Methylmercury, amalgams, and children's health.

In their excellent article, Bjornberg 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 Bjomberg 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--presnmably as methylmercury (C[H.sub.3][Hg.sup.+])--in saliva.

Previous studies have reported that mouth air levels of elemental mercury ([Hg.sup.0]) significantly correlate with the number of occlusal surfaces (Lorscheider et al. 1995; Clarkson 2002). Hence, when mercury vapor ([Hg.sup.0]) is released from amalgams and dissolved into the saliva, it exists mainly as [Hg.sup.0] and partly as inorganic divalent mercury ([Hg.sup.2+]).

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 [micro]g/L] to 780 [micro]g/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 methyimercury in fish. Leistevuo et al. (2001) reported that the levels of methylmercury in saliva ranged from 0 to 174 nmol/L (0-37.523-[micro]g/L), with a mean methylmercury level estimate of 14.0 nmol/L (3.019-[micro]g/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 [micro]g/day (Leistevuo et al. 2001). Perhaps Bjomberg 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.

REFERENCES

Bjornberg KA, Vahter M, Berglund B, Niklasson B, Blennow M, Sandborgh-Englund G. 2005. Transport of methylmercury and inorganic mercury to the fetus and breast-fed infant. Environ Health Perspect 113:1381-1385.

Clarkson TW. 2002. The three modern faces of mercury. Environ Health Perspect 110(suppl 1):11-23.

Guzzi G. Minoia C, Pigatto PD, Lucchiari S, Severi G. 2005. Mercury and dental patients: toxicology, immunology and genetic connection [Abstract]. Toxicol Lett 158S:S239.

Heintze U, Edwardsson S, Derand T, Birkhed D. 1983. Methylation of mercury from dental amalgam and mercuric chloride by oral streptococci in vitro. Scand J Dent Res 91:150-152.

Leistevuo J, Leistevuo T, Helenius H, Pyy L, Osterblad M, Huovinen P, et al. 2001. Dental amalgam fillings and the amount of organic mercury in human saliva. Caries Res 35:163-166.

Lorscheider LF, Vimy MJ, Summers AO. 1995. Mercury exposure from "silver" tooth fillings: emerging evidence questions a traditional dental paradigm. FASEB J 9:504-508.

Lyttle HA, Bowden GH. 1993. The level of mercury in human dental plaque and interaction in vitro between biofilm of Streptococcus mutans and dental amalgam. J Dental Res 72:1320-1324.

Pigatto PD, Guzzi G, Severi G. 2005. Oral lichen planus: mercury and its kin. Arch Dermatol 141:1472-1473.

Gianpaolo Guzzi

Italian Association for Metals and

Biocompatibility Research

A.I.R.M.E.B.

Milan, Italy

E-mail: [e-mail]

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

The authors declare they have no competing financial interests.