Mercury is a metal, a chemical element that is found everywhere. Because mercury is everywhere, it is not possible to prevent all exposure to mercury. Humans and wildlife are exposed to both. The inorganic form is quickly released from our bodies. Humans are usually exposed to the organic form of mercury by eating some fish which have accumulated it in their muscle tissue.
Thimerosal is used as an extra safeguard against vaccine contamination.
It may be used during the manufacturing process or added to the final container to prevent the contamination that may occur after vials that contain more than one dose of vaccine are opened and used. Thimerosal is very effective in helping to prevent a vaccine from spoiling or from being contaminated by outside bacteria. Although mercury is found everywhere in the environment, including food and household products, exposure to mercury, when possible, should be avoided as it can be toxic in large doses. In recent years, several federal agencies have been addressing the health risks of mercury. Other than the occurrence of localized allergic reactions in sensitive persons, no harmful effects have been reported from thimerosal at doses found in vaccines.