The type of mercury used in thimerosal is generally cleared from the body within six weeks, which in theory would render it harmless. But according to those researchers who believe that the preservative causes autism, babies born during that 20-year window were injected with many times the "safe" level as determined by the FDA -- and some, they feel, were genetically incapable of clearing the doses of mercury from their bodies. Mercury is, in fact, a neurotoxin, and the theory is that the recent leap in autism diagnoses can be directly tied to thimerosal.