evidence: Mercury poisoning can be dangerous and even fatal. Children might be exposed to mercury from several sources, but the majority of exposure in the United States comes in the form of methylmercury, which is found in foods such as certain fish. Methylmecury is known to be toxic to fetuses. Thimerosal, a preservative once commonly used in vaccines, contains another form of mercury called ethylmercury. Some people believe that mercury in vaccines and from other sources can affect development of the nervous system and contribute to autism. However, extensive research has found no conclusive evidence that any part of a vaccine or combination of vaccines causes autism. Even so, aside from some flu vaccines, mercury compounds have been reduced or eliminated from most routine childhood vaccines.