Discovery Fit & Health
With one exception, vaccines today contain at best only tiny amounts of thimerosal. Thimerosal is a preservative used in vaccines going as far back as the 1930s, and the ethylmercury it contains has been blamed by some critics for the rise in autism rates. These critics mistakenly believed that ethylmercury had the same dangerous properties as methylmercury, the type of mercury found in fish, which can be toxic. These critics, led by prominent figures such as Robert F. Kennedy Jr., and some celebrities, launched a campaign to have thimerosal removed from vaccines and were largely successful in their efforts. Today, by mandate of the U.S. Food and Drug Administration, almost all vaccines in the United States contain no thimerosal or only contain trace amounts of less than one microgram. (One prominent exception is the vaccine for influenza, although thimerosal-free versions are available.)
The autism dustup began largely because of this mercury misunderstanding, helped along by the natural human desire among parents to find a cause for a problem as heartbreaking as autism. (And once pop culture personalities got involved, the risk of losing sight of the facts during an information stampede increased.) When people began to search for an explanation for why cases of autism were on the rise, they noticed that vaccines contained levels of ethylmercury that exceeded the recommended levels for methylmercury. These people, however, were not aware that the two kinds of mercury have different properties and are handled by the human body differently. In addition to this misinformation, many activists claimed that the federal government knew thimerosal was dangerous but tried to cover it up.
Today, still, there is no known cause of autism, and thimerosal, it seems, got a raw deal. Indeed its ethylmercury metabolizes in the body and is passed through the system much faster than methylmercury, making it much less likely to build up in a child's body; comparison attempts that have been made between the symptoms of autism and those of mercury poisoning are simply inaccurate; and no long-term studies have been able to causally link mercury with autism [source: Texas Children's Hospital].
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