Thimerosal is a preservative that has been used in vaccines and other products since the 1930s. It contains a form of mercury known as ethyl mercury.

Methyl mercury is found in the environment in contaminated water and polluted air, and it accumulates in certain fish. Standards for mercury toxicity are based on methyl mercury. Ethyl mercury is the type of mercury found in thimerosal. The ethyl mercury in thimerosal is eliminated from the body about six or seven times more rapidly than the environmental methyl mercury.

Most of the knowledge of mercury toxicity is based on information pertaining to methyl mercury. Exposure to large concentrations of methyl mercury can be harmful to various body organs. Preliminary study results of mercury metabolism note that the half-life of ethyl mercury in infants who received thimerosal-containing vaccines is only six to eight days and eliminated in stool, compared with a half-life of 45 days for methyl mercury.

An Institute of Medicine committee of scientific experts concluded there was not enough evidence to accept or reject a causal relationship between thimerosal exposures from vaccines and neuro-developmental disorders of autism, attention-deficit hyperactivity disorder, and speech or language delay. Additional studies are ongoing, but a considerable body of evidence indicates there is no causal relationship between receiving thimerosal-containing vaccines and neuro-developmental disorders.

No. Since March 1, 2001, all vaccines recommended for routine use in children are available in formulations that are essentially free of thimerosal. Depending on the manufacturer, some vaccines have been thimerosal-free for as long as a decade. A survey in February 2002 found that 98 percent of supplies in providers’ offices were thimerosal free. Influenza vaccine still contains thimerosal, but there is no longer concern about a cumulative effect from other vaccines.

No. Measuring mercury levels in children who previously received thimerosal-containing vaccines is not helpful in determining if a child’s behavioral disorder was caused by the exposure to mercury in vaccines, according to the American Academy of Pediatrics.

Absolutely. Vaccines remain the safest way to ensure that preventable diseases such as measles do not return and become a serious threat to children. By successfully preventing disease, we do not see the illness, deaths and permanent disabilities these diseases can cause.