Human toxicology of mercury

This article describes the health effects of the two principal forms of mercury to which humans are exposed, namely mercury vapor and methylmercury compounds. Mercury vapor is the monatomic gas that vaporizes from liquid metallic mercury. It has a long history of human exposure dating back to ancient times. The earliest life forms on this planet may have received substantial exposure and developed defense mechanisms that we inherit to this day. Severe forms of poisoning from inhalation of mercury vapor are now rare. More commonly observed in occupational exposures are subtle neurobehavioral effects and preclinical changes in biochemical markers of kidney function. Methylmercury compounds may also date back to Archean times. The methylation of inorganic mercury by methanogenic bacteria is believed to be a protective mechanism for these cells that appeared early in evolution. Unfortunately, for life forms dependent on a central nervous system, what is protective for simple cells becomes a threat to the more complex life forms. Today, the main source of human exposure is from methylation of inorganic mercury in bodies of fresh and ocean water, the ensuing bioaccumulation in the aquatic food chain, and the consumption of fish or sea mammals by humans. Methylmercury selectively damages the brain. In adult humans, damage is focal, affecting specific cell types in certain anatomical areas such as the visual cortex and the cerebellum. Prenatal exposure disrupts the normal developmental processes of the fetal brain. Indeed, the pre natal period is widely believed to be the most susceptible stage of the life cycle. At present, two large epidemiological studies are underway to determine human health risks from methylmercury in fish. J. Trace Elem. Exp. Med. 11:303-317, 1998. (C) 1998 Wiley-Liss, Inc.