Mercury Use in Dentistry

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 silvery liquid, extracted from cinnabar ore, was volatile and would quickly disappear as vapor 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 was eventually introduced into North America in the 1830's. (reference)

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. Other dentists were concerned about mercurial poisoning, because it was already widely recognized that Hg exposure resulted in many overt side effects, including dementia and loss of motor coordination. By 1845, as a reflection of these concerns, the American Society of Dental Surgeons and several affiliated regional dental societies adopted a resolution that its members sign a pledge not to use amalgam.

Consequently, during the next decade some members of the society were suspended for the malpractice of using amalgam. But the advocates of amalgam eventually prevailed and membership in the American Society of Dental Surgeons declined, forcing it to disband in 1856. In its place arose the American Dental Association, founded in 1859, based on the advocacy of amalgam as a safe and desirable tooth filling material. Shortly thereafter, tin was added to the Hg/silver paste to counteract the expansion properties of the previous amalgam formula.

There were compelling economic reasons for promoting dental amalgam as a replacement for the other common filling materials of the day such as cement, lead, gold, and tinfoil. Amalgam's introduction meant that dental care would now be within the financial means of a much wider sector of the population, and because amalgam was simple and easy to use, dentists could readily be trained to treat the anticipated large number of new patients. By 1895, the dental amalgam mixture of metals had been modified further to control for expansion and contraction, and the basic formula has remained essentially unchanged since then.

Scientific concerns about amalgam safety initially surfaced in Germany during the 1920's, but eventually subsided without a clear resolution. At the present time, based on 1992 dental manufacturer specifications, amalgam (at mixing) typically contains approximately 50% metallic Hg, 35% silver, 9% tin, 6% copper, and a trace of zinc. Estimates of annual Hg usage by U.S. dentists range from approximately 100,000 kg in the 1970's to 70,000 kg today. Hg fillings continue to remain the material preferred by 92% of U.S. dentists for restoring posterior teeth. More than 100 million Hg fillings are placed each year in the U.S. Presently, organized dentistry has countered the controversy surrounding the use of Hg fillings by claiming that Hg reacts with the other amalgam metals to form a "biologically inactive substance" and by observing that dentists have not reported any adverse side effects in patients. Long-term use and popularity also continue to be offered as evidence of amalgam safety.

Potential toxicity from exposure to mercury vapor (Hg) from dental amalgam fillings is the subject of current public health debate in many countries. We evaluated potential central nervous system (CNS) toxicity associated with handling Hg-containing amalgam materials among dental personnel with very low levels of Hg exposure (i.e., urinary Hg < 4 ug/l), applying a neurobehavioral test battery to evaluate CNS functions in relation to both recent exposure and Hg body burden.

New distinctions between subtle preclinical effects on symptoms, mood, motor function, and cognition were found associated with Hg body burden as compared with those associated with recent exposure. The pattern of results, comparable to findings previously reported among subjects with urinary Hg > 50 ug/l, presents convincing new evidence of adverse behavioral effects associated with low Hg exposures within the range of that received by the general population.

Presently, dental amalgam accounts for 10% of Hg used in the USA and one third of Hg used in Sweden. Multinational political actions, similar to those against the use of Hg in batteries, have not been directed towards the use of amalgam for dental fillings, despite economically viable alternatives and large and well-documented emissions to both air and water. Mercury emissions to the air from cremation are estimated to 0.28 t yr-C in Sweden,75 or 0.03 g per capita per year from a population of 8.5 million with 40-100 t of Hg in dental fillings and a cremation rate around 65%. Mercury losses to waters from amalgam fillings via human feces amount to about one third of that to the air, while losses to waters from dental practices are not quantified in Sweden but could be larger and are of concern for sewage treatment plants. In Denmark with 5.4 million inhabitants, Hg losses to water from dental practices were estimated to between 83 and 120 kg per year in 1992/93. Losses of Hg from Danish dentistry is expected to continue, because dental amalgam is until further notice excluded from the general ban on Hg use from 1994. (reference)

An estimated 20% of Hg consumed as dental amalgam is lost within a 10-year period. This indicates that of 108 t of Hg consumed for dental applications in Sweden during the 1970's, about 20 t could have polluted the environment until the end of the 1980's. Dental use of Hg has been significant also in Japan and most European countries. Part of the demand in the EU has been supplied by Swedish companies, exporting dental amalgam when the demand in Sweden decreased during the latest decade. Dental amalgam is classified as a medical product according to EU legislation and consequently exempted from the Swedish export ban of Hg, compounds, mixtures, and products containing Hg. Swedish efforts to reduce the use of dental amalgam beyond voluntary agreements by legislation have been hindered by economic interests profiting on the continued use of amalgam. Considering that some persons evidently suffer from dental amalgam and that researchers have been able to show effects of inorganic Hg at lower concentrations than before, it seems appropriate that the amalgam producers set aside funds for treatment and convalescence of patients affected by amalgam, instead of leaving this costs to the public.