Mercury from dental amalgam can accumulate in many body tissues, including the brain. Eggleston and Nylander in 1987 examined brain tissue from 38 adult cadavers and found a correlation between the number of occlusal surfaces of amalgam and the amount of Hg in the brain tissue . In 1986, Nylander reported on three dentists at necropsy had high levels of mercury in the pituitary glands versus four nondentist controls . In 1989, Nylander et al showed high levels of mercury accumulation in the thyroid, pituitary, occipital lobe, and kidneys in dentists and dental staff .
It has been alleged that mercury from dental amalgam can play a role in the development of Alzheimer's disease. Wenstrup et al. reported that at autopsy, the brains of Alzheimer's patients had higher levels of Hg compared to control patients and cited amalgam fillings as a possible source . Other studies have failed to confirm a correlation. In a 1999 study, investigators studied the autopsied brains of a group of 68 subjects with Alzheimer's disease and 33 control subjects without Alzheimer's disease and found no differences in brain Hg levels between the groups . They also found no association of Alzheimer's disease with the number, surface area, or history of dental amalgam placement.
A 2001 study from the University of Calgary showed disruption of "the membrane structural integrity of neurites and the growth cones of identified neurons" after exposure to mercury . Anti-amalgamists have stated that this study "should remove all doubt regarding the role that dental mercury from amalgam fillings plays in the development of Alzheimer's Disease."  But the study was done on the tissue of dead snails, and its relevance to humans is questionable.
A 1997 study from the Universities of Calgary and Kentucky exposed rats to high concentrations of mercury vapor for four hours per day up to 28 days . Some of the rats showed brain lesions similar to those found in humans with Alzheimer's disease. Anti-amalgamists assert that this study provides evidence that dental amalgam fillings can cause Alzheimer's disease . However, humans with an average of 25 surfaces of amalgam fillings would only inhale 2 µg Hg/m3 during four hours of stimulated conditions  versus the 250 or 300 µg Hg/m3 vapor used in this study. The rats were therefore exposed to over 100 times greater concentrations of mercury vapor than humans with 25 amalgam surfaces would typically inhale, even under stimulated conditions. The rat study therefore has little relevance to whether mercury from amalgams causes Alzheimer's disease.
Controlled human study has failed to confirm a link between dental amalgam fillings and Alzheimer's Disease. A relatively homogenous group of Roman Catholic nuns were studied in 1995 and studied for their performance on a battery of neuropsychological tests, including one from the Consortium to Establish a Registry for Alzheimer's Disease . Their scores on these tests could not be correlated with the presence or absence of amalgam fillings or the number or surface area of such fillings.
Huggins has stated that patients with multiple sclerosis can improve after having their amalgam fillings removed . Clausen found no significant differences in total mercury content of 8 autopsied brains of with multiple sclerosis (MS) versus 8 controls . Case-control studies found no relationship between the number or duration of exposure of amalgam fillings and a risk for MS [13,14]. The National Multiple Sclerosis Society has stated, "There is no scientific evidence to connect the development of MS with dental fillings containing mercury." 
To test the hypothesis that dental amalgam fillings are a causal factor in tumors of the central nervous system, Rodvall et al. in 1997 matched 333 patients with glioma, meningioma, or acoustic neurinoma by age, gender, and location with 343 controls. There was no association between the number of amalgam fillings and tumors of the central nervous system and concluded that there was no evidence that amalgam fillings are a cause of central nervous system tumors. 
Dr. Wahl practices dentistry in Wilmington, Delaware. This article was originally published in Quintessence International 32:696-710, 2001, and is reproduced here with the kind permission permission of Quintessence Publishing Co. The author thanks Drs. J. Rodway Mackert, Ivar A. Mjör, and Fred Eichmiller for reading the manuscript and offering several helpful suggestions.