Amalgam Debate: Distorted Science

Amalgam Debate: Distorted Science

Excerpt from: The US Dental Amalgam Debate, 2010 Meeting of the FDA Dental Products Panel
By Robert Cartland
©2011 Robert F. Cartland

Mercury is among the most studied chemical substances. Unfortunately, the passion fueling the amalgam debate has led members of both the IAOMT and the ADA to make statements with more rhetorical than scientific value. Two commonly touted statements, one from each side of the debate, will be considered to illustrate the problem. Both statements were made at the December 2010 FDA Dental Products Panel meeting held to consider current FDA policy regarding the safety of dental amalgam.

During the public presentation at the December 2010 FDA meeting John Kall, reading a statement by IAOMT President Matthew Young, repeated an often made claim by those opposing dental amalgam that mercury “is the most toxic nonradioactive element on earth.”¹

Some opponents of dental amalgam unintentionally substitute the word “substance” for the word “element” making the statement unequivocally false. There are many nonradioactive substances that are much more toxic than mercury. As is, the statement exaggerates the toxicity of mercury by only comparing elemental mercury with other nonradioactive elements.

Elements are the fundamental building blocks of all chemicals and are conveniently classified on the periodic table.  As of 2011, there are 118 chemical elements but only 94 are found naturally on Earth. The others have been produced in particle accelerators. Only 80 of the 94 naturally occurring elements are nonradioactive (bismuth, with a half-life longer than the age of the universe is currently considered radioactive). Describing mercury as the most toxic nonradioactive element limits the comparison to only 79 other elements and limits the type of mercury considered to its elemental form.

How does mercury compare with the other 79 nonradioactive elements?  Mercury is the only metal element that is liquid at room temperature and standard atmospheric pressure. Elemental mercury has low toxicity when ingested as less than 0.001% enters the body of a healthy person through the stomach or intestines.² Ingestion of half a teaspoon (about 204 grams) of liquid mercury with little toxic effect has been reported.² Arsenic is more toxic than mercury when ingested and iodine, an element essential to life, can be lethal if 2 g are ingested.³ Sodium is not considered toxic, but elemental sodium metal, because of its violent reactivity with water, will explode or burst into flames if ingested.  The IAOMT is not considering ingested mercury when claiming it to be the most toxic nonradioactive element.

The primary route of toxic exposure for elemental mercury is through inhalation. About 80% of inhaled mercury vapor is absorbed by the lungs and enters the bloodstream.²  Occupational Safety and Health Administration (OSHA) sets the permissible exposure limit (PEL) for mercury vapor at 0.1 mg/m³ (milligrams per cubic meter).⁴ The PEL is the maximum level of exposure permitted in occupational settings. While regulatory agencies periodically adjust PELs, they do provide some method of comparing relative toxicities; more toxic substances should have lower PELs. The PEL for mercury, however, while lower than arsenic or cadmium is higher than lead or beryllium.⁴ Mercury has a lower PEL than chlorine gas but both can be extremely harmful in similar concentrations (40 mg/m³).^4,5 Chlorine is a gas under normal conditions. It becomes lethal at sufficient concentrations and was used in chemical warfare during World War I.⁵ Because of mercury’s low vapor pressure (low tendency to go from liquid to a gas), typical room ventilation is sufficient to prevent conditions in which mercury vapor is lethal. The unique properties of each chemical element complicate comparisons of toxicity. Any method of comparison, including PEL levels, is problematic. The claim that mercury is the most toxic nonradioactive element requires significant qualification regarding how the comparison is made and has strong potential to be misleading.

Some proponents of dental amalgam are also guilty of distorting the science.  Historically, it was argued that the mercury in dental amalgam is bound in such a way as to render it safe. An often made analogy between dental amalgam and table salt was repeated during the public presentations by Dr. Dennis Charlton, president-elect of the Pennsylvania Dental Association (a constituency of the ADA).

Elements are the fundamental building blocks of chemistry and can be combined chemically to form an essentially unlimited number of molecules. Compounds are molecules formed from at least two different elements. Elements and compounds can be combined, non-chemically, to form an even larger number of mixtures.⁶ Mercury can exist in ionized (charged) or neutral (metallic) states.  Mercury can also form inorganic compounds, including mercury salts and sulfides, and organic compounds, including methylmercury, found in some seafood, and ethylmercury, used as a germicide. Unlike elemental mercury, these forms of mercury are toxic when ingested; 95% of methylmercury is absorbed when ingested. Dental amalgam is a complex mixture of elemental mercury and mercury compounds as well as other metals and metal compounds. The elemental mercury vapor that escapes the amalgam is the primary concern.

Sodium chloride illustrates how some toxic elements can be combined to create compounds that are less toxic. However, combining elements into compounds does not always render them safe. Compounds, including compounds of mercury, can be far more toxic than elements. For example, a few drops of dimethylmercury, Hg(CH₃)₂, penetrated a protective glove contacting the skin and ending the life of Karen Wetterhahn, Professor of Chemistry at Dartmouth College. Chemical descriptions of amalgam and table salt are also quite different. Sodium chloride is best compared with other compounds like mercury salts (mercury salts, however, are toxic). Amalgam is more like salt water, a mixture. Solutions, suspensions, colloids and alloys are types of mixtures. Compounds tend to be more tightly bound than mixtures and usually maintain stoichiometry; the relative proportion of each element is fixed. Table salt is always 50% sodium and 50% chlorine and forms an ionic solid, a crystal of positively charged sodium ions and negatively charged chlorine ions.  Any amount of salt may be mixed with water until the solution saturates. Similarly, you can have varying amounts of mercury in amalgam.

Amalgam is typically defined as being an alloy containing mercury and other metals. Alloys are mixtures of elements and compounds, typically metals, forming a metallic matrix. Chemical reactions occur when amalgam is mixed (a process called trituration) creating chemical bonds between the mercury and other metals called intermetallic compounds. Dental amalgam consists of regions containing different intermetallic compounds called phases; the mercury in dental amalgam is not necessarily completely bound within the matrix. Because of the inability of the various phases to mix, dental amalgam is sometimes classified as a solid emulsion (a mixture of olive oil and vinegar is an emulsion). Alloys, like bronze and steel, have physical, mechanical and corrosion properties that are tailored by composition and fabrication processes. Similarly, the properties and relative stability of dental amalgam, the amount of mercury released and the release mechanisms, as vapor, particulate or through corrosion, can depend on composition and fabrication techniques. One cannot draw a conclusion regarding the toxicity or stability of a substance based on the existence of chemical bonds or by analogy with another substance.

Both sides of the amalgam debate have a responsibility to the public to carefully consider the scientific merit of their claims and arguments as well as the potential for a statement to be misleading. Economics, litigation and emotion can distort science.

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1.     December 14, 2010: Meeting Transcript, 2010 Meeting Materials of the Dental Products Panel, FDA Generated, Gaithersburg, MD, December 14-15, 2010. On-line: www.fda.gov/AdvisoryCommittees/CommitteesMeetingMaterials/MedicalDevices/MedicalDevicesAdvisoryCommittee/DentalProductsPanel/ucm235085.htm

2.     Agency for Toxic Substances and Disease Registry (ATSDR) and Research Triangle Institute, Toxicological profile for mercury, U.S. Dept. of Health and Human Services, Public Health Service, Atlanta, Georgia, 1999.

3.     Agency for Toxic Substances and Disease Registry (ATSDR) and Research Triangle Institute, Toxicological profile for iodine, U.S. Dept. of Health and Human Services, Public Health Service, Atlanta, Georgia, 2004.

4.     PELs determined by OSHA are available on-line: www.osha.gov

5.     Agency for Toxic Substances and Disease Registry (ATSDR) and Research Triangle Institute, Toxicological profile for Chlorine, U.S. Dept. of Health and Human Services, Public Health Service, Atlanta, Georgia, 2010.

6.     Pauling, L, General Chemistry, 3 edition, Dover Publications; (April 1, 1988).

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