Mercury ("quicksilver") is a metal that is liquid at room temperature. Most compounds containing mercury are poisonous. Methylmercury is an organic form of mercury that has been used to preserve seed grain. Methylmercury can also be produced from metallic mercury or mercury compounds in bodies of water by the action of bacteria. Outbreaks of methylmercury poisoning have occurred following ingestion of treated seed grain, meat from animals fed treated seedgrain, or fish from waters contaminated with methylmercury, such as Minamata Bay in Japan.

Fetuses and young infants are very sensitive to methylmercury's effects. Methylmercury causes central nervous system (brain and spinal cord) damage and the severity of the damage depends on the extent of the exposure. Many of the CNS effects of mercury poisoning are similar to those seen in cerebral palsy, and methylmercury is thought to cause a form of cerebral palsy.

In January 2001, the FDA issued a warning that pregnant women, women who may become pregnant, nursing mothers, and small infants to avoid fish that may contain unsafe levels of methylmercury. These fish include large, longer-lived fish, such as swordfish, king mackerel, shark, or tilefish. The FDA especially warns against noncommercial fish, or fish caught by friends and family, and the agency recommends that consumers check their local or state health departments for warnings against locally caught, noncommercial fish.

Some physicians have raised concerns about ethyl mercury (thimerosal), a preservative used in some vaccines. Though individual vaccines do not contain high levels of thimerosal, the fear was that multiple childhood vaccines might cause mercury levels in children to rise to dangerous levels. Research indicates that this does NOT, in fact, happen. The highest levels of ethyl mercury found in vaccinated children did not reach a level deemed to be toxic. Research also shows that thimerosal-containing vaccines have NOT been shown to cause autism or ADHD. Currently in the U.S., routine pediatric vaccines that contain thimerosal have only trace amounts. Further, all of the routine vaccines are also available in thimerosal-free formulas.

• history of exposure to methylmercury (possibly in the womb)
• cerebral palsy
• growth deficiency
• microcephaly (small head)
• deafness
• blindness