Health Risks from Increases in Methylmercury Exposure

• Published in: Environmental health perspectives Vol. 63; pp. 133 - 140
• Main Authors: Mottet, N. Karle, Shaw, Cheng-Mei, Burbacher, Thomas M.
• Format: Journal Article
• Language: English
• Published: United States National Institute of Environmental Health Sciences. National Institutes of Health. Department of Health, Education and Welfare 01-11-1985
• Subjects: Animals; Blood; Chemical hazards; Cognition - drug effects; Dosage; Dose-Response Relationship, Drug; Environmental Exposure; Female; Female animals; Fetal Death - chemically induced; Humans; Impact of Acid Precipitation on Metals Toxicity; Infants; Intellectual Disability - chemically induced; Lesions; Male; Methylmercury Compounds - toxicity; Nervous System Diseases - chemically induced; Poisoning; Pregnancy; Pregnancy - drug effects; Primates; Sperm Motility - drug effects; Spermatogenesis - drug effects; Toxicity
• ISSN: 0091-6765; 1552-9924
• DOI: 10.1289/ehp.8563133

Our present knowledge of the human health effects of methylmercury exposure is derived from study of major outbreaks of human poisonings in Japan and Iraq and experimental studies on primates. Methylmercury readily passes through such physiological barriers as the blood-brain barrier, blood-testes barrier, and the placenta. Its major pathological effects are on the nervous and reproductive systems and the developing embryo/fetus. The neurotoxicity of methylmercury is well established in both humans and non-human primates. Lesions in the cerebral and cerebellar gray matter consist of necrosis and lysis of neurons, phagocytosis and gliosis. The changes are most prominent in the deep sulci and may have a vascular component. A late effect is cerebral atrophy. At high dose levels the liver, kidneys, and other organs may also have degenerative changes. Although not yet described in humans, a major effect of exposure of female primates is an adverse effect on pregnancy. Maternal female M. fascicularis blood mercury levels above 1 ppm are associated with a decreased pregnancy rate and increased abortion rate. To date our experimental data lack sufficient numbers to detect infrequent pregnancy effects below 1 ppm. Preliminary studies also reveal that methylmercury may also decrease the number and function (swim speed) of sperm. Both human and primate studies demonstrate deleterious effects of methylmercury on the developing embryo/fetus. Autopsies on human and primate infants reveal retarded brain development and the occurrence of a cerebral palsy-like behavior in the newborns, whereas the mother may be free of signs and symptoms of methylmercury toxicity. The fetal blood level of mercury is higher than the maternal level. Many features of physical and behavioral development of the newborn and infant have been reported from relatively high exposure levels. Behavioral tests of infant primates (object permanence; visual preference) revealed a retardation of cognitive development. Further research is needed to define the level at which methylmercury begins to have significant neurotoxic, reproductive, or fetal developmental effects. The increased level of methylmercury in some fish due to acid precipitation and the demonstration of significant lesions at clinically nontoxic levels suggest that the margin of safety may be narrow.