Dental amalgam controversy
The dental amalgam controversy is a debate over the use of amalgams containing mercury as a dental filling. A minority of dentists has always been opposed to the use of amalgam in dental fillings, since they first came into use over 150 years ago.
The essence of the controversy is that it is intrinsically difficult to scientifically demonstrate either conclusive safety or conclusive harm without very large-scale long-term epidemiological studies of the type that have not so far been performed, and might in any case be impossible due to ethical considerations.

# Background
Dental amalgam is a mixture, rather than a true alloy, of Mercury (metal)|mercury along with other metals like silver, tin, copper, and zinc. Cadmium, indium, palladium and lead were historically found in some amalgam products. Amalgams have been used in dentistry for over 150 years because they are malleable, durable, and more affordable than gold or composites.
While it is accepted that amalgam fillings release mercury, it is generally argued that the amount of mercury released by amalgam fillings is negligible, thus there is no significant danger from mercury leaking from fillings into the body.  Opponents of amalgam argue that long-term exposure to the low levels of mercury vapor causes neurodegenerative diseases, birth defects, and mental disorders. Experts acknowledge the risks of exposure to mercury in any form, while noting that effects of exposure to dental amalgam are not evident at the population level despite its past ubiquity.
Better dental health overall coupled with increased demand for more modern alternatives such as resin composite fillings (which match the tooth color), as well as public concern about the mercury content of dental amalgam, have resulted in a steady decline in dental amalgam use, so the issue may eventually become moot.

# History and overview
Minority dissent over amalgam has been a stable feature of informed debate among dentists almost since its invention.
In 1840, the American Society of Dental Surgeons was founded by a group of dentists who met in New York city.  It was the first national organization of dentists in existence at the time. Chapin A. Harris Artium Magister|A.M.,M.D.,D.D.S.  , who in 1840 co-founded the ASDS and the first dental school in the US, the Baltimore College of Dental Surgery, said in his opening address: "It is one of the most objectionable articles for filling teeth that can be employed, and yet from the wonderful virtues ascribed to this pernicious compound by those who used it, thousands were induced to try its efficacy".  "At that time there were only about three hundred trained and scientific dentists in the entire country; the rest were relatively untrained operators, outright quacks, or charlatans".   In 1845, the ASDS had members sign a mandatory pledge promising not to use mercury fillings because of fear of mercury poisoning in patients and dentists (at the time, dentists made amalgam by mixing liquid mercury and the other components of amalgam themselves in their office, a practice which continued until pre-filled amalgam capsules became generally available in the 1960s). During the next decade some members of the society were suspended for the use of amalgam. Because of its stance against dental amalgam, membership in the American Society of Dental Surgeons declined, and due to the loss of members, the organization disbanded in 1856. 
In 1859, the American Dental Association (ADA) was founded by twenty-six delegates representing various dental societies in the United States at a meeting in Niagara Falls, New York.  The ADA did not forbid use of amalgams. The ADA position on the safety of amalgam has remained consistent since its foundation.  As of 2006, ADA has over 152,000 members and is the largest and longest-standing professional association of dentists in the world.
Amalgam formulations and properties were gradually improved, notably by Dr. G.V. Black in 1895.  Despite these changes, debate over the use of amalgams persisted in the dental profession.  The ADA maintained until 1984 that mercury was bound in amalgam and did not release mercury vapor. In the 1970s studies demonstrated that a small amount of mercury vapor was constantly being released from amalgam, corroborating the first such study published in 1882 in the Ohio State Journal of Dental Science by Dr. Eugene S. Talbot.
The majority of dentists maintain that dental amalgams are not only safe for use but desirable, since they are cheap, easy to use, fairly durable and strong, can be quickly inserted into the oral cavity and that any release of mercury vapor is negligible.  Opponents, however, argue that repetitive exposure to a very small amount of mercury can be cumulative, so amalgams can cause health problems.
Those in favor of amalgam fillings (the majority view in the USA and elsewhere) indicate that significant measurable adverse health effects from them have not been proven conclusively. Those against amalgam fillings (the minority view in the USA) indicate that their safety has not been proven conclusively. Because both of these statements are objectively true, the controversy continues without resolution to date.

# Regulation and governmental involvement
Some legislators have introduced legislation to prohibit or restrict use of amalgam fillings.
In 2001 in a lawsuit involving California Proposition 65 and amalgams, a California Superior Court judge ruled that all dental offices with more than nine employees must provide notices on the contents of dental fillings.  The mandated notice reads:
Some countries, such as Japan, Norway, and Sweden, have banned or regulated the use of mercury in dental amalgams, citing health or environmental concerns. The Swedish Chemicals Inspectorate (KemI) maintains a web site containing a report on the investigation for a general ban on mercury on which it states, "KemI judges that there are strong grounds for banning amalgam for environmental reasons. From a health point of view there is every reason to apply a precautionary approach." Some countries, notably Austria and Germany, have some restrictions on dental amalgam, such as permitting its use for the general population, but not for children, pregnant women, people with kidney problems, when in contact with other metals, such as braces, and in people with mercury sensitivity.
In 2002, eight members of the U.S. House of Representatives introduced the Mercury in Dental Filling Disclosure and Prohibition Act  (H.R. 4163, 107th Congress, 2nd Session, April 10th 2002), which would have prohibited any mercury in dental fillings starting in 2007.  In an interim period between July 1, 2002 and 2007, the bill would have required labeling of amalgam with a warning.  The bill was referred to a subcommittee, which Table (verb)|tabled it. It has since been re-introduced as bill HR-4011 by congresswoman Diane Watson, who issued a public statement after the September 2006 decision of the FDA Review panel on dental amalgam. 
In most European countries (for example, the United Kingdom, France, and Italy) and the United States, amalgam use is unrestricted.  In the United States, amalgams are classified as a "device," not a "substance," by the Food and Drug Administration (FDA). Under the U.S. Code of Federal Regulations, amalgams are a prosthetic device:
As a result of this classification, amalgams have not been subject to official government testing in the United States such as is required for medicines. However, the FDA maintains a web page on the use of amalgam, last updated in December 2002, on which it states, "no valid scientific evidence has shown that amalgams cause harm to patients with dental restorations, except in the rare case of allergy." As described above, this earlier conclusion by the FDA was called into question by a panel of scientists in September 2006, when a joint meeting of three FDA committees reviewed and discussed peer-reviewed scientific literature on dental amalgam devices. Prior to the joint meeting, some scientists and consumer advocates had raised questions about the fairness of this process, as planned by the FDA.
Following the meeting of the joint committees on 6-7 September 2006, when the panel of outside advisers that the FDA had asked to assess the conclusions of its report on amalgam safety rejected the FDA report in a 13-7 vote, they stated the report's conclusions were "unreasonable", given the quantity and quality of information currently available. Panelists said remaining uncertainties about the risk of so-called silver fillings demanded further research, in particular, on the effects of mercury-laden fillings on children and the fetuses of pregnant women with fillings and the release of mercury vapor on insertion and removal of mercury fillings. Michael Aschner, a professor of pediatrics and pharmacology at Vanderbilt University and a panel consultant said "There are too many things we don't know, too many things that were excluded."   
Shortly after the decision of the joint advisory panel, the president of the International Academy of Oral Medicine and Toxicology (IAOMT) wrote to the FDA to ask for an expanded review of current science on dental mercury amalgams, a definitive date for such a hearing, and a format that will assure that the full breadth of health effects is assessed.  In a press release the ADA wrote that it " welcomes the call by a U.S. Food and Drug Administration (FDA) panel for additional review of scientific studies on the safety of dental amalgam fillings." and reiterated that "the overwhelming weight of scientific evidence supports the safety and efficacy of dental amalgam, and it should continue to be made available to dentists and their patients " and " dental amalgam contains elemental mercury combined with other metals such as silver, copper, tin and zinc to form a safe, stable alloy. " 

# Points of contention

## Available alternatives
One argument against the use of amalgams in dentistry is that a number of less controversial, safer alternatives exist, like  composite resin (or "white") fillings that have been available since the 1980s.  Other alternatives include gold, porcelain, and glass ionomers. Amalgams are stronger, more durable, and less expensive than most of the available alternatives, though some newer composite materials have demonstrated durability on par with amalgam. Most of these materials, except gold which has been used longer than amalgam, have not had as long a period of use and study as amalgam, and some of them are known to contain other potentially hazardous compounds.  This is one reason why biocompatibility testing is recommended for all dental materials as per ADA/ANSA or ISO standards, and can be performed by specialized laboratories. Some experts also caution against a too-sudden shift towards composite resin to allow time for dentists to acquire the skills needed to properly place resin fillings since " clinical studies on the longevity of amalgam and resin composite restorations placed by dental students and dentists who are experienced in both composite resin and amalgam placement show a comparable and acceptable annual failure rate for those restorations ".  Teaching of amalgam techniques to dental students is declining in some schools in favor of composite resin, and at least one school, University of Nijmegen in the Netherlands, had eliminated dental amalgam from the curriculum entirely as of 2001.

## Environmental impact
Mercury is an environmental toxin and the World Health Organization, OSHA, and NIOSH have established specific occupational exposure limits.  Amalgam removed from teeth is classified as toxic waste in various countries.  Mercury from improperly disposed of amalgam may be released into sewage water. Crematoria are to establish recovery of mercury from flue gases as soon as reasonable since mercury from amalgam is released into air during cremation of cadavers with amalgam fillings.
Environmental risks are mitigated provided that amalgams are disposed of properly. ISO has issued standards regarding the proper handling and disposal of amalgam waste, and legislation to enforce these standards is being adopted in some states.

## General health effects
Dental amalgams have been suspected by some medical practitioners, particularly of integrative or alternative medicine, of causing many physical and/or psychological problems.  They reason that, since mercury is poisonous and neurotoxic, amalgams known to release mercury into the mouth are indisputably a source of poison.  These persons argue that amalgams can cause neurodegenerative diseases, birth defects, and mental disorders.  While it is proven that mercury exposure can cause health problems such as these, the potential adverse effects of amalgam itself on health has not yet been resolved.  Two recently released comprehensive reviews of papers published in peer-reviewed journals arrived at opposite conclusions.  One released in December 2004 in the US found little evidence to link mercury fillings to health problems while the other, release in March 2005 in Germany found removal of dental amalgam leads to permanent improvement of various chronic complaints in a relevant number of patients in various trials.
Two recent randomized clinical trials in children showed no statistically significant differences in adverse neuropsychological or renal effects observed over the five-year period in children whose  caries were restored using dental amalgam or composite materials, although one study could not rule out the possibility of a small adverse effect on IQ in children with amalgam. In contrast, one study showed a trend of higher dental treatment need later in children with composite, and thus, claimed that amalgam fillings are more durable. However, subsequent study published in JAMA cites increased mercury blood levels in children with amalgam fillings. The study states, "during follow-up  1.0 to 1.5 microg/g higher in the amalgam group than in the composite group." EPA considers high blood mercury levels to be harmful to fetus, and also states that "exposure at high levels can harm the brain, heart, kidneys, lungs, and immune system of people of all ages." Currently, EPA has set the "safe" mercury exposure level to be at 5.8 micrograms of mercury per one liter of blood. While mercury fillings themselves do not increase mercury levels above "safe" levels, they been shown to contribute to such increase. However, such studies were unable to find any negative neurobehavioral effects, and once again assured that amalgams are safe.
In response to the studies that assert amalgam safety, anti-amalgam groups, such as the International Academy of Oral Medicine and Toxicology (IAOMT), state that these studies are poorly designed and raise ethical questions. An analysis of the data collected during the studies showed that the authors of the studies ignored the drop in mercury excretion, after two years, in the urine in the children with amalgam fillings, even though the mercury exposure from amalgam remained the same or increased.  This is explained in a response by Dr. Boyd Haley, to the 2006 publication, in JAMA, of the NIDCR-funded children's amalgam study.  According to Haley, this is evidence that these children are losing the ability to excrete mercury with increased exposure.
Toxicologists have catalogued a large number of health effects caused by mercury poisoning, thus the range of possible symptoms is very diverse and all of them can be produced by other causes, making it difficult to draw definite conclusions.  While most dentists acknowledge that a small proportion of patients may suffer from local symptoms due to mercury sensitivity or allergy, they would not agree with alternative practitioners who suspect a broad range of local and non-local health effects in a larger proportion of people.  There is no substantive evidence to suggest that nearly two centuries of amalgam use has resulted in measurable health effects in patients. A 2003 report from a group of international experts reviewing various studies on mercury concludes that:
- Studies on human and animals have demonstrated that dental amalgam contribute significantly to mercury body burden in humans with amalgam fillings.
- dental amalgam is the most common form of exposure to elemental mercury in the general population, constituting a potentially significant source of exposure to elemental mercury, with estimates of daily intake from amalgam restorations ranging from 1 to 27 g/day, the majority of dental amalgam holders being exposed to less than 5 g mercury/day.
- other sources of exposure include food, particularly fish, drinking water and airborne mercury
- intestinal absorption varies greatly among the various forms of mercury, with elemental mercury (as found in amalgam) being the least absorbed form (<0.01%)
- absorption also varies according to individual factors such as gum chewing and bruxism (tooth grinding ).
- the number of restorations - amalgam or otherwise - is declining, largely due to improved dental hygiene, in all industrialised countries examined declining by 38% since the 1970s in the USA and over 65% in the ten years from 1986 in the UK
- although several studies have demonstrated that some mercury from amalgam fillings is absorbed, no relationship was observed between the mercury release from amalgam fillings and the mercury concentration in basal brain.
- However, in the same report it was concluded that "...even at very low mercury levels, subtle changes in visual system function can be measured." In multiple sclerosis patients with amalgam fillings, red blood cells, haemoglobin, hematocrit, thyroxine (T4), T-lymphocytes and T-8 (CD8) suppressors cells levels are significantly lower, while blood urea nitrogen and hair mercury levels are significantly higher.
- The report also notes that regarding elemental mercury exposure, the main form of exposure from dental amalgam," most studies rely on assessment of exposure at the time of study, which may not be fully informative, as mercury has a long half-life in the body and thus accumulates in continuous exposure ", making the evaluation of effects on health uncertain. 
Other, lesser findings were reported, including a cross-sectional study in which cognitive function was not related to the number or surface area of occlusal dental amalgams, a case-control study in which patients with numerous amalgam fillings exhibited higher levels of neurological symptoms than the controls, and a study of self-referred patients who believed they were suffering ill-effects from dental amalgams which found no correlation between number of dental fillings and symptomatology, but higher mean neuroticism than two comparison groups (the authors concluded that self-referred patients with health complaints attributed to dental amalgam are a heterogeneous group of patients who suffer multiple symptoms and frequently have mental disorders).
Overall, the results are inconclusive in respect of adverse effects from exposure to mercury in dental amalgam.
In a 2005 policy paper on mercury in health care the World Health Organisation (WHO) states that dental amalgam is the main source of mercury exposure in non industrialized settings, exposing the concerned population to mercury level significantly exceeding those set for food and air.  
Recent studies suggest that mercury may not have a threshold below which some adverse effects do not occur.   Research on monkeys has shown that mercury released from dental amalgam restorations is absorbed and accumulates in various organs such as the kidney, brain, lung, liver, gastro-intestinal tract, the exocrine glands. It was also found to have crossed the placental barrier in pregnant rats and proven to cross the gastrointestinal mucosa when amalgam particles are swallowed after amalgam insertion or after removal of old amalgam fillings. 
Various diagnostic methods exist to detect the level of mercury in the body, including blood tests, urine tests, stool tests, saliva tests, DMPS or DMSA chelation urine tests, a hair analysis and others.  Opinions differ on which of these tests, if any, is the most accurate, although mainstream scientific research tends to place the most weight on chelation urine tests or stool tests when trying to assess chronic levels, or on blood or urine tests when trying to assess recent acute exposure.  None of these tests, however, can link mercury levels to dental amalgams for methodological reasons, except (a) on an epidemiological scale; or (b) through measuring levels before and after dental work.  Studies have investigated both angles and results have differed, fueling the controversy since the scientific data remains inconclusive and has not yet proven either safety or danger.
Most dentists, however, still feel that while, when placing the fillings, both patient and dentist are exposed to a small amount of mercury and mercury vapor, once the amalgam has hardened (which takes less than a minute), most of the mercury is captured in the filling.  Further, being trapped in the amalgam, they contend that the mercury cannot escape into the mouth except in small amounts, which they would not deem significant except in the case of hyper-sensitive patients.  Opponents and some practitioners disagree, and prefer the use of alternative fillings.  Because of such concerns, according to the Consumers for Dental Choice, fully one third of the dentists in the US are currently 'mercury free'.  Some practitioners may also recommend that patients with amalgam fillings have them removed and replaced, particularly if the patient is experiencing the symptoms that they attribute to the use of amalgam. These professionals recommend that unprotected amalgam removal should be avoided even in routine dental procedures to avoid exposure to mercury vapor and amalgam particulate matter. 
Organizations opposed to amalgam use, such as Consumers for Dental Choice, claim to have over 65,000 studies on file implicating amalgam fillings in as the cause of a diversity of health disorders.  These studies have been used in various lawsuits, and were the main contributing factor to the passage of Californian legislation concerning the issuing of warnings to patients about mercury, and to the federal bill introduced in 2002 proposing that amalgam fillings be made illegal after 2006. One recent meta-study has concluded that after studies with methodological flaws are discounted, the evidence indicates that amalgams should not be used.

## Health effects for dentists
Among modern dentists who are exposed to mercury amalgam and vapor on a daily basis, no evidence of mercury poisoning has been demonstrated.  Some studies have indicated that mercury from amalgams affect some dentists mildly.  Dentists in several large-scale studies performed multiple cognitive and behavioural tests and, compared to a normal population, lagged behind in many areas. In one study this included 14% worse scores in memory, co-ordination, motor speed and concentration.  The study did not demonstrate any link between mercury exposure and these lagging scores, however.
A study examining the health effects of mercury on dentists in the UK published in the Occupational and Environmental Medicine Journal  concluded that 180 dentists had on average 4 times the urinary mercury excretion levels of 180 people in a control group.  Dentists were significantly more likely than control subjects to have had disorders of the kidney or memory disturbance.  No direct correlation between urinary mercury levels and the disability, however, was found.  Urine testing is unreliable for showing lifetime mercury accumulation rather than recent exposure.  Research on living humans has proceeded slowly.

# Footnotes