, and animals, in a variety of chemical forms. Natural forces move mercury through the environment, from air to soil to water, and back again. Industrial activities have increased the portion of mercury in the atmosphere and oceans, and have contaminated some local environments. Coal-fired electric utilities are the largest single source of U.S. mercury emissions, according to EPA, but mobile sources also are important. The chemical form of mercury generally determines how it moves through the environment, but mercury can and does change form relatively rapidly where bromine and other oxidizing substances (e.g., ozone) are abundant. In soil or sediments of lakes, streams, and probably oceans (especially where water is oxygen-poor and acidic, and sulfate is present), bacteria convert inorganic mercury to more toxic methylmercury, which can accumulate in fish. Newly deposited mercury seems to be more readily converted than older deposits.
People and wildlife who eat contaminated fish can be exposed to toxic levels of methylmercury. In people, methylmercury enters the brain, where it may cause structural damage. Methylmercury also crosses the placenta. The National Research Council has reported that the human fetus is sensitive to methylmercury exposure, and the current risk to U.S. women who eat large amounts of fish and seafood during pregnancy is "likely to be sufficient to result in an increase in the number of children who have to struggle to keep up in school." Some studies indicate that the cardiovascular system may be even more sensitive. Mercury concentrations generally are low, but the estimated safe blood-mercury level is exceeded in about 6% of U.S. women between the ages of 16 and 49 years. EPA and the Food and Drug Administration advise women of child-bearing age to avoid certain large fish, and to limit the amount eaten of other fish. In making choices about fish consumption, the health benefits of eating fish also should be considered. Fish-eating wildlife also are exposed to methylmercury, but it is not clear whether typical current levels of environmental contamination are harmful. This report will be updated as warranted by significant scientific discoveries.