Although it can accumulate in many organs, the target organs for mercury in the body are the kidney and the brain [1,2]. High levels of mercury are known to cause renal damage in occupationally exposed patients. A group at the University of Calgary showed high Hg levels in various body tissues 29 days after twelve amalgam fillings were placed in one sheep . A later experiment by the same group in one monkey showed similar results . In addition to other organs, there were particularly high levels of mercury accumulated in the stomach and gastrointestinal systems of the animals studied, particularly the sheep. The mercury accumulation was therefore most likely more from swallowing large amounts of amalgam scrap during placement than of the restorations . As a ruminant animal chewing 15 hours per day, the sheep probably swallowed particles of amalgam from the fillings as they chewed. These factors combined with the fact that each experiment was done only on a single animal with no controls make the studies' relevance to humans questionable.
The Calgary group reported in 1991 that mercury from amalgam placement impaired kidney function . Kidney function was measured in sheep for two weeks before and 30 and 60 days after twelve large occlusal amalgams were placed. The authors reported that the renal glomerular filtration rate as measured by the clearance of inulin was shown to be lowered by 60% afterward. Anti-amalgamists have claimed that this study shows that dental amalgam placement in humans leads to kidney damage.
However, the study was criticized by a group of renal physiologists  for several reasons. There were questions about whether the inulin could truly have "equilibrated" before the glomerular filtration rate was measured, and methods "so poorly described that they are not possible to understand." In addition, although Boyd et al  described 60% decreased GFR (chronic renal failure), other data actually showed improved kidney function. For example, plasma urea and urinary albumin levels decreased. Finally, the authors did not show any evidence of kidney damage since renal histology was normal.
In contrast, a human study of 100 adults in 1995 measured urine N-acetyl-beta-glucosaminidase (NAG) along with urine mercury . Sixty-six subjects had amalgam fillings; 34 did not. Those with amalgam fillings excreted slightly more mercury and slightly more NAG than those without, but the authors conclude that this small increase in NAG excretion is probably of no clinical significance.
Other studies on humans have failed to show any renal dysfunction. In one study, all amalgam fillings of ten healthy people were replaced with gold inlays . Blood and urinary mercury output was measured before and after amalgam removal and compared periodically for twelve months with a control group of another ten people without amalgam removal. Blood and urinary output of mercury increased immediately after amalgam removal, and then declined to significantly lower than preremoval levels. Although urinary albumin was significantly higher twelve months after amalgam removal, the levels were still within reference intervals. An analysis of many biochemical indices showed there were no differences in organ function before or after amalgam removal.
In a study on ten subjects, glomerular filtration rate was the same one week before and sixty days after amalgam removal (an average of 18 surfaces), and there were no detectable effects on the excretion of N-acetyl-ß-glucosaminidase (NAG), ß2-microglobulin, or albumin, indicating that dental amalgam had no effect on renal function . In another study of 48 randomly selected young men, 23 had no amalgam fillings, 23 had 1 to 6 surfaces of amalgam fillings, 1 had 9 surfaces, and 1 had 17 surfaces . The investigators found no relationship between the number of amalgam surfaces and urinary protein levels normally indicative of tubular and/or glomerular lesions. They concluded that amalgam fillings did not cause renal dysfunction in this study.
Dentists typically have higher mercury concentrations than other people with amalgam fillings. A 1991 study of dentists screened in the United States in 1985 and 1986 with urinary mercury concentrations as high as 115 µg Hg/L showed no renal dysfunction as measured by serum and urine ß2 microglobulin concentrations, serum creatinine and creatinine clearance . There was also no relationship between the level of urinary mercury and potential kidney dysfunction. Similar results were reported in a 1997 study of Swedish dentists screened in 1990 .
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.