The FDA limits IV solutions to 25 mcg (or .025 mg) because aluminum was causing neurological delays in premature babies and dementia in kidney dialysis patients. The toxic dose of aluminum for infants is 10 - 20 mcg. The Hepatitis B Vaccine given to newborns contains 250 mcg. Also, depending on the brand of vaccines given, at a typical 2 month old checkup a child receives anywhere from 295mcg - 1225mcg, and these vaccines are repeated at 4 and 6 months.  Doctors believe those high levels are okay since the aluminum has to be absorbed by the muscle tissue before it enters the blood stream where it is then (supposedly) filtered out by the kidneys.
However, The NNii concedes that after being vaccinated, childrens' aluminum levels are above the minimal risk level. They then say that is not considered a problem because 50% - 70% is filtered out the next day. They haven't done any studies to determine what happens to the remaining 50% -30%.  Does this left over aluminum accumulate in brain and skeletal tissue?
The CDC says that large amounts of aluminum have been shown to cause neurological and skeletal delays in unborn and developing animals. 
The CDC also cites a study that found a "statistically valid" association between children with autism and thin bones. This study said that even the boys who were NOT on a casein-free diet had thinner bones than the control group. This surprised the researchers because they expected the group with autism to have thicker bones since they on average weighed more than the control group. 
I think this correlation justifies further studies on using aluminum in vaccines.
 The Vaccine Book, by Dr. Sears