Methylmercury is a type of mercury, a metal that is liquid at room temperature. A nickname for mercury is quicksilver. Most compounds containing mercury are poisonous. Methylmercury is a very poisonous form of mercury. It forms when bacteria react with mercury in water, soil, or plants. It has been used to preserve grain that is fed to animals.
Methylmercury poisoning has occurred in people who have eaten meat from animals that ate grain that was treated with this from of mercury. Poisoning from eating fish from water that is contaminated with methylmercury has also occurred. One such body of water is Minamata Bay in Japan.
Methylmercury is used in fluorescent lights, batteries, polyvinyl chloride, and latex paint. It is a common pollutant of air and water.
Unborn babies and 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 symptoms of 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. This includes swordfish, king mackerel, shark, and tilefish. Infants should not eat these fish, either. No one should eat any of these fish caught by friends and family. Check with your local or state health department for warnings against locally caught, noncommercial fish.
Some health care providers have raised concerns about ethyl mercury (thiomersal), 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 thiomersal. Thiomersal-free vaccines are available.