ABSTRACT
Mercury is widespread in the environment, occurring in its elemental state (mainly as a vapor), as inorganic mercurous or mercuric salts, and as methylmercury. All three states are interconverted with relative ease in the environment. Biological and environmental conversion mechanisms include reduction to metallic mercury, oxidation to mercuric or mercurous ion, and methylation (1). The latter is a natural process occurring microbially in aquatic sediments. This bacterial biotransformation of inorganic mercury compounds probably accounts for the significant mercury levels seen in fish and leads to significant body burdens at the higher end of the food chain (1,2). In the mammalian organism, all forms of mercury are preferentially converted to bivalent mercury, which attaches to sulfhydryl ligands. Of the organomercurials, the alkyl derivatives are the more environmentally persistent. In the elemental state, mercury has a relatively high vapor pressure, 0.67 Pa at 37°C, and it is rapidly taken up via the lungs. In North America, pre-industrial annual atmospheric mercury deposition has been estimated at 3.7 µg/m2 annually (3). However, atmospheric concentrations are steadily increasing; by some estimates modern levels are now four times higher (4). The major sources of the estimated 159 tons of mercury emitted annually into the atmosphere in the United States are coal-fired electric utilities, municipal waste incinerators, boilers, and chloralkali plants. Further, mercury levels in soil can attain significant values, due to degassing of unstable amalgams and volatile mercury salts such as mercuric chloride (HgCl2) of natural occurrence. According to the U.S. Department of Energy’s Oak Ridge National Laboratory, plants and soils discharge gaseous elemental mercury at rates significant enough to have a measurable impact on environmental levels. Gas phase emissions can be as high as 100 ng/m2/hr in a heavily forested area, and up to 7.5 ng from contaminated soil. Plants absorb mercury and then release the metal through transpiration in amounts inversely related to ambient concentrations (5).
