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Thimerosal, which is approximately 50% mercury by weight, has been one of the most widely used preservatives in vaccines. It is metabolized or degraded to ethylmercury and thiosalicylate. Ethylmercury is an organomercurial that should be distinguished from methylmercury, a related substance that has been the focus of considerable study (see "Guidelines on Exposure to Organomercurials" and "Thimerosal Toxicity", below).

At concentrations found in vaccines, thimerosal meets the requirements for a preservative as set forth by the United States Pharmacopeia; that is, it kills the specified challenge organisms and is able to prevent the growth of the challenge fungi (U.S. Pharmacopeia 2004). Thimerosal in concentrations of 0.001% (1 part in 100,000) to 0.01% (1 part in 10,000) has been shown to be effective in clearing a broad spectrum of pathogens. A vaccine containing 0.01% thimerosal as a preservative contains 50 micrograms of thimerosal per 0.5 mL dose or approximately 25 micrograms of mercury per 0.5 mL dose.

Prior to its introduction in the 1930's, data were available in several animal species and humans providing evidence for its safety and effectiveness as a preservative (Powell and Jamieson 1931). Since then, thimerosal has been the subject of several studies (see Bibliography) and has a long record of safe and effective use preventing bacterial and fungal contamination of vaccines, with no ill effects established other than minor local reactions at the site of injection.

While the use of mercury-containing preservatives has declined in recent years with the development of new products formulated with alternative or no preservatives, thimerosal has been used in some immune globulin preparations, anti-venins, skin test antigens, and ophthalmic and nasal products, in addition to certain vaccines. Under the FDA Modernization Act of 1997, the FDA compiled a list of regulated products containing mercury, including those with thimerosal (Federal Register 1999). It is important to note that this list was compiled in 1999; some products listed are no longer manufactured and many products have been reformulated without thimerosal. Updated lists of vaccines and their thimerosal content can be found in Table 1 (routinely recommended pediatric vaccines) and Table 3 (expanded list of vaccines).

Mercury is the second most toxic element on earth to plutonium. Toxicity of mercury has been linked to many different diseases, including autism, learning disabilities, Alzheimer’s, multiple sclerosis, fibromyalgia, lupus, chronic fatigue syndrome, arthritis, depression, and bipolar disorder. The amount of mercury found in one mercury thermometer is enough to pollute a small lake.

Health effects of mercury toxicity have been a concern because of the potential for it to act as a poison. Toxic doses of mercury can cause developmental effects in the fetus, as well as affecting the kidney and the nervous system in children and adults. Mercury exists in a number of different chemical forms, each one consisting of different levels of toxicity. The forms of mercury can also be converted from one to another in the environment and in the body, so symptoms caused by mercury poisoning depends on the precise chemical forms involved.

Mercury can be toxic when inhaled, eaten, or when placed on the skin. Low concentrations of mercury may appear to have no effect but signs of toxicity can develop later or become more noticeable with continued exposure. When toxicity in humans takes place loss of feeling or a burning sensation in arms and legs, psychological effects, loss of memory, loss of vision, loss of hearing, paralysis, congenital malformations, kidney toxicity, and death may occur. Prenatal toxicity can result in a child with normal appearance at birth but who later exhibits a developmental delay in the ability to walk and/or talk. Because of the long latent period for observable effects, the need for treatment may be recognized too late.

Health effects vary according to the amount of mercury exposure is taken into the body. The health risks of mercury at low levels of exposure remain uncertain, but this continues to be a highly debatable topic with ongoing scientific investigation. Fetuses, infants and small children appear to be particularly sensitive to mercury because their brains are still developing. Vaccines with mercury have been considered to contribute to autism, learning disabilities, Alzheimer’s Disease, and other neurological conditions, and an FDA review conducted in 1998 determined that, at the time, children who received the full complement of childhood vaccines were potentially exposed to levels of mercury that were sometimes 30 to 50 times the acceptable levels established by the EPA.

High-level exposures to mercury can cause serious effects or even be lethal. Several historical examples of epidemic mercury poisonings in other parts of the world provide classic examples of investigative epidemiology and toxicology and serve to highlight the reasons why regulators are concerned about mercury. Effects on the brain and nervous system are frequently seen with high-level exposures to mercury and can be quite severe.