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Even one amalgam dental filling exposes pregnant women to potentially harmful mercury vapor levels compared with women with no fillings — and multiple fillings increase exposures and risks proportionally, according to a paper published Feb. 5 in Human & Experimental Toxicology.
All pregnant women with one or more fillings exceed at least one published “safe” exposure limit for mercury vapor.
Dental amalgam fillings are made from a mixture of mercury and other metals. Mercury — the silvery substance inside old-fashioned mercury thermometers — is the only metal that is liquid at room temperature.
Like all other liquids, mercury evaporates, turning from liquid to gas. Inhaling mercury vapor is a significant source of mercury exposure.
Other sources include fish and seafood, industrial processes and products, including mining, and mercury-containing products like thermometers.
Vaccines are another source of mercury exposure. Many seasonal influenza vaccines contain thimerosal, an organic mercury preservative that negatively affects several organs, particularly the nervous system.
Thimerosal was an ingredient in many childhood vaccines but was removed from those products in 2001. However, it is still present in flu vaccines administered to children and pregnant women.
The more fillings, the higher the exposure
Lead researcher Dr. Mark Geier, an advocate for mercury-free pharmaceuticals, tapped into data on 1,665,890 pregnant women from the 2015-2020 National Health and Nutrition Examination Survey (NHANES).
Of those, 606,840 had at least one dental amalgam filling and 1,059,050 did not.
Pregnancy was determined during the NHANES intake exam through a rapid pregnancy test and a confirmatory clinical-grade blood test for human chorionic gonadotropin, a hormone released during early pregnancy. The number of amalgam dental fillings was noted through a standard dental examination.
By measuring mercury levels in the study subjects’ urine and their daily urine output, the authors calculated each subject’s daily mercury exposure from inhaling mercury vapor released from their fillings.
They expressed these values as micrograms of mercury per kilogram of body weight per day (µg of Hg/kg/day), which is how regulators and agencies express toxic metal exposures.
Geier then compared calculated daily mercury exposures to “safe” exposure limits from four government agencies and one other study (Richardson et al.). Those results are summarized in Table 5 from the study.
Table 5 lists exposure limits by increasing stringency, with the highest U.S. Environmental Protection Agency (EPA) exposure limit at the top and California’s standard, which is almost tenfold lower, at the bottom.
Boosting already-suspect mercury levels
Table 2 in the study shows that even women without fillings experience mercury exposure, which must arise from sources other than dental fillings. Mercury levels for women with and without fillings were 0.99 µg and 0.41 µg of Hg/kg/day, respectively.
Table 2’s second key takeaway is that exposure through fillings, when combined with exposures from non-amalgam sources, “promotes” many women with low-level non-dental mercury exposure into high-exposure categories.
For example, column 2 in Table 5 indicates that 28% of all pregnant women exceed daily mercury exposure levels, according to the least-stringent EPA standard, while 36% exceed California’s strict standard.
But for women with fillings 77% — more than double — exceed the EPA standard and 100% (nearly 3 times) fall short of California regulations.
California’s standards are so strict that, according to Geier, pregnant women with just one filling “would have to weigh more than 100 Kg [220 lbs] to not receive a Hg vapor dose from their amalgam not in excess of the most restrictive California EPA Hg vapor safety limit.”
Geier also reported that the more fillings a pregnant woman had, the higher the mercury levels in her urine. To quantify this value Geier calculated a “β-coefficient” which relates the number of fillings to excreted mercury levels.
Among pregnant women, this relationship was highly statistically significant and likely clinically significant as well. Interestingly, Geier wrote, that the β-coefficient or sensitivity to the number of fillings was double what he had found in a previous study on mercury exposure in adults.
“This difference may indicate that pregnancy status has an important physiological impact on increasing exposure/distribution of Hg vapor from amalgams. More American pregnant women exceeded the US EPA Hg vapor safety limit (30% vs 10%) and the US ATSDR Hg vapor safety limit (32% vs 21%) in this study as compared to American adults in our previous study.”
Unsafe at any exposure level?
Interest in prenatal mercury exposure is a hot-button issue, with more than 200 papers on the subject between 2019 and the present. Some studies show adverse effects but only at high exposure levels, while others show modest or no effect at typical exposure levels.
Assessing the effects of mercury exposure during pregnancy comes down to whether maternal exposures transfer to the fetus and what levels of fetal exposure are safe.
The first issue is not in dispute, as mercury crosses both the placental and blood-brain barrier, which is one of the body’s critical lines of defense. So it’s known that maternal exposures will involve fetuses.
Also well established are the harms of high exposures, particularly of inhaled mercury, which include damage to the lungs. Because mercury vapor easily passes from the lungs to the bloodstream, exposure eventually harms the nervous system, kidneys, liver and immune system.
The issue comes down to whether babies can tolerate agency-recommended exposure levels and whether safe fetal exposure levels exist at all.
This question is more difficult to answer than one might think, as many exposure studies report no apparent harm from low-level exposures, even during pregnancy.
For example, one study found that total maternal exposure during gestation did not affect a child’s performance in math and science. Another found the “association between mercury and neurodevelopment was weak.” And another study “did not identify strong evidence that mercury exposure leads to impaired prenatal growth.”
However, other studies reported harms. One found a negative association between exposure and growth — the more mercury, the less growth. Another showed harms to both male and female reproductive function.
And another noted that for prenatal exposures “while the mother’s symptoms usually improved, the damage to the fetal nervous system appears to be permanent.”
The simplest answer is that mercury is highly toxic and “there is no known safe level of exposure. Ideally, neither children nor adults should have any mercury in their bodies because it provides no physiological benefit.”
A more nuanced view is that the effects of mercury exposure are linear for a wide range of harms, with higher exposure always leading to greater harm — the corollary being that lower exposures may lead to less severe injury but lead to injury nevertheless.
As one study author put it: “The available literature indicates a linear relationship with mercury levels and IQ deficit, and therefore a safe limit of mercury cannot be calculated.”
