Methylmercury has long been associated with serious neurological disorders, demyelinating diseases, gut disease, and visual damage. There's no blood-brain barrier in infants. Mercury accumulates in brain cells and nerves. Infants don't produce bile, which is necessary to excrete mercury. Once it is in nerve tissue, converted irreversibly to its inorganic form.
Thimerosal is converted to ethylmercury, an organic form that has a preference for nerve cells.
Without a complete blood-brain barrier, an infant's brain and spinal cord are sitting ducks. Once in the nerve cells, mercury is changed back to the inorganic form and becomes tightly bound. Mercury can then remain for years, like a time-release capsule, causing permanent degeneration and death of brain cells. Bernard also notes that the body normally clears mercury by fixing it to bile, but before six months of age, infants don't produce bile. Four separate government agencies have set safe levels for methylmercury, but no safe levels have ever been set for thimerosal, because thimerosal isn't included in toxicity studies.