Thimerosal is a very effective preservative that contains mercury. It has been used in some vaccines and other products like cosmetics, eye drops and contact lens solutions since the 1930s.
Thimerosal is the most widely used preservative in vaccines.
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.