This document summarizes the health basis for the national emission standard for mercury, originally set in 1973 and currently under review. Mercury is unique among metals as the only metal in liquid form at room temperature. It exists in three oxidative states - metallic (HgÀ), mercurous (Hg2++), and mercuric (Hg++) mercury - and a wide variety of chemical forms, the most important of which are compounds of methyl mercury, mercuric mercury, and the vapor of metallic mercury. The global cycle of mercury involves the emission of HgÀ from land and water surfaces to the atmosphere, transport of HgÀ in the atmosphere on a widespread basis, possible conversion to unidentified soluble species and return to land and water through various depositional processes. The major source of human exposure to methyl mercury is by diet through the consumption of fish and fish products. Mercury vapor is inhaled, whereas uptake of inorganic and methyl mercury compounds is primarily through oral ingestion. Once absorbed, mercury in all forms is distributed by the bloodstream to all body tissues; however, tissue distribution of methyl mercury is more uniform. Mercury vapor and methyl mercury readily cross the blood-brain and placenta! barriers. Chronic exposure to mercury compounds primarily affects the central nervous system and kidneys. Depending upon the form of mercury and level of intake, effects on the adult nervous system can range from reversible paresthesias and malaise to irreversible destruction of neurons in the cerebellar and visual cortices, leading to permanent signs of ataxia and constriction of the visual field. The fetus is most sensitive to methyl mercury poisoning, with effects in infants ranging from psychomotor retardation to a severe form of cerebral palsy. All prenatal effects have to date been found irreversible.