While researching the Thimerosal debate, I noticed claims that Thimerosal is 50% Mercury. Sounds scary right? Mercury is one of those dangerous chemicals and anything that is 50% Mercury must be bad. Well, what the more honest or better informed will say is 50% by weight. Still sounds like a lot until you actually look at the chemical in question. The chemical formula of Thimerosal is this C9H9HgNaO2S, which is short hand for 9 Carbon (C), 9 Hydrogens (H), 1 Mercury (Hg), 1 Sodium (Na), 2 Oxygens (O), 1 Sulpher (S). Please note that there is only one Mercury atom out of a total of 21 atoms and strickly speaking they are correct to say that Mercury is half the weight. Why? Mercury is huge compared to the others, Mercury has a molecular weight of 200.59 where Sodium the next largest is 22.99. What they are not telling you is that by concentration Mercury makes up a minute amount of thimerosal and concentration is the key in determining toxicity levels not molecular weight. For example, Potassium is quite small compared to Mercury at 39.09 but at sufficient concentration is quite lethal. So, while all of these people are making thimerosal sound scary they are actually lying to you by telling what amounts to the truth told incorrectly.
Now, I am not saying go and get yourself some thimerosal, I am only saying it is not as dangerous as they make out. However, Mercury as a heavy metal is not something our bodies are equipped to expel readily so it builds up over time. Thus repeated exposure in a short period of is bad.