Thimerosal

As my I approach the deadline for my annual TB test, I am trying to find answers to my questions about the chemicals used in the formula.  One of the questions I was trying to find an answer to is: "If mercury is so bad for us via fish consumption, then why is it okay in vaccines?".  Honestly, I did not find a comforting answer to that question.  What I learned is that there are a few different forms of mercury.  The kind in our vaccines is called ethylmercury, and the kind we ingest through eating fish is methylmercury.  Most studies to date have focused on methylmercury.  The results of these studies have revealed many toxic effects on developing fetus' and young children.

When reviewing the chemical summary on organic mercury provided in the TEACH database (which is a database of information about chemicals, provided by the Environmental Protection Agency)  I discovered that there is very little research on ethylmercury, while there is quite a bit of research on methylmercury.  For methylmercury, the U.S. EPA Reference Dose (RD) for chronic oral exposure is .00001 mg/kg-day and the ATSDR Minimal Risk Level (MRL) is .00003 mg/kg-day.  These exposure limits were last updated in 2001 and 1999, respectively.  To my shock, there were no toxicity references even available for ethylmercury.  I am assuming that is due to the lack of research that is readily available.  The summary of methylmercury studies reported the following findings:

• methylmercury has been shown to cross the placenta and has been measured in breast milk
• frequent eating of fish has been linked to mercury levels being 40 times the national average
• studies reported mild-to-moderate impairments detected in visual and behavioral tests of children that were associated with mercury or methylmercury blood concentrations
• high-dose, prenatal exposure to methylmercury has been linked to increased incidence of still births and miscarriages, several developmental disabilities, and other adverse neurological effects in children of exposed mothers
• children and adults exposed to methylmercury have developed a disorder called acrodynia, manifested by symptoms of: leg cramps, irritability, peeling of skin and hands, nose, and feet; fever, sweating, excessive salivation, sleeplessness, photobia, and/or weakness
• impaired growth of children was significantly associated with maternal exposure to methylmercury during pregnancy

(The source for the above findings is: www.epa.gov/teach/chem_summ/mercury_org_summary.pdf)

Another interesting thing is that the CDC states on their website that Thimersol (ethylmercury) is eliminated from the body easily and that data from studies shows no convincing evidence of harm.  When I went to examine their references, they are all AT LEAST 10 years old, with some references dating back to 1980!!!  Not to mention, there are only 7 references listed.  First of all, there are few studies that examine ethylmercury alone, which might be a reason for the lack of data suggesting harm.  Like I mentioned before, there is so little data that even the EPA doesn't have enough evidence or research to make a recommended toxicity reference.  Secondly, in a newer article published in 2005 they did report that ethlymercury cleared more rapidly than methylmercury... BUT the proportion of inorganic mercury in the brain was twice as high in the ethylmercury (the type of mercury in Thimerosol) group.  It was further noted that inorganic mercury remains in the brain much longer than organic mercury, and has a half-life of MORE THAN A YEAR!  It goes on to state that "it's not currently known whether inorganic mercury presents any risk to the developing brain".  So, it looks to me that while there might not be any compelling evidence to show us that Thimerosol is harmful, there most certainly isn't any evidence showing us that it's effects are safe. 

References:
1) http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1280369
2) http://ehp03.niehs.nih.gov/article/info%3Adoi%2F10.1289%2Fehp.7712