Dental Amalgam Safety

What is amalgam?

A dental amalgam is produced by reacting the liquid metal, mercury, with metal alloys. Conventional silver-tin amalgam usually also contains small amounts of copper and zinc. High copper amalgams are prepared from either a mixture of silver-tin and silver-copper alloys or from a silver-copper-tin alloy. High copper amalgams have better clinical properties with a higher resistance to corrosion and marginal breakdown but release mercury 30 times faster than older style amalgams of 20 years ago. Amalgams used to be considered the ideal filling material since it is easy and cheaper than the alternatives now available.

Filling materials need to last as long as possible because replacement weakens the remaining natural tooth and may make more complex treatment necessary. As more dentist turn to better solutions the number of amalgam fillings being inserted is falling.

Can mercury from amalgam fillings reach the rest of the body?

Amalgam fillings release mercury vapour, especially when teeth are chewed on or brushed. This vapour can easily reach the rest of the body through inhalation and part of it will also dissolve in saliva and will be swallowed.

Much of the mercury entering the body is excreted but some accumulates in certain organs - especially the kidneys but also, all be it to a lesser extent, in the brain, lungs, liver and gastrointestinal tract. Experts do not believe that the levels of mercury exposure resulting from amalgam fillings alone are of any general health significance but linked with the increasing levels of mercury in the atmosphere could increase the chance of ill health.

A bit of a history lesson

As early as the 7th century, the Chinese used a "Silver Paste" containing mercury (Hg) to fill decayed teeth. Throughout the Middle Ages, alchemists in China and Europe observed that this mysterious silver liquid, extracted from cinnabar ore, was volatile and would quickly disappear as vapour when mildly heated. Alchemists were fascinated that at room temperature Hg appeared to "Dissolve" powders of other metals such as silver, tin and copper. By the early 1800's the use of a Hg/Silver paste as a tooth filling material was being popularized in England and France and it eventually introduced into North America in the 1830's. Some early dental practitioners expressed concerns that the Hg/Silver mixture (amalgam) expanded after setting, frequently fracturing the tooth or protruding above the cavity preparation and thereby prevented proper jaw closure.

In 1998, the Department of Health's Committee on Toxicity (COT) reviewed the evidence in response to an expert report from the European Commission, and concluded that dental amalgam is free from risk of systemic toxicity and only a very few cases of hypersensitivity occur.

Whether amalgam can be called 'safe' is a matter for manufacturers of amalgam and for the Department of Health, and for the toxicologists and other scientists who advise them. Dentists comment on the dental properties of the material. If amalgam was found to harm general health dentists would stop using the material immediately. Dentists and their staff are concerned about their own safety too and research into their health can give insights into the likely safety of amalgam for patients.

It is never possible to say categorically that anything is safe in all circumstances. With amalgam, as with all bio-materials, there are risks and benefits to be balanced. Until all dental decay can be prevented, decayed teeth either have to be filled or, ultimately, extracted. Loss of natural teeth impairs eating, speaking and socialising. Also, use of an alternative to amalgam may raise the possibility of other hazards, and if the alternative is less durable it will fail more quickly and need to be replaced more often.

All dentists can do is keep abreast of the research and be prepared to change prescribing practices if new evidence emerges. However, it is worth pointing out that the establishment of a link between amalgam use and a condition such as dementia in elderly people is, in principle, a feasible subject for research if long-term dental records are available and the study population is large enough. An important long-term study of ageing is currently taking place in an American convent. Findings so far show no link between mental cognitive performance and numbers of amalgam fillings³.

But mercury is toxic, isn't it?

Like most bio-materials, dental amalgam is not inert. Amalgam fillings release mercury vapour, especially when teeth are chewed on or brushed. Some of the vapour is exhaled but some reaches the rest of the body through inhalation. Part of it also dissolves in saliva and is swallowed.

Mercury is certainly toxic at large enough doses but the research does not indicate that the small exposures which result from the use of dental amalgam are harmful. Many substances are toxic at certain dose levels and for certain people, depending on individual susceptibility. Some people are even allergic to ingredients in foods such as bread and milk.

Reactions sometimes occur in the soft tissues of the mouth next to fillings, not only with amalgam⁴ but with other restorative materials too. Amalgam is not especially allergenic and true sensitivity reactions are very rare. They may resolve spontaneously or after a change of restorative material. Suspected allergies are investigated by dermatologists/allergists and by oral medicine departments of dental hospitals, on referral from the dentist.

Who regulates dental filling materials?

If products are to be used within the European Economic Area (EEA), they are now required to be CE marked to demonstrate compliance with the Essential Requirements of the Medical Devices Directive. The system requires an assessment of toxicological risks, with identification of all hazards being the first stage of this process. In the end, no product can be proved safe, and new hazards can always be discovered. The courts ultimately decide the issue, rather than scientists. For example, breast implants were the subject of successful litigation, even though they had achieved wide acceptance following extensive use prior to the introduction of regulatory controls.

How much of our day-to day mercury intake comes from dental fillings and how much comes from other sources?

Exposure to mercury depends on diet, any occupational exposure, and environmental mercury levels, as well as on amalgam fillings. On average, a UK adult absorbs about 9 millionths of a gram of mercury a day from all sources. About a sixth of this amount comes from amalgam fillings. Certain foods have high mercury content - fish, for example. Baseline mercury levels in Icelanders are about ten times higher than in the UK population because of the amount of fish eaten there.

How can patients find out about mercury exposure?

The mercury content of urine, hair, blood, breath or finger nails can be measured and dentists and their staff can monitor their own exposure by sending specimens for laboratory testing. Patients concerned about exposure to mercury could ask dentists to arrange testing in the same way but the test could not be provided under National Health Service dental arrangements. Also, the test would not necessarily identify the source of any mercury identified.

Is it possible to remove mercury absorbed into the body?

Patients with severe mercury poisoning are sometimes prescribed 'chelating agents' which combine with mercury to produce a substance which can be more readily excreted. However, there is no evidence that removal of mercury through the use of these agents has any beneficial effect for patients with symptoms believed to be caused by low exposure from amalgam fillings. The use of Chelating agent's has side effects of its own.

How should amalgam fillings be removed?

While there is no proof of a toxic effect during removal of amalgam fillings, there are the risks associated with the removal of any filling material - namely, inhalation of particles. With amalgam removal, there is also a potential hazard from inhaled mercury vapour. Risks can be minimised with copious use of water and adequate suction. The turbine spray can be supplemented with a 3 in 1 syringe plus suction with a high volume wide bore tube. A rubber dam may also be used. Removal by piecemeal sectioning also helps to minimise risks.

Because the risk during amalgam removal is concerned mainly with particle inhalation, there is not an additional risk in removal of many amalgams at one session. However, subsequent restoration may be made more difficult because of occlusal contact problems, so removal of amalgams may take place over several visits.

Should amalgam be used during pregnancy?

It is known that mercury can cross the placenta from mother to foetus and can also be detected in breast milk but there is no evidence of any link between amalgam use and birth defects or still births. Generally, it is sensible to minimise health interventions during pregnancy, where this is clinically feasible. Dentists would approach the placement or removal of amalgam fillings from the same precautionary standpoint.

The 1998 report from the Committee on Toxicity² already referred to said that there was no reason to think that the placement or removal of amalgam fillings during pregnancy was harmful. COT agreed with the European Commission's expert group, however, that 'it may be prudent to avoid, where clinically reasonable, the placement or removal of amalgam fillings during pregnancy'. More research was recommended.

This does not constitute a ban on the use of dental amalgam during pregnancy. There will be occasions when patients will decide, on advice from a dentist, that the benefit of using amalgam (in terms of permanency, for example) outweigh the as yet theoretical risk of systemic toxicity. Dentists must always have a patient's informed consent before undertaking any treatment.

Should amalgam be used for children?

Children who have a good diet and oral hygien can usually be treated successfully using sealant resins and glass ionomer cements. But once a posterior cavity approaches one third of the occlusal width and once any mesial or distal extension is not bound by enamel, glass ionomer fillings are not normally recommended and amalgam is an acceptable restorative material.

Is there any group for whom amalgam should not be used?

Patients with proved amalgam sensitivity are the only group for whom the placement of new amalgam fillings is not advised.

Should amalgam fillings be kept below a safe maximum related to body weight?

Safety thresholds apply in many areas of health care and were considered for amalgam fillings in 1995 by a report commissioned for the Canadian Government⁶. The concept is logical but the data available about rates of mercury release and about potential toxicity do not permit an amalgam filling safety threshold to be reliably identified. The Canadian Government has since said that 'current evidence does not indicate that dental amalgam is causing illness in the general population' and no safety threshold has been approved.

What do other governments and health bodies say?

No government or reputable scientific medical or dental body anywhere in the world accepts, on present published evidence, that amalgam is a hazard to general health. The World Health Organisation agrees that amalgam should continue in use. In America, the Public Health Service, the National Institute for Dental Research and the American Dental Association all support continued use. The Swedish Medical Research Council found no connection between health problems and amalgam use and the phasing out of amalgam in that country is for environmental reasons.

Should dentists explain current concerns about amalgam safety when suggesting its use?

Law and medical ethics require that patients are told enough about a proposed treatment and any associated material risks to enable them to reach an informed decision on whether to accept the treatment. The information given by the dentist will be a matter for personal judgement. In the case of a child with multiple allergies, the possibility of mercury or amalgam hypersensitivity might be raised, for example.

It is in the end for the courts to decide what is a reasonable level of information. However, based on recent legal decisions about the requirements for consent, and in the light of current scientific opinion and statements by the Department of Health, the BDA's advice to dentists is that it is not necessary to discuss the alleged links between amalgam use and health problems with generally healthy patients, except in relation to pregnancy. Until there is a reputable body of opinion which believes that there are material risks in amalgam use, consent given without discussion of side-effects can be regarded as 'informed'. Dentists should, of course, always be prepared to answer patients' questions about the safety of dental amalgam.