Science in the News
New analysis of mercury and dental surface restorations in the U.S. population
September 30, 2016
People may be exposed to mercury through a variety of sources, one of which is dental amalgam.1
Perhaps in part because the FDA has concluded that there is insufficient evidence to support an association between exposure to mercury from dental amalgams and adverse health effects2
, interest remains in examining the relationship between amalgam use, mercury exposure levels, and health outcomes. A recent study by Yin, et al.3
explored whether there was an association between the number of dental surface restorations and the levels of total blood mercury (THg), inorganic mercury (IHg) or methyl mercury (MeHG). The American Dental Association issued a press statement on the study
and the ADA’s position on dental amalgam remains unchanged.
The study's authors explored whether there was an association between the number of dental surface restorations and levels of total blood mercury (THg), inorganic mercury (IHg) or methyl mercury (MeHG). They separately analyzed two data sets from the National Health and Nutrition Examination Survey (NHANES), one collected in 2003-2004 and the other in 2011-2012. NHANES is designed to be nationally representative and combines interviews and physical exams to assess the health and nutrition status of adults and children in the US. Although this study finds statistically significant difference in circulating levels of mercury, all the levels observed were within the lower 95% confidence limit set forth as safe by the National Research Council of the National Academy of Sciences.4
For the purpose of this analysis, NHANES participants from 2003-4 and 2011-12 were divided into three groups based on dental surface restorations (DSR): those with no DSRs, those with between 1-8 DSRs, and those with 9 or more DSRs. These DSRs included both amalgam and composite restorations, with no distinction between the two materials. Teeth with 5 surfaces restored were not included, as these were said to involve a fabricated dental crown.
The number of DSRs rose with age. In the group with no DSRs, 61.2% were 21 or younger, whereas in the group with >8 DSRs, 52.7% were 46 or older. In addition, total blood mercury tends to increase with age.4
To account for this, NHANES participants were further subdivided into 4 age groups: 3-12, 13-21, 22-65, and > 66. For each age group, a larger proportion of subjects had DSRs in 2011-2012 than those in 2003-2004, for an overall approximate 10% increase in DSRs.
Two-way analysis of variance was used to examine the impact of DSRs on blood mercury levels with the covariates of age, gender, race/ethnicity, education level, tobacco, and diet. They found significant effects of age and DSRs on increased blood total mercury in the data sets from 2003-4 and 2011-12; but only DSR, not age, was significantly associated with inorganic mercury. The number of DSRs and blood total mercury and methyl mercury had a nonmonotonic association, in which mercury increased with increasing age until between 45-60, at which point it plateaued and then decreased with increasing age. Non-Hispanic Asians had the highest concentrations of THg, MeHg, and IHg of all racial or ethnic groups, regardless of number of dental surface restorations, while Mexican Americans had the lowest (all p-values <0.005). Those with higher levels of educational attainment had higher mean THg, MeHg, and IHg than those with less education (all p-values <0.001 except for IHg in the 2003-4 dataset). The impact of gender on the relationship between DSRs and blood mercury concentrations differed between the 2003-4 and 2011-12 datasets. Smoking was not a significant factor in most models, except for THg in the 2011-12 dataset. Those who ate fish or shellfish in 30 days before examination had significantly higher mean THg and MeHg (but not IHg) than those who did not.
Multivariate generalized linear models were constructed to examine the relationships between DSRs and mercury while controlling for these covariates. With each additional dental surface restoration, there was a significant increase in total mercury, inorganic mercury, or methyl mercury (all p-values ≤ 0.01).
Individuals with 9 or more dental surface restorations had greater circulating levels of total, inorganic and methyl mercury levels than those with no restorations or those with between 1 and 8 restorations. The results also pointed to the impact of fish consumption in blood total and methyl mercury levels. Unexpectedly, when those with no dental restorations were compared between 2003-4 and 2011-12, over time the blood total mercury mean significantly decreased in those under age 12 and significantly increased for those between the ages of 20-45, indicating that non-dental sources of exposure to mercury in the 20-45 age group may have changed over time. While finding that dental restorations are not the only contributor to circulating levels of mercury, these data suggest that the presence of 9 or more dental surface restorations may contribute to circulating levels of mercury. However, these average blood mercury levels are below the safety threshold established by the World Health Organization and the Environmental Protection Agency.3
This study did not examine any health effects associated with higher mercury levels or DSRs. A limitation of the study is that the data sets used did not include categorization of restoration materials used. Although excluding teeth with more than 5 restored surfaces because these were likely a fabricated crown3
, being able to distinguish between amalgam and composite restorations would have been useful. However, a strength of this study is that by using NHANES data, the results should be reflective of the U.S. population’s blood mercury levels.
Prepared by: Center for Scientific Information, ADA Science Institute
- Centers for Disease Control and Prevention. Mercury Fact Sheet. 2009. Accessed September 29, 2016.
- FDA Consumer Health Information A Guide to Drug Safety Terms at FDA. U.S. Food and Drug Administration. November 2012. Accessed August 3, 2016.
- Yin L, Yu K, Lin S, Song X, Yu X. Associations of blood mercury, inorganic mercury, methyl mercury and bisphenol A with dental surface restorations in the U.S. population, NHANES 2003-2004 and 2010-2012. Ecotoxicol Environ Saf 2016;134P1:213-25.
- National Research Council. Toxicological effects of methylmercury. Washington, DC: National Academies Press; 2000.