Mercury and Human Health

Once deposited in or discharged to water bodies, mercury can be converted by bacteria into organic mercury compounds, such as methyl mercury, that accumulate in the food chain. Human exposure to methyl mercury is predominantly through fish consumption.

Methyl mercury compounds can cross biological membranes, are soluble in lipids and adipose tissues, and can bind to various cell receptors and enzyme sites. Methyl mercury has not been found to be a carcinogen and has not been conclusively established as a teratogen (a chemical which causes a birth defect). Without cancer as a complicating factor, scientists have been able to conduct relatively straightforward analyses of the risks posed by human exposure to mercury compounds. At sufficient levels of accumulation of methyl mercury compounds, toxic effects occur. Serious toxic effects include neurotoxicity (brain and nerve tissue damage) and nephrotoxicity (kidney damage). These toxic effects can impact organisms from birds to mammals, including humans.

At very high levels of methyl mercury contamination, such as observed in Minimata Bay, Japan, in the 1950's, serious health effects occur.⁹ Recently, scientists have been exploring the effects of chronic low doses of methyl mercury, particularly for higher risk populations including children, fetuses, and women of child-bearing age. Developing fetuses may be at greatest risk because of methyl mercury's ability to pass through the placenta.

Several cohort studies have been conducted on children who were exposed to methyl mercury before and after birth in the Seychelle Islands and in the Faroe Islands. No neurodevelopmental deficits were identified in the Seychelle Islands children, while some neuropsychological effects were identified in the Faroe Islands children. Notable differences exist between the two populations that may explain the differing results, including diet. (ocean fish in the Seychelles versus the higher levels of methyl mercury in pilot whale meat in the Faroe Islands)¹⁰ The studies also raise questions concerning the complicating factor of selenium, its interaction with mercury, and subsequent health effects. ¹¹ Selenium, which is found in some ocean fish, provides a substitute for sulfur that permits a weaker bond with mercury, allowing the human body to remove mercury more easily and excrete mercury in greater quantities, reducing both the exposure period and the dose. ¹² No comparable studies to these international efforts have been undertaken in the Great Lakes area. However, recent work intended to investigate the effects of PCB levels on the development of children whose mothers consumed large amounts of fish during pregnancy in the Oswego, New York, area, have also raised questions concerning effects of mercury.¹³

Studies reviewed by the U.S. National Academy of Sciences associate chronic low-dose prenatal methyl mercury exposure with poor performance by children on neurobehavioral tests that measure such things as attention, language ability, fine motor skills, and intelligence. ¹⁴ Further research is required to investigate methyl mercury exposure and coronary disease. The majority of epidemiological studies performed has been retrospective, in which linkages are inferred from past events; prospective studies are needed that make a hypothesis and then follow events to observe actual linkages.

Several organizations have established a "reference dose" for methyl mercury. A reference dose is an estimate of a daily exposure to the human population that is likely to be without an appreciable risk of deleterious effects during a lifetime. Different agencies and organizations have established different reference doses, some of which are shown in Table 3. Different Great Lakes states also have different threshold levels for the general public and sensitive populations. ¹⁵

Table 3. Organization Reference Doses for Methyl Mercury (notes as given)