“Mercury in the thimerosal molecule is in the form of ethylmercury for which there is limited toxicologic information” (Clarkson, 2002). Thimerosal is the preservative commonly used in many vaccinations and given to children in amounts of ethylmercury that exceed the U.S. Environmental Protection Agency safety level for adults (Burbacher et al, 2005). Thimerosal has been withdrawn from many pediatric vaccines since 1999 as a result of concerns over the neurodevelopmental toxicity of organic mercury although it is still used in influenza, diphtheria, and pertussis vaccinations (Goth et al, 2006). It is plausible that the withdrawal of thimerosal is indicative of its potential as a neurotoxicant. Children vaccinated before 1990 time may have received cumulative doses of mercury exceeding 200µg/kg and because thimerosal is organic mercury there is suspicion among scientists that it acts as methylmercury does in the brain though the two forms vary in the way they are distributed and eliminated from the brain (Spzir, 2006). Health risk estimates from thimerosal in vaccines originally assumed that ethylmercury is toxicologically similar to methylmercury (Ball et al, 2001). Studies have since found this to be misleading as methylmercury is distributed differently than methylmercury (Burbacher et al, 2005).
Reports indicating infants can be given ethylmercury in the form of thimerosal above the guidelines for safe exposure set by the U. S. Environmental Protection Agency were challenged in an experiment (Burbacher et al, 2005) in which monkeys were exposed to methylmercury or thimerosal and tested at intervals to determine how long the mercury remained in the brains of the monkeys. Findings showed that methylmercury is not a proper reference for risk assessment from exposure to thimerosal-derived mercury as ethylmercury clears the brain faster than methylmercury (Burbacher et al, 2005). The deposition kinetics of the two forms of mercury varies greatly requiring further investigation into the safety and efficacy of ethylmercury from thimerosal as a vaccine preservative. “Although the initial distribution volume of total mercury is similar for the two groups, a biphasic exponential decline in total blood mercury is observed only after intramuscular injections of thimerosal. This suggests continual distribution into and localization in tissue sites over time” (Burbacher et al, 2005) where the mercury is stored and accumulated. More “knowledge of the toxicokinetics and developmental toxicity of thimerosal is needed to afford a meaningful assessment of the developmental effects of thimerosal-containing vaccines. We need knowledge of biotransformation of thimerosal to interpret the potential developmental effects of immunization with thimerosal-containing vaccines in infants. This information is critical if we are to respond to public concerns regarding the safely of childhood immunizations” (Burbacher et al, 2005).
Booker, S (2001). NIEHS investigates links between children, the environment, and neurotoxicity. Environmental Health Perspectives. 109:6, A258-A261.
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