Mercury is ubiquitous in the environment. It is in the air we breathe, the food we eat, and the water we drink.[1,2]. Dental amalgams contain about 43 to 50.5% mercury by weight before mixing , although less after condensation. There is no doubt that amalgam restorations release mercury vapor [4-6] -- the question is how much. According to the World Health Organization, the threshold at which subtle toxic effects may occur is 30 µg Hg/g creatinine . The maximum exposure limit for occupational exposure is 25 µg Hg/m3 air for long term exposure and 500 µg Hg/m3 for short-term peak exposure. The maximum recommended individual urinary mercury level is 50 µg/g creatinine .
In 1985, Vimy and Lorscheider, investigators from the University of Calgary in Canada [9,10] purported to show that patients with amalgams had an average intra-oral Hg concentration of 4.91 µg/m3 before chewing and up to 12.7 µg/m3 after chewing gum. They then determined that all these subjects received an average daily dose of 20 µg Hg from amalgam fillings, but those subjects with 12 or more occlusal amalgam surfaces received a daily Hg dose of 29 µg, many times higher than the recommended maximum daily Hg dose of some countries.
These estimations were soon shown to be erroneous. The most serious flaw in the Vimy and Lorscheider intraoral Hg vapor studies was the misuse of the Jerome Hg detector (which was designed to measure entire rooms of mercury vapor, not intraoral vapor) and subsequent misinterpretation of its data. Several investigators showed that Vimy and Lorscheider's calculations assumed that the subjects were breathing room air with equivalent concentrations of mercury as the intra-oral air, but in fact room air would normally dilute the inhaled mercury vapor from amalgam fillings. Mackert , Olsson and Bergman , and Berglund et al.  showed in separate reports that Vimy and Lorscheider grossly overestimated the mercury vapor inhalation. Mackert's recalculated average daily dose was 1.7 µg.
There were other serious flaws in the Vimy and Lorscheider studies [9,10], including their assumption that mercury vapor release was dependent on the flow rate during respiration. Berglund et al.  showed that mercury vapor release is time-dependent but not dependent on flow rate, and their recalculation was also 1.7 µg. Olsson and Bergman's recalculated daily dose was 1.3 µg .
Another mistaken assumption was that chewing food stimulates as much mercury release as chewing gum. Mercury release has been shown to increase after chewing some types of food, but it can actually decrease with other types of food. Drinking hot coffee was not shown to affect mercury release . In a detailed study of his own over a 24-hour period, Berglund estimated an average daily intraoral mercury vapor dose to be 1.7 µg . In a comprehensive review of mercury exposure from dental amalgams, Mackert and Berglund stated that brain, blood, and urinary levels of mercury from dental amalgam are consistent with the low absorbed doses of 1 to 3 µg/day shown by these studies .
Because mercury vapor release is dependent on the number and surface area of all amalgams, occlusal amalgams, and on other factors like temperature change, bruxism , toothbrushing, chewing gum, and chewing food, it can be difficult to determine an average daily mercury dose. The amount of mercury released from dental amalgam decreases as the amalgam ages .
Although mercury vapor is absorbed by the lungs, mercury from dental amalgam can also be swallowed and absorbed by the gastrointestinal tract. In a study of 21 subjects with amalgams, Halbach  estimated an average daily dose of Hg from amalgam to be 4.8 µg Hg, which the author points out is well below the acceptable daily intake of 40 µg per day for the general population [20,21]
In a study of 1127 healthy males in 1998, Kingman et al. estimated that for each ten-surface increase of amalgam fillings, there is a corresponding 1-µg/L increase of mercury concentration in urine . Berdouses et al. estimated that a single amalgam filling releases 0.03 µg/day . A patient would have to have 2,740 amalgam fillings in order to reach the threshold limit value of 82.20 mµ Hg/day considered to be dangerous for occupational exposure in the United States .
Dr. Wahl practices dentistry in Wilmington, Delaware. This article was originally published in Quintessence International 32:696-710, 2001, and is reproduced here with the kind permission permission of Quintessence Publishing Co. The author thanks Drs. J. Rodway Mackert, Ivar A. Mjör, and Fred Eichmiller for reading the manuscript and offering several helpful suggestions.