Methylmercury, Amalgams, and Children's Health

Referencing: Transport of Methylmercury and Inorganic Mercury to the Fetus and Breast-Fed Infant

In their excellent article, Björnberg et al. (2005) stated that exposure to methylmercury in humans occurs primarily through fish consumption. We would like to make one observation about the sources of potential exposure to methylmercury in the general population.

We were surprised that Björnberg et al. (2005) failed to mention saliva as a plausible biologic source of methylmercury in individuals who have mercury dental fillings. Leistevuo et al. (2001) found a correlation between the total amalgam surfaces and organic mercury--presumably as methylmercury (CH₃Hg⁺)--in saliva.

Previous studies have reported that mouth air levels of elemental mercury (Hg⁰) significantly correlate with the number of occlusal surfaces (Lorscheider et al. 1995; Clarkson 2002). Hence, when mercury vapor (Hg⁰) is released from amalgams and dissolved into the saliva, it exists mainly as Hg⁰ and partly as inorganic divalent mercury (Hg²⁺).

Consistent with this background, saliva has high levels of inorganic mercury associated with the total number of amalgam surfaces, which markedly increased during mastication and bruxism. In approximately 270 individuals with amalgams, we used inductively coupled plasma-mass spectrometry to measure a wide range of possible values of total mercury in saliva. Mercury levels ranged from the limit of detection [LOD; 0.1 µg/L] to 780 µg/L in both salivary baseline flow rate in unstimulated condition and in a post-chewing-gum test (Guzzi et al. 2005).

Trace amounts of elemental and inorganic mercury from saliva are taken up by oral bacteria, which in turn release methylmercury as their by-product. Heintze et al. (1983) and Lyttle et al. (1993) reported direct evidence that organic mercury in saliva is due to the transformation of bacteria. As shown in our article (Pigatto et al. 2005), the proximate cause of mercury alkylation in oral microbial communities--which occurs in dental plaque--appears to be associated with the presence of some bacteria.

Furthermore, our ongoing investigation seems to support the work of Leistevuo et al. (2001), suggesting evidence that subjects with dental amalgams have shown higher levels of methylmercury compared with controls (Guzzi et al. 2005).

Once ingested in the gastrointestinal tract, the methylmercury in saliva is therefore nearly all absorbed (> 95%), as is methylmercury in fish. Leistevuo et al. (2001) reported that the levels of methylmercury in saliva ranged from 0 to 174 nmol/L (0-37.523-µg/L), with a mean methylmercury level estimate of 14.0 nmol/L (3.019-µg/L). (Leistevuo et al. 2001). Assuming that daily adult salivary secretion is at least 800 mL, speciation analyses indicate that exposure to methylmercury through ingestion--apparently derived from oral bacteria biomethylation of inorganic mercury--is about 2-3 µg/day (Leistevuo et al. 2001). Perhaps Björnberg et al. (2005) did not deem this exposure significant?

Considering that the relevant feature of methylmercury in humans is accumulation in both adult and fetal brain, it is quite clear that, over time, the extensive exposure to methylmercury associated with dental amalgams should be taken into account. We believe that organic mercury found in saliva may indeed represent a potential nondietary source of methylmercury.

The authors declare they have no competing financial interests.

Gianpaolo Guzzi
Italian Association for Metals and Biocompatibility Research
A.I.R.M.E.B.
Milan, Italy
E-mail: gianpaolo_guzzi@fastwebnet.it

Claudio Minoia
Laboratory of Environmental and Toxicology Testing "S. Maugeri"
IRCCS
Pavia, Italy

Paolo D. Pigatto
Department of Dermatological Sciences
University of Milan
IRCCS Maggiore Hospital Foundation Policlinico
Milan, Italy

Gianluca Severi
Cancer Epidemiology Centre
The Cancer Council Victoria
Melbourne, Australia

References

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Methylmercury, Amalgams, and Children's Health: Björnberg et al. Respond

We acknowledge the points raised by Guzzi et al. regarding our recent publication on the transport of methylmercury and inorganic mercury to the fetus and breast-fed infant (Björnberg et al. 2005). The issue is whether the methylation of inorganic mercury from dental amalgam is of sufficient size to significantly contribute to the exposure to organic mercury.

Demethylation of methylmercury into inorganic mercury is the key step in the excretion process of methylmercury. This process occurs through microbial activity within the intestine. To a limited extent demethylation may also take place in the blood (Berglund et al. 2005). The kinetics of mercury in the human body may also include methylation of inorganic mercury, but the present knowledge of this process is rather limited. Based on findings from in vitro studies (Heintze et al. 1983; Lyttle et al. 1993), Guzzi et al. postulate that organic mercury in saliva is due to bacterial transformation in the oral cavity. It is of course of toxicologic interest to further investigate the biotransformation of mercury in both directions.

Inorganic mercury has been shown to accumulate in exocrine glands, and saliva is also one excretion pathway for inorganic mercury (Joselow 1968). It should be pointed out that the saliva samples used by Leistevuo et al. (2001), to which Guzzi et al. refers, consists of paraffin stimulated whole saliva. Therefore it is not possible to ascertain to what extent the sample reflects excreted mercury from the central circulation (which could originate from both inorganic mercury and methylmercury exposure) or mercury derived directly from the fillings in the oral cavity.

In the study by Leistevuo et al. (2001), 15-18% of total mercury in saliva (5-12.5 nmol/L) was organic in a group of subjects with amalgam fillings. These subjects had, on average, 22 amalgam-filled surfaces (range, 2-51). In the non-amalgam group, the organic mercury was 2-5 nmol/L. As calculated by Guzzi et al., the subjects with amalgam would ingest about 2-3 µg/day of methylmercury derived from oral bacteria biomethylation of inorganic mercury.

Our study group of pregnant women (Björnberg et al. 2005) was exposed to low levels of both methylmercury and inorganic mercury, as reflected in the low concentrations found in blood. They consumed small amounts of fish and had few amalgam fillings, on average five amalgam-filled surfaces (range, 0-24). Therefore, the exposure to methylmercury possible originating from bacterial methylation of inorganic mercury in the oral cavity is far lower than that reported by Leistevuo et al. (2001).

It should also be pointed out that a meal of fish (200 g) containing 500 µg/kg methylmercury would result in the ingestion of 100 µg methylmercury. Also, consumption of fish with more moderate levels (50 µg/kg) would give rise to significant exposure (10 µg methylmercury).

Even though a small exposure to methylmercury may occur from bacterial methylation of inorganic mercury in the oral cavity, this exposure would be far lower than methylmercury exposure via fish consumption.

The authors declare they have no competing financial interests.

Karolin Ask Björnberg and Marie Vahter
Institute of Environmental Medicine
Karolinska Institutet
Stockholm, Sweden

Gunilla Sandborgh Englund
Institute of Odontology
Karolinska Institutet
Huddinge, Sweden
E-mail: Gunilla.Sandborgh@ki.se

References