J Expo Sci Environ Epidemiol. 2008 May;18
Maternal amalgam dental fillings as the source of mercury exposure in developing fetus and newborn.
This study found a strong correlation between maternal and cord blood mercury (Hg) levels. The Hg levels were associated with the number of maternal mercury amalgam fillings. While the Hg levels were below the EPA reference dose of 5.8 μg/l of Hg in cord blood (a level at which the EPA considers hazardous for neurodevelopmental effects in children exposed to Hg in utero), it would be prudent for women of reproductive age to limit their exposure to mercury as from amalgam fillings.
Dental amalgam is a mercury-based filling containing approximately 50% of metallic mercury (Hg0). Human placenta does not represent a real barrier to the transport of Hg0; hence, fetal exposure occurs as a result of maternal exposure to Hg, with possible subsequent neurodevelopmental disabilities in infants.
This study represents a substudy of the international NIH-funded project “Early Childhood Development and polychlorinated biphenyls Exposure in Slovakia”. The main aim of this analysis was to assess the relationship between maternal dental amalgam fillings and exposure of the developing fetus to Hg. The study subjects were mother–child pairs (N=99). Questionnaires were administered after delivery, and chemical analyses of Hg were performed in the samples of maternal and cord blood using atomic absorption spectrometry with amalgamation technique. The median values of Hg concentrations were 0.63 μg/l (range 0.14–2.9 μg/l) and 0.80 μg/l (range 0.15–2.54 μg/l) for maternal and cord blood, respectively. None of the cord blood Hg concentrations reached the level considered to be hazardous for neurodevelopmental effects in children exposed to Hg in utero (EPA reference dose for Hg of 5.8 μg/l in cord blood).
A strong positive correlation between maternal and cord blood Hg levels was found (ρ=0.79; P<0.001). Levels of Hg in the cord blood were significantly associated with the number of maternal amalgam fillings (ρ=0.46, P<0.001) and with the number of years since the last filling (ρ=−0.37, P<0.001); these associations remained significant after adjustment for maternal age and education. Dental amalgam fillings in girls and women of reproductive age should be used with caution, to avoid increased prenatal Hg exposure.
The aim of our study was to evaluate the association between the number of maternal dental amalgam fillings and prenatal exposure to Hg, using cord blood Hg levels as the biomarker of prenatal Hg exposure.
The strongest predictor of cord blood Hg levels was the concentration of Hg in maternal blood, confirming previous reports (Vahter et al., 2000; Bjornberg, 2005). Levels of Hg were higher in cord than in maternal blood, also in accordance with other studies (Drasch et al., 1994; Bjerregaard and Hansen, 2000; Ramirez et al., 2000), indicating partial trapping of Hg in fetal tissues, once Hg was transferred from maternal side through the placenta to the fetal side.
The Hg concentrations in cord and maternal blood in our study were approximately 2–10 times lower than those observed by Bjerregaard and Hansen (2000), Vahter et al. (2000) or Walker et al. (2006). This is likely the result of differences in dietary habits and other characteristics of the populations studied; residents of Arctic Canada, Greenland and presumably Sweden too are more likely to eat seafood especially predatory fish and marine mammals. Seafood in the diet is the most important source of MeHg.
In our study, information on the speciation of Hg was not available; but in general, the diet of the Slovak population contains only low amounts of fish and seafood. In the women enrolled in this study, only 29% reported consuming fish during pregnancy and no association between fish intake in pregnancy and maternal, or cord blood Hg concentrations was found. Given that the Hg level in whole blood reflects current inorganic and organic mercury exposure (Loftenius et al., 1998) and that exposure to organic Hg from fish consumption was minimal in our population,
Hg exposure from amalgam dental fillings likely represented a major contribution to the total Hg concentration. The relatively strong association of the number of fillings and cord total Hg supports this conclusion; furthermore, the strength of the association was similar for the concentrations of Hg in maternal blood, and significant difference was observed between the median Hg levels in maternal blood samples for mothers without amalgam fillings and mothers having at least 1 amalgam filling, with no significant effect of other environmental characteristics on Hg levels in maternal or cord blood samples.
Final multivariate analyses included adjustment for maternal education and age. Although the effects of other variables such as ethnicity, parity and active/passive exposure to smoking during pregnancy on prenatal Hg exposure were examined (Figure 2), they were excluded from the final model, since controlling for maternal age and education was shown to be sufficient to control confounding, and ethnicity, was strongly correlated with both maternal education and age.
On the basis of the associations among the selected variables in the model, ethnicity, smoking and parity appear to be proxy markers for other unspecified socioeconomic factors rather than covariates with direct effect on the cord blood Hg levels, or the number of maternal amalgam dental fillings in our study. Variable ethnicity had two categories Slovak or other Eastern European (e.g. Hungarian, Czech, Polish) and Romani and it was a determinant of all variables in our model. In general, Romani population is characterized by living in poorer hygiene conditions, having lower socioeconomic status, lower level of education and a higher prevalence of smoking (Koupilova et al., 2001). In our study,
being Romani was strongly associated with having lower age and education, a smaller number of maternal amalgam fillings, lower concentrations of Hg in cord blood, and higher parity and smoking. Smoking exposure showed a negative association with both cord blood Hg levels and the number of maternal fillings in bivariate analyses; and at the same time, it was strongly associated with the level of maternal education, younger age and Romani ethnicity, suggesting the indirect effect of this variable on the predictor and outcome of interest; hence, controlling for the level of education and age helped control for smoking exposure at the same time (Figure 2). The same reasoning was used for the variable parity.
Taking into consideration the biological half-life of Hg and given that Hg exposure from dental fillings is highest around the time of placement of the filling (Pleva, 1994), the most recently inserted dental fillings are expected to have a more important impact on prenatal Hg exposure than, for example, maternal fillings placed a few years ago. The further passage of Hg would be expected to have little impact. For these reasons, instead of a simple linear term, we used an inverse exponential relationship in the assessment of the role of the time since the latest amalgam placement.
Our study had several limitations. The sample size of the study population was relatively small and only the total Hg concentrations were measured, with no speciation of Hg during the analytical process. Data on maternal dental fillings were obtained using only a questionnaire, with no objective information from a dentist; hence, no information was available about the number and area of amalgam surfaces. Information on fish consumption was also based on questionnaire. Thus, given a relatively low Hg in blood in our population and our use of self-reported information about dental amalgam fillings, the significant positive relationship between the number of maternal amalgam fillings and cord blood Hg concentrations and the significant negative association between the years since the most recent amalgam dental filling insertion and cord blood Hg concentrations are more striking.
On the other hand, vaccination rate in Slovakia represents almost 99%, so the differences in maternal and cord Hg concentrations due to different level of vaccination were negligible. Ethylmercury is an organic Hg compound that has been used in the form of thimerosal as preservative in some vaccines. While the use of mercury-containing preservatives has declined in recent years, some vaccines containing thimerosal are still available in Slovakia (e.g. Diptheria-Tetanus-Pertusis, DTP) and represent one of the possible sources of Hg exposure in the general population.
We found a relationship between maternal amalgam fillings and prenatal exposure to Hg. Further studies need to focus on the adverse health effects of Hg from dental fillings, mainly in sensitive populations for example, pregnant women and infants and to establish evidence-based guidelines for the use of dental materials during the reproductive age and pregnancy. In our cohort, children are being followed up to school age, and regular examinations include an evaluation of neurobehavioral development; hence, further assessment of the potential negative effect of prenatal Hg exposure will be possible.