Why is exposure to mercury a concern?

Mercury is an element that is dispersed widely around the earth. Most of the mercury in the water, soil, plants and animals is found as inorganic mercury salts. Mercury accumulates in the aquatic food chain, primarily in the form of the methylmercury, an organomercurial. Methylmercury is more easily absorbed and is less readily eliminated from the body than inorganic mercury. Exposure to one chemical with mercury, i.e., methylmercury, has been shown to pose a variety of health risks to humans. Extremely high levels, such as that observed in poisoning episodes in Japan and Iraq has caused neurological damage and death. The fetus is considered more sensitive to health effects of methylmercury than adults. In recent years some studies have found adverse health effects of methylmercury at levels previously thought to be safe. Other studies, however, have shown conflicting results.

It is important to note that the preservative thimerosal contains ethylmercury, a related though distinct chemical from methylmercury. Moreover, recent studies in animal models exposed to thimerosal containing vaccines or oral methylmercury suggest that methylmercury may not be a suitable reference to assess the risk from exposure to thimerosal (Burbacher et al, 2005). In addition, data from studies in human infants that were given routine immunizations with thimerosal-containing vaccines showed that mercury levels in blood and urine were uniformly below safety guidelines for methyl mercury and that unlike methylmercury excretory profiles, infants excreted significant amounts of mercury in stool after thimerosal (ethylmercury) exposure, thus removing mercury from their bodies (Pichichero ME, et al, 2002).