This appendix provides a brief overview of key historical aspects of the ongoing debate within the dental profession, and in society more generally, about the continuing use of dental amalgam as a restorative material. Understanding and appreciating this history has implications for the kinds of recommendations that may gain moral traction in the current debate, as well as in developing implementation strategies for such recommendations.

Early experimentations with various combinations of mercury (Hg) amalgam were conducted in France and Britain in the early part of the 19th century, and amalgam was eventually introduced to America in the 1830s.^133,143 From the outset, there was debate among dentists as to the safety of mercury amalgams. When the American Society of Dental Surgeons was formed in 1840, its members were required to sign a pledge never to use amalgam because of the known toxicity of mercury. Enforcing that pledge proved problematic, however, leading to dissension within the American dental profession. Eventually the controversy led to the dissolution of the American Society of Dental Surgeons in 1856.^133,134,143

Then, as now, there were conflicting opinions as to the motives of the parties holding opposing views. Amalgam detractors maintained that monetary self-interest was the primary motive for amalgam supporters, who in turn downplayed the potential toxic effects of mercury.¹⁴³ Amalgam supporters, on the other hand, claim that those against amalgams were driven primarily by jealously, prejudice, and poor judgment.^134,245

When the American Dental Association was formed to replace the defunct American Society of Dental Surgeons, it expressed no opinion on the safety of dental amalgam. In the meantime, there were ongoing efforts to develop a better amalgam and, in the late 1870s, a movement began within dentistry to promote amalgam as a valuable filling material even as reports of its potential deleterious effects were debunked.¹³⁴ By1895, the American Dental Association was expressing support for the use of amalgam, a position it has held consistently until the present.^198,246

Detractors to amalgam were active throughout the 20th century,¹⁴³ and speculation about potential links between amalgam and various ailments were ongoing.^247,248 Concerns were also raised about potential occupational hazards for dentists and dental assistants who were exposed to mercury on an ongoing basis.^249–252 For the most part, however, the safety of amalgam was largely assumed until the 1980s when methods were developed that confirmed the steady release of mercury vapours from amalgam fillings.^253,254

Although the American Dental Association acknowledged the persistent off-gassing of mercury vapour in the mouths of patients with amalgam fillings, it maintained that any mercury levels were clinically insignificant, while reasserting its confidence in amalgam.²⁰⁶ In December of 1990, the American news program 60 Minutes aired an exposé that proposed potential links to multiple sclerosis and other ailments due to poisoning from amalgam, placing the debate squarely in the public sphere once again.^255–257

The 1990s was a decade of heightened activity in the amalgam debate. While some within dentistry maintained that no scientific studies showed amalgam to be unsafe,²⁵⁸ complained that media hype was undermining a good product,^155,257 and even went so far as to equate amalgam concerns with witchcraft and astrology,²⁵⁹ others doggedly questioned the evidence in support of amalgam safety. Indeed, a persistent theme throughout the debate involves conflicting interpretations both of what constitutes evidence and what any supposed evidence means. Although numerous studies and supporting statements throughout the 1990s from North America and abroad affirmed the supposed safety of mercury amalgam, while debunking any connections to chronic diseases,^{160,181,217,218,258,260–264} others questioned those conclusions: “The comparison of mercury exposure levels from dental amalgam with occupational exposure is illusive,” states one commentator. “Occupational exposure is 40 hours per week (while amalgam exposure is 154 hours per week)… and continues uninterrupted during the entire lifetime of the restoration.”²⁶⁵ Another detractor argued that the interpretation of mercury toxicity is extremely difficult because of the variable half-life of mercury which can vary between tissues in the same individual.¹³² Yet another refers to “good evidence” for delayed neurotoxicity from mercury exposure that may only be manifested many years later.²⁶⁶ Others simply question the long-term safety of amalgam.²⁶² The potential connection between amalgam and chronic diseases such as multiple sclerosis²⁶⁷ or mental illness^255,268 is also frequently raised.

Given the media attention and apparent lack of consensus, some patients insisted that their amalgams be removed. Dentists struggled to know how to respond.²⁶⁹ Contrary to available evidence, one leading professional journal advised that, if asked, patients should be informed that when combined with other metals mercury becomes “a biologically inactive substance.”²⁰⁶ Some dentists simply refused to comply with patient requests, resulting in a 1993 case in Canada in which a dentist was charged with malpractice for refusing to replace a patient’s amalgam fillings. While the Ontario Health Disciplines Board found that dentist innocent,²⁷⁰ other dentists were more willing to grant their patients’ requests, leading to charges of quackery and suggestions of exploitation.^180,181,271 The ongoing issue prompted the American Dental Association to revise its Principles of Ethics and Code of Professional Conduct to state: “The removal of amalgam restorations from the non-allergic patient for the alleged purpose of removing toxic substances from the body, when such treatment is performed solely at the recommendation or suggestion of the dentist, is improper and unethical.”¹⁷¹ The Canadian Dental Association followed suit with similar statements, maintaining that amalgam removal was unwarranted and unprofessional.^{19,139,141,272} Meanwhile, dentists who questioned the use of amalgam continued to voice concerns and in some cases questioned the professional competency of those who maintained the status quo. Inasmuch as amalgam is relatively easy to work with compared to resin, some speculated that it was lack of skill that in part motivated many to resist the move to resin. “Amalgam is a material that is ideal for mediocre dentistry,” opined one dentist.²⁷³

The Canadian contribution during this particular period was significant. While the official position of the Canadian Dental Association, in support of amalgam, has been documented, there were strong dissenting voices within the Canadian scientific community. University of Calgary researchers M.J. Vimy and F.L. Lorscheider were instrumental in developing techniques to measure concentrations of mercury vapour released by amalgams^254,274 and published a number of papers in medical and scientific journals throughout the 1980s and 1990s that raised concerns about mercury toxicity.^167,275 Their consistent conclusion was that research evidence does not support the notion of amalgam safety.²⁷⁶ It should also be noted that professor Vimy was one of the scientists interviewed in the 60 Minutes exposé of 1990.

As the public debate grew, the Medical Devices Bureau of Health Canada started its own investigation.¹⁹ Dr. Mark Richardson was commissioned to attempt a calculation of the fraction of total exposure and relative risk due to mercury exposure from amalgam.

Richardson’s report, released in 1995, was the first comprehensive risk assessment in Canada of mercury exposure from amalgam.²⁷⁷ Richardson’s study did not include laboratory research or clinical investigations but relied instead on sophisticated computer modelling techniques to arrive at a tolerable daily intake level for mercury. His initial simulations and calculations indicated that amalgam contributes about 50% of the daily mercury exposure for the average Canadian.^19,277

Before releasing Richardson’s study, Health Canada asked a group of international experts in toxicology, public health, and risk assessment to review it. While the reviewers generally agreed Richardson’s methodology was sound, concerns were expressed over the lack of data on many of the crucial factors in his assessment model. Doubts were raised about whether probabilistic estimation techniques that relied on assumptions in lieu of data could provide a reliable tolerable daily intake.¹⁹ Health Canada subsequently convened a committee of stakeholders to review the report. That committee initially included professor Vimy, but when it became apparent the Committee would not recommend accepting Richardson’s calculation of the tolerable daily intake, Vimy resigned, complaining that the committee was stacked in favour of those supporting the use of amalgams.²⁷⁸ Health Canada subsequently decided not to follow Richardson’s recommendation,¹⁹ and the Canadian Dental Association declared it “good news on amalgam.” “Science, not misinformation and zealotry, must be the determining factors,” declared the then-president of the Canadian Dental Association.²⁷⁸

Although Health Canada did not endorse Richardson’s tolerable daily intake estimate, the stakeholder committee did approve eight recommendations including one related to potential amalgam toxicity. That recommendation is carefully phrased, however, and emphasizes that “there is no evidence that dental amalgams contribute to immunological, neurological, or kidney disease.” However, given that there is some evidence that mercury exposure from all sources could have potential negative effects, dentists and physicians were advised to consider these concerns in their choice of dental materials,¹⁹ although even these somewhat innocuous recommendations were challenged by Canadian dentists.²⁷⁹ This Canadian response contrasted with what was occurring in many European countries.

Even as WHO and the FDI were issuing a 1995 consensus statement reaffirming the safety of amalgam, while emphasizing its cost-effectiveness,¹⁶⁴ the conversation had taken a somewhat different turn and tone in Europe. Already in 1987, the Federal Office of Public Health in Germany issued a series of recommendations against the use of amalgam for pregnant women, children, and people suffering from kidney disease. By 1992, the Swedish parliament was considering a total ban on amalgam, and had already disallowed its use for patients less than 20 years of age.¹⁹ The total Swedish ban did not occur, however, until 2009 and, when announced, was primarily out of environmental as opposed to patient safety concerns.²⁸⁰ This shift in focus to emphasize public health and environmental concerns was to become a common theme as the amalgam issue moved into the new millennium.^187,281 Nevertheless, in the 1990s, patient safety was still the motivating factor throughout Europe. In 1998, the Department of Health in Britain advised dentists against using amalgam during pregnancy, following the leads of Sweden and Norway, where such restrictions had been in place since the late 1980s. While Finland and Denmark did not specifically highlight pregnancy, they had issued general recommendations against amalgam use. Germany and Austria followed suit, issuing recommendations to reduce amalgam use in young children, pregnant women, and in individuals with kidney disease ²⁸² — this last ostensibly based on evidence that mercury accumulates in solid organs of the body and especially the kidneys and liver. ²⁷⁵

As the amalgam controversy moved into the 21st century, the lines of disagreement regarding patient safety have remained essentially the same. While various studies maintaining either that mercury toxicity from amalgam is not clinically significant,²⁸³ or studies purportedly demonstrating that those exposed to mercury vapours did not exhibit any particular deleterious effects from such exposure,^47,48 others continue to dispute both the findings and the methods used in reaching those conclusions.^144,284 “Although the issue of amalgam safety is still under debate,” says one recent review, “the preponderance of evidence suggests that mercury exposure from dental amalgams may cause or contribute to many chronic conditions.”²¹² Yet the temptation to cast aspersions on the opposing position is ever-present: “Google amalgam,” complains one amalgam supporter, “and you’ll be overwhelmed by junk science and fraud.”²²¹ Nevertheless, the calls for additional research on the long-term effects of mercury exposure remain constant.^285–288 Despite the Canadian Dental Association’s continuing support for the use of amalgam, a 2002 survey of Canadian dentists identified the development of materials other than amalgam to be a research priority.²⁸⁹

Other areas of potential research have emerged in recent years including the role of genetics in identifying patients who may be more susceptible to mercury toxicity,^210,211 as well as the potential impact of electromagnetic fields including magnetic resonance imaging (MRI) scanners in elevating mercury toxicity levels for those with amalgam fillings.^{97,162,290,291}

While the ongoing questions regarding patient safety have remained consistent, there are three areas of heightened activity in the 21st century worth noting. The first concerns the increased level of litigation. Due in part, no doubt, to the heightened public awareness of amalgam throughout the 1990s, a number of lawsuits were launched in a various jurisdictions (primarily in the US) against dental associations, either claiming harms from the continued use of amalgams or seeking legislative restrictions on such use.^{146,147,151,152} Virtually all such cases were dismissed. However, not all cases were decided in favour of those supporting the use of amalgams. Cases in both Oregon and California challenged the relevant dental associations’ attempts to restrict the kinds of information dentists could share with their patients about potential amalgam toxicity, which the plaintiffs perceived as “gag orders.” In both cases, the courts ruled in favour of the plaintiffs.^148,149 Such legal proceedings were instrumental in the FDA’s 2009 decision to reclassify dental amalgam.^150,292 In particular, the FDA documentation reports that 70% to 80% of inhaled mercury vapour is absorbed by the lungs and distributes to several organ systems in the body, including a fraction that crosses the blood-brain barrier. Although the FDA reclassification document concludes there is inadequate evidence to conclude that vulnerable populations are at risk, it includes “special controls” for developing fetuses, breastfed infants, and children under six.²⁹²

The second development, which bears noting, is the rise in the use of composite resins as an alternative to amalgam. Whether out of concern for safety or simply as a matter of aesthetic preference, composite resins have been gaining in popularity throughout the past two decades. While concerns have also been raised about the potential toxic effects of BPA as a by-product of composites,^170,293,294 the evidentiary basis for these concerns is also disputed.²⁹⁵

Finally, a rise in concerns about environmental protection in general, and about mercury toxicity from all sources in particular, has had a significant impact on the amalgam discussion in the 21st century. Canada has recently ratified the Minamata Convention — an international effort to reduce human-generated mercury emissions.¹³¹ Such international efforts have raised questions about the future role for amalgam in dentistry,²⁹⁶ and about the potential impact on dental patients.^297,298 While international bodies still maintain the safety of amalgam as a dental material, it nevertheless supports a phase-down in use ^165,166

The debate over the safety of dental amalgam as a restorative material has been long and sustained. If there is any semblance of common or neutral ground, it is around the growing consensus that dental amalgam contributes to the overall environmental load of mercury toxicity, and efforts to limit and reduce its impacts are appropriate.

Finally, it is worth mentioning the precautionary principle which is often invoked “in circumstances where there is some evidence that a particular activity may result in health or ecosystem damage, but great uncertainty as to the potential magnitude or nature of those impacts.” ¹⁸² Tickner and Coffin summarize the relevance of this principle to the mercury amalgam issue, as follows:

A precautionary approach to mercury amalgams would consider the clear evidence of mercury toxicity, the lifecycle risks of amalgams, and a broad range of alternatives. While the potential health risks from mercury amalgams to healthy adults with fillings is likely low, one must consider the lifecycle of the amalgam, including cumulative exposures and the potential for greater impacts when exposures occur at sensitive times in development — pregnancy, childhood. This case illustrates the challenges of alternative assessment — in finding those that are both effective and safe — and the trade-offs that are often involved. However, the lack of currently available alternatives should not stall intermediate action to minimize exposures to mercury. (p12)¹⁸²