Dental "Silver" Tooth Fillings: A Source of Mercury Exposure Revealed by Whole-Body Image Scan & Tissue Analysis

by Fritz L. Lorscheider Departments of Radiology, Medicine & Medical Physiology University of Calgary, Alberta, Canada FASEB

Abstract

Mercury (HG) vapor is released from dental "silver" tooth fillings into human mouth air after chewing, but its possible uptake routes and distribution among body tissues are unknown. This investigation demonstrates that when radioactive 203Hg is mixed with dental Hg/silver fillings (amalgam) and placed in teeth of adult sheep, the isotope will appear in various organs and tissues within 29 days. Evidence of Hg uptake, as determined by whole-body scanning and measurement of isotope in specific tissues, revealed three uptake sites: lung, gastrointestinal, and jaw tissue absorption. Once absorbed, high concentrations of dental amalgam Hg rapidly localize in kidneys and liver. Results are discussed in view of potential health consequences from long-term exposure to Hg from this dental material.

Hahn, L.J.; Kloiber, R.; Vimy, M.J.; Takahashi, Y.; Lorscheider, F.L. Dental "silver" tooth fillings: a source of mercury exposure revealed by whole-body image scan and tissue analysis. FASEB J. 3:2641-264; 1989.

Mercury (Hg) has been the major component of tooth filling materials for the past 150 years and its use has met with continuing controversy, as clear experimental evidence regarding its safety has not been demonstrated. Dental "silver" tooth fillings typically have a weight composition that is approximately 50% pure elemental Hg, 35% silver, 13% tin, 2% copper, and a trace amount of zinc when mixed as an amalgam. A newly placed multisurface dental silver filling involving an occlusal (grinding) surface of a molar tooth contains between 750-1000 mg of Hg and has an average serviceable life span in the human mouth of 7-9 years. Approximately 80% of all tooth restorations employ this Hg/silver dental amagam.

The traditional view in dentistry maintains that the Hg component of dental amalgam becomes inert once the fillings have been allowed to set for several days, and that long-term danger to the patient from Hg vapor is therefore remote. However, more recent clinical studies in subjects with amalgam fillings who chewed gum for 10 minutes have demonstrated that quite substantial amounts of rig vapor are released into intra-oral air from dental amalgam, being sixfold higher than pre-chewing levels. The intra-oral Hg vapor concentration remained elevated during 30 minutes of continuous gum chewing; and after cessation of chewing, the mouth Hg vapor concentration declined slowly to pre-chewing levels over a period of 90 minutes. Control subjects with no amalgams had insignificant intra-oral air Hg vapor levels that did not change as a function of chewing. Brushing the teeth with commercial toothpaste will also stimulate the release of Hg vapor from amalgam surfaces. Although a positive correlation has been demonstrated between the number of dental amalgams and the levels of rig vapor in the mouth, it remains uncertain how much of this Hg is absorbed into body tissues. A current review, addressing whether Hg usage in dentistry constitutes a potential public health hazard, has concluded that further experimental evidence is needed, particularly regarding the metabolic fate of Hg vapor. The objective of this investigation was to determine possible sites of uptake and patterns of tissue distribution for Hg released from in situ dental amalgams. Qualitative information by whole-body scanning and quantitative tissue measurements by scintillation detection were determi ned using radioactive 203Hg in a sheep experimental model.

Discussion

The results of this study clearly demonstrate that substantial quantities of rig from amalgam will appear in various body tissues as early as 29 days after placement of amalgam fillings in teeth. This Hg can be readily visualized by scintigraphy and can be easily quantified by analysis of tissue radioactivity. The experimental design of this in vivo isotope study has the advantage that all of the Hg measured originates only from dental amalgam and cannot be attributed to food, water, or background environmental sources.

Our findings indicate at least three principal sites for absorption of Hg from amalgam. First, the lungs absorbed Hg as did the cilia lining the trachea because of continual breathing of intra-oral air that had a Hg vapor concentration ranging from 19-50 [mu]g/m3 throughout this study. In humans, approximately 80% of inhaled elemental Hg vapor is absorbed into blood and becomes available for tissue retention. Second, the gastrointestinal tract contained a large amount of Hg likely due to mixing of intra-oral Hg vapor, amalgam microparticles, and dissolved mercuric ions with saliva and food before swallowing. About 10% of the elemental Hg in the human gastrointestinal tract can be absorbed into blood. Even though the efficiency of Hg absorption in the gut is low, large amounts of Hg in feces seen in the present study may signify a substantial pathway for uptake of rig in its elemental or vapor forms. Amalgam microparticles containing Hg would not likely be susceptible to gut absorption . Third, some tissues in the jaw such as gum mucosa and the tooth root and surrounding bone also absorbed Hg. The Hg absorbed into the jaw could be transported from bone marrow directly into blood by venous routes radiographically demonstrated for human circulation. The highly vascularized oral mucosa may likewise afford a route for some Hg vapor transport directly into the systemic circulation.

We are confident that the Hg uptake observed in this animal was not the result of procedural contamination during dental surgery because serial blood measurements taken for 24 hours after surgery had no measurable radioactivity. This indicates that the endotracheal tube prevented inhalation of Hg vapor. Any amalgam particles not removed from the mouth by surgical rinsing would have passed through the gastrointestinal tract well before 29 days when the imaging was performed.

After the Hg released from dental amalgam is absorbed into blood, the two principal target organs of rapid accumulation are kidney and liver. Based on organ weights for kidneys (250g) and liver (1000 g) in the adult ewe, the total Hg concentrated in the kidney in this animal was 1.86 mg, and in the liver it was 0.77 mg, after only 29 days. Even during this relatively short time, the brain and several endocrine glands (pituitary, thyroid, adrenal, pancreas, and ovary) also showed evidence of Hg accumulation from the dental amalgams.

Since Hg/silver fillings remain in human teeth for 8-10 years, this would allow an extended opportunity for body tissues to be continuously exposed to Hg. Other investigators have recently reported that Hg concentrations in autopsied human brain and kidney are significantly higher in those subjects with dental amalgams than in subjects with no amalgams.

Each molar tooth of this sheep contained approximately 425 mg Hg, only one-half the amount of Hg used in the average human occlusally involved molar filling. In humans, occlusally involved Hg/silver dental fillings frequently encompass additional tooth surfaces such as buccal, lingual, mesial, and distal aspects. Thus, such complex human tooth restorations have a greater surface area exposed to grinding forces from which Hg may vaporize. This is in contrast to occlusal restorations in this sheep that are limited only to the occlusal surface and are totally supported circumferentially by solid tooth structure. The natural bovine molar is multiridged for forage grinding. Technical reproduction of these ridges to their original exact functional occlusal level in the amalgam fillings was not possible. Therefore, the restorations were purposely overcarved, which created a concave occlusal surface, ensuring that the amalgams would not be functionally too high and thus subject to abnormally rapid wear. None of the Hg/silver fillings were lost from the mouth during the course of this study.

We believe the sheep is a suitable experimental model for the purpose of our investigations because it exhibits molar chewing mechanics that are similar to those of humans. Moreover, intra-oral air Hg vapor levels in the sheep are very similar to those reported in humans with the same number of amalgams. Although sheep may chew more than the average human does, it is likely that humans who are chronic gum chewers or who exhibit bruxism (chronic grinding of teeth) would have daily periods of chewing that are comparable to sheep fed two meals per day. The sheep body weight also compares favorably with humans, and the sheep is the most widely used obstetrical model in research today.

In other studies of sheep that were not imaged we have established that Hg vaporized from dental amalgam fillings will progressively accumulate in both maternal and fetal tissues as a function of time, and tissue Hg levels will remain elevated in experiments run for as long as 140 days. Exposure of newborn lambs to milk suckled from ewes with dental amalgams results in Hg uptake into tissues of the young.

In North America 5.4% of the population display contact hypersensitivity to Hg. The pathogenesis of a variety of immediate or delayed Hg-induced hypersensitivity responses by the immune system resulting in glomerulonephritis has been postulated. Experimental evidence supports this contention because Hg is capable of inducing autoreactive T lymphocytes and specific autoantibodies resulting in Hg-induced auto-immunity, indicating a potential for Hg to precipitate antibody-mediated tissue injury and autoimmune disease. The kidney and endocrine glands are known sites of autoimmune disorders, which brings into question the long-term implications of Hg concentration in these tissues from dental amalgams as demonstrated by the present study.

Our laboratory findings in this investigation are at variance with the anecdotal opinion of the dental profession, which claims that amalgam tooth fillings are safe. Experimental evidence in support of amalgam safety is at best tenuous. From our results we conclude that dental amalgams can be a major source of chronic Hg exposure. As it has been estimated that in North America 100,000 kg of Hg are used each year in dentistry, continuing research in this area is essential and may have an effect on public health.

Correspondence: Fritz L. Lorscheider Department of Medical Physiology Faculty of Medicine Health Sciences Centre University of Calgary 3300 Hospital Dr. N.W. Calgary, Alberta T2N 4N1 Canada

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By Leszek J. Hahn; Reinhard Kloiber; Murray J. Vimy and Yoshimi Takahashi

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