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Medical Alert:  Mercury Based Thimerosal Injury Risk

Thimerosal was the most widely used preservative used in vaccines and is still a common additive. ¹ Thimerosal has also been used in eye and nose drops. Its common use was a result of its ability to kill specific organisms and prevent the growth of certain fungi, even under aseptic conditions. ² Thimerosal is most useful where a vaccine is accessed multiple times or manufactured without strict antiseptic controls, which increases the risk of contamination and subsequent bacterial infection. The antiseptic character of thimerosal is related to the composition of the solution, which is largely mercury. By weight, thimerosal is approximately 50% mercury.

Thimerosal has been used as a preservative for decades. In 1931, a study offered evidence that its use was not only effective but safe for use in humans. ³  Subsequent studies on thimerosal, referenced by the Centers for Disease Control and Prevention (CDC), seem to back up the initial study. The main finding of these studies is that thimerosal use has "no ill effects . . . other than minor local reactions [and irritation] at the site of injection." ⁴ 

Even though the CDC has adopted the view that there is 'little reason for concern,' exposure to thimerosal has declined with the development of non-mercury containing alternatives and preservatives having less mercury. This is being done as concern about exposure to mercury and the effects of overexposure have grown, especially in the United States. This concern culminated in the issuance of a joint statement, on July 7, 1999, by the American Academy of Pediatrics (AAP) and the US Public Health Service (USPHS) alerting clinicians and the public about the concerns related to thimerosal use. To view a list of thimerosal containing and thimerosal-free vaccines, please see the table below.

Metabolisis turns the mercury-based preservative into ethyl-mercury and thiosalicylate. Ethyl-mercury is an organomercurial (organic form of mercury) similar, but not identical, to methyl-mercury. Methyl-mercury is toxic to humans and exposure should be prevented where possible. This toxicity has led to concern about exposure to ethyl-mercury and, therefore, the use of thimerosal. The table below contains information regarding the methods of exposure and risks from exposure to both methyl-mercury and ethyl-mercury.

	Methyl-mercury	Thimerosal / Ethyl-mercury
Exposure Paths	Exposure results primarily from consumption of contaminated fish.	Exposure results from injection of a vaccine or use of other thimerosal-containing products.
Exposure Pattern	Consumption of contaminated fish.	Injection for a vaccine or contact for eye and ear drops.
Exposure Rate	Variable, depending on rate of consumption of contaminated fish.	Variable, depending on frequency and amount of thimerosal containing vaccine injected.  Higher rates of exposure more likely in infants and children.
Sensitivity to Exposure	Consumption of contaminated fish in large quantities creates risk.  Risk is greatest for a fetus and developing child.	No definitive data, but potential for similar sensitivities.
Accumulation of Contaminant	Methyl mercury accumulates in the body.  This means levels of the contaminant rise over time with prolonged, continued consumption.	No definitive data, but potential for similar accumulation of the contaminant.

Unnecessary exposure to mercury should be avoided where possible. This is imperative as the effects of 'mercury poisoning' are severe and irreversible. Effects of mercury poisoning and exposure to high levels of methyl-mercury include:

• Death, especially to an exposed fetus.
• Mental retardation
• Cerebral palsy
• Autism
• Seizures
• Tremors

Exposure to low doses is presumably less toxic and less dangerous to humans. However, lower doses accumulate in the body and can, over time, cause damage similar to that caused by large doses.

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1. The information on this page is accurate as of July 22, 2002.

2. The requirements for a preservative are set by the United States Pharmacopeia (USP).

3. Powell HM, Jamieson WA. Merthiolate as a Germicide. Am J Hyg 1931;13:296-310.

4. Publications used by the U.S. Food and Drug Administration to prove these statements.  Claims located at http://www.fda.gov/cber/vaccine/thimerosal.htm.

Studies on Safety and Effectiveness of Thimerosal:

1. Batts AH, Narriott C, Martin GP, et al. The effect of some preservatives used in nasal preparations on mucociliary clearance. Journal of Pharmacy and Pharmacology 1989; 41:156-159.
2. Batty I, Harris E, Gasson A. Preservatives and biological reagents. Developments in Biological Standardization 1974;24:131-142.
3. Beyer-Boon ME, Arntz PW, Kirk RS. A comparison of thimerosal and 50% alcohol as preservatives in urinary cytology. Journal of Clinical Pathology 1979;32:168-170.
4. Gasset AR, Itoi M, Ishii Y, Ramer RM. Teratogenicities of ophthalmic drugs. II. Teratogenicites and tissue accumulation of thimerosal. Archives of Ophthalmology 1975;93:52-55.
5. Goldman KN, Centifanta Y, Kaufman HF, et al. Prevention of surface bacterial contamination of donor corneas. Archives of Ophthalmology 1978;96:2277-2280.
6. Keeven J, Wrobel S, Portoles M, et al. Evaluating the preservative effectiveness of RGP lens care solutions. Contact Lens Association of Ophthalmologists Journal 1995;21:238-241.
7. Naito R, Itoh T, Hasegawa E, et al. Bronopol as a substitute for thimerosal. Developments in Biological Standardization 1974;24:39-48.
8. Wozniak-Parnowska W, Krowczynski L. New approach to preserving eye drops. Pharmacy International 1981;2(4):91-94.

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