Methylmercury is a type of mercury ("quicksilver"), a metal that is liquid at room temperature. Most compounds containing mercury are poisonous. Methylmercury has been used to preserve seed grain, which is fed to animals. Methylmercury may also form in water when other forms of mercury in the water react with certain bacteria. Methylmercury poisoning has occurred after people have eaten meat from animals fed seed grain or fish from waters contaminated with methylmercury (such as Minamata Bay in Japan).
Unborn babies and young infants are very sensitive to methylmercury's effects. Methylmercury causes central nervous system (brain and spinal cord) damage. How bad the damage is depends on how much poison gets into the body. Many of the symptoms of mercury poisoning are similar to those seen in cerebral palsy. In fact, methylmercury is thought to cause a form of cerebral palsy.
The FDA recommends that women who are pregnant, or may become pregnant, and nursing mothers avoid fish that may contain unsafe levels of methylmercury. Such fish includes swordfish, king mackerel, shark, or tilefish. (Young infants should not be given these fish, either.) You should not eat any type of these fish caught by friends and family. Check with your local or state health departments for warnings against locally caught, noncommercial fish.
Some health care providers have raised concerns about ethyl mercury (thimerosal), a chemical used in some vaccines. However, research shows that childhood vaccines do not lead to dangerous mercury levels in the body. Vaccines used in children today only contain trace amounts of thimerosal. Thimerosal-free vaccines are available.