Thimerosal contains ethylmercury (mercury). 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. 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.
Thimerosal is added to some vials of vaccine that contain more than one dose (multi-dose vials) to prevent growth of germs, like bacteria and fungi. Thimerosal was commonly used as an antiseptic such as Merthiolate or Mercurochrome®. Over the past few decades, thimerosal has been replaced with other preservatives. Now, thimerosal is rarely included in antiseptics or medications, including merthiolate.
A contact allergy to thimerosal should not affect your ability to be vaccinated because most modern vaccines do not contain thimerosal. Vaccines given to children under the age of 6 are made without thimerosal. A few adult vaccines are still manufactured with thimerosal as a preservative, but thimerosal-free alternatives can usually be found if needed.