Thimerosal contains ethylmercury.
Mercury is a naturally occurring element found in the earth’s crust, air, soil, and water. Two types of mercury to which people may be exposed — methylmercury and ethylmercury — are very different.
Methylmercury is the type of mercury found in certain kinds of fish. At high exposure levels methylmercury can be toxic to people. In the United States, federal guidelines keep as much methylmercury as possible out of the environment and food, but over a lifetime, everyone is exposed to some methylmercury.
Thimerosal contains ethylmercury, which is cleared from the human body more quickly than methylmercury, and is therefore less likely to cause any harm.