How dangerous is thiomersal (thimerosal)? Some people blame the preservative thiomersal in vaccines for all kinds of illnesses, but mainly for autism. As thiomersal is a mercury compound, there is some plausibility in suspecting it may cause some damage, but is there some hard data on how dangerous it really is? How comparable are the effects of thiomersal to equal amounts of elementary mercury or methyl mercury? Is there evidence that the amount that was typically used in vaccines (it is seldom used now) is harmful?

Thiomersal itself is toxic in quantity, degrading to ethylmercury (C2H5Hg+).

As a vaccine preservative, however, it is harmless. (Global Advisory Committee on Vaccine Safety (2006-07-14): "Thiomersal and vaccines".) This hasn't stopped anti-vaccine activists claiming that the Hg+ atom has nigh-magical powers of autism-causing evil, however, despite no link whatsoever having been found with much research.