Chemistry. Mercury (Hg) is a metal that exists in three basic forms: (a) elemental mercury (part of the earth's natural geochemistry), (b) inorganic mercury (as mercuric salts), and (c) organic mercury (methylmercury compounds). Inorganic mercury comprises the vast majority of the total mercury found in Hudson River sediments, but it is the small fraction of mercury that is methylated by microorganisms in the aquatic environment, enters the food chain, and bioaccumulates in fish that is of greatest toxicologic concern (ATSDR, 1999).

Environmental distribution. Mercury is found widely in the environment across the United States. It is encountered at 46 percent of Superfund sites on the NPL (ATSDR, 1999). High concentrations of mercury are found in the Hackensack Meadowlands, a tidal estuary at the western margin of New York Harbor. The highest levels of methylmercury in fish from the lower Hudson are found in large striped bass (mean: 517ng/g), and two of thirty-eight large striped bass tested in 1993 exceeded the regulatory criterion of 1,000 ng/g (Skinner et al., 1996).

Human exposure. Chronic poisoning by mercury occurs in two distinct clinical forms, depending upon whether exposure has been to inorganic or to organic compounds of mercury (Hunter, 1969; Clarkson, 1997; ATSDR, 1999). Only organic mercury toxicity is relevant to this discussion of exposure from the aquatic environment. Poisoning by the organic compounds of mercury, including methylmercury, produces an almost purely neurologic illness (Clarkson, 1997; ATSDR, 1999). Early symptoms include paresthesias, perioral numbness and other manifestations of sensory neuropathy. With continued exposure, the syndrome progresses to a triad of dysarthria, ataxia, and visual field constriction (Hunter, 1969).

Acute devastating outbreaks of organic mercury poisoning have been reported. Infants and children have been most seriously affected. Diet has been the usual route of exposure. The first of these epidemics occurred in Minimata Bay, Japan, where exposure resulted from ingestion of contaminated shellfish (Kurland et al., 1960). Later epidemics occurred in Iraq (Bakir et al., 1973), Pakistan (Hag, 1976), and Guatemala (Ordonez, Carillo, and Miranda, 1966), where exposure was caused by consumption of seed grain that had been treated with mercurial fungicides. An episode of poisoning by organic mercury occurred in the United States among members of a New Mexico family who ate pork from hogs that had been fed mercury-treated seed grain (Pierce, Thompson, and Likosky, 1972).

Toxicity. A major focus of current research surrounds the question of whether low-level exposure to organic mercury is capable of causing subclinical developmental neurotoxicity. To answer this question, three major prospective epidemiologic studies have beeninitiated and are ongoing: one among children in the Seychelles Islands in the Indian Ocean (Marsh et al., 1995), the second among children in the Faroe Islands in the North Atlantic (Grandjean et al., 1998), and the third among children in New Zealand (Kjellstrom et al., 1989;Crump et al., 1998). Maternal exposure to methylmercury during pregnancy has been assessed in each study through determination of mercury levels in scalp hair, and neurological development in their children has been tested postnatally. To date, the Seychelles study has foundno associationbetweenlow levels of exposure and any sign of neurodevelop-mental toxicity. By contrast, the Faroe study, which employed more sensitive assessment instruments, has found evidence of subclinical impairment. There was concomitant exposure to PCBs in the Faroe Islands, but this does not seem to account for the findings. The New Zealand study also has found that prenatal exposure to low levels of methylmer-cury is associated with neurobehavioral impairment in six- and seven-year-old children. After a careful weighing of the data from these three studies, the United States National Academy of Sciences has declared methylmercury a fetal neurotoxin (NRC, 2000). The USFDA recently announced an advisory and comprehensive educational program to warn pregnant women, women of childbearing age, and their health care providers about the hazard posed to the unborn child from consuming fish that may contain high levels of methylmercury (USFDA, 2001).

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