The Tübingen study has already been criticized by some German dental organizations who, among other things, consider the use of the WHO exposure limit inappropriate. Here are my own conclusions:
It is not clear from the materials how the daily ingested mercury has been calculated. However, the results agree closely with earlier studies of mercury in feces. Svare and Engqvist [1] measured a mean fecal mercury excretion of 60 micrograms.
The use of the WHO exposure limit (300 micrograms per week = 43 micrograms per day) has been criticised on the grounds that it is based partly on methyl mercury, which is more easily absorbed and more toxic than inorganic mercury compounds. This is a valid point. On the other hand, the WHO limit is not specifically intended for methyl mercury. It addresses total mercury, which is necessarily a mixture of different forms of mercury. (The WHO limit for methyl mercury is 200 micrograms per week.) "This provisional limit value was originally extrapolated from data related to methylmercury exposures from food; however the guideline for the intake of total Hg is in force." [1]
Using the WHO limit to evaluate amalgam exposure is not without precedent. In 1981, toxicologists at the Norwegian Institute of Public Health noted that mercury release from 10 copper amalgam fillings (as calculated by Ivar Mjør of the Scandinavian Institute of Dental Materials) exceeded the WHO limit by a factor of 150.
(Anyone who knows that Mjør has since been one of the leading supporters of amalgam might find this incredible. All I can say is that this was expressed in letters which were not intended for public consumption. Perhaps they were speaking more freely than usual. Copper amalgam is usually assumed to be a thing of the past, but it was in use by some dentists in Norway at least until 1994.)
Ingested mercury from amalgam is probably a mixture of amalgam particles and various forms of inorganic and elemental mercury, but the composition has not been measured directly. However, the analysis method used by the Tübingen researchers does not measure the part of the mercury that is in particle form.
Mercury can be methylated by oral and intestinal bacteria, but Skare and Enqvist's data do not indicate large-scale methylation.
The usual view is that amalgam particles are poorly absorbed, but since the size of the particles is unknown, there is no way to be sure. There are practically no data on absorption of inorganic mercury in humans. Rodent data indicate anything from a few per cent to nearly 40 per cent. There is some evidence that the absorption may vary depending on age and diet.
Again there are practically no human data, mainly since there is no occupational exposure to ingested inorganic mercury compounds. These substances are usually considered a great deal less toxic than methyl mercury and mercury vapor, but again it is uncertain by how much.
In my opinion, yes. The critics may be correct that ingested mercury is a smaller problem than inhaled mercury vapor from fillings. On the other hand, the uncertainties are many and easy to overlook. There are also indications that the mercury may have effects even without being absorbed. In animals, amalgam has been shown to promote antibiotic resistance in intestinal bacteria [2]. Although later investigations have not confirmed this, amalgam may be one the factors in antibiotic resistance. More importantly, high mercury levels have been found in the tissues of the mouth and gastrointestinal tract both in human beings and in experimental animals with amalgam fillings.
Mercury levels in gum biopsies close to amalgams seem to be among the highest ever measured in tissues from living organisms (mean 147 mcg/g) [3]. They are much higher than the concentrations measured by X-ray fluorescence in the kidneys of chloralkali workers by Borjesson et al (24 mcg/g) [4] and higher than in patients who died of Minamata disease and the livers of of cats found dead in Minamata [5]. They are approximately equal to the level achieved in the kidneys of rats exposed to 570 mcg/m3 of mercury vapor (156.7 mcg/g) [6]. Other tissues in the same rats were much lower; none of them were above approximately 30 mcg/g.
Clearly, no estimate of whole-body exposure can be used to assess the effects of such astronomical local concentrations. Ignoring everything but mercury vapor exposure from amalgam, as is commonly done, is not really justified.
Also, the amount of absorbed mercury vapor alone is as high as or higher than limits determined in recent risk assessments.