Breastfeeding and Mercury Exposure

By Kelly Bonyata, IBCLC

Since mercury is all around us in the environment, we all have small amounts of mercury in our bodies. There are different forms of mercury, and each form is absorbed into the body differently:

• Elemental/metallic mercury (like you find in old thermometers) is very poorly absorbed orally (0.01%) but well absorbed via inhalation (>80%).
• Inorganic mercury compounds are poorly absorbed orally (7-15%). Inorganic mercury compounds are found in mercury disk batteries and are used as antibacterials (Silvadene cream, for example).
• Organic mercury (methylmercury fungicides, phenyl mercury, ethylmercury) is well absorbed orally (90%). One of the organic mercury compounds, methylmercury, is the type we worry about when it comes to eating mercury contaminated fish and seafood. Methylmercury tends to bioaccumulate — build up in the body over time — in humans and animals. Thimerosal (a preservative) contains approximately 49% ethylmercury, which is also an organic mercury. According to the World Health Organization, “ethyl mercury is actively excreted via the gut unlike methyl mercury that accumulates in the body.”

As with other environmental contaminants, our exposure to mercury begins before birth. Most of a baby’s mercury exposure from the mother occurs during pregnancy, via the placenta.

Mercury in human milk

Although mercury does pass into breastmilk, the amount of mercury in breastmilk is not expected to be a problem under normal circumstances. Per Lawrence & Lawrence, in Breastfeeding: A Guide for the Medical Profession (2016, p. 399), “Acute exposures to methylmercury from industrial or environmental sources should be evaluated on a case-by-case basis, although it appears breastfeeding is safe.” A 2004 review on mercury and lead during breastfeeding (Dorea 2004) noted that “cows’ milk-based formulas pose a greater risk of infant exposure to neurotoxic substances.”

The mother’s diet appears to be, by far, the primary source of mercury in breastmilk — studies have shown higher levels of mercury in the breastmilk of mothers who eat large amounts of fish. Mercury levels in breastmilk are about one-third the level of mercury in the mother’s blood. The amount of mercury that reaches the milk depends upon the form of the mercury: inorganic mercury compounds enter the milk easily but are poorly absorbed by baby; organic mercury compounds do not enter the milk easily but the small amounts that do get through are easily absorbed by baby. Studies have shown that mercury levels in breastfed babies are highest right after birth and decline significantly by 2-3 months of age (Sakamoto 2002, Sandborgh-Englund 2001, Drexler 1998).

Amalgam dental fillings

A German study (Drexler 1998) found that during the first week after birth, the amount of mercury in the study mothers’ milk was associated with the number of amalgam (“silver”) dental fillings; at two months, the amount of mercury in breastmilk was much lower and associated with the mothers’ fish consumption (rather than the number of amalgam fillings). These authors concluded that “the additional exposure to mercury of breast-fed babies from maternal amalgam fillings is of minor importance compared to maternal fish consumption.” Some studies have found similar correlations between amalgam fillings and mercury levels (Drasch 1998, Oskarsson 1996), but another German study (Klemann 1990) found no correlation between the total surface area of the mother’s amalgam dental fillings and mercury in amniotic fluid, maternal blood, newborn blood or breastmilk, and concluded that “maternal amalgam fillings are of no importance for the mercury load of the fetus and the neonate.”

Hale (Medications and Mothers’ Milk, 2017, p. 611) recommends that “the replacement of amalgam fillings while pregnant or breastfeeding could potentially increase the transfer of mercury to the infant.” If replacement of an amalgam filling is necessary, Hale recommends that the dentist take routine precautions when removing the old amalgam: “use copious amounts of cold water irrigation to minimize heat, use a rubber dam to isolate her mouth from the particles, and use an alternate source of air (oxygen) to minimize mercury vapor inhalation.”

Development of children exposed to mercury via breastmilk

In the two long-term studies of children exposed to methylmercury via breastmilk, no adverse effects were documented; in fact, the breastfed children scored better on developmental tests. A study in the Seychelles islands (Grandjean 1995) found that babies who were exposed to methylmercury via their mothers’ milk had higher developmental scores than formula fed babies, suggesting that “if methylmercury exposure from human milk had any adverse effect on milestone development in these infants, the effect was compensated for or overruled by advantages associated with nursing.” Another study in the Faroe Islands (Jensen 2005) found “marginally better neuropsychological performance” in 7 year olds who had been exposed to methylmercury via breastmilk.

How do I minimize my child’s exposure to mercury?

Since mercury levels in babies begin to accumulate before birth, the US Food & Drug Administration advises pregnant women to avoid eating several types of fish that tend to have higher levels of mercury. Per the FDA, “While it is true that the primary danger from methylmercury in fish is to the developing nervous system of the unborn child, it is prudent for nursing mothers and young children not to eat these fish as well.” See Should I be avoiding certain kinds of fish? for more information.

 

Additional Information

Mercury Guide from the Natural Resources Defence Council

References (most recent listed first)

Hale TW, Rowe HE. Medications and Mothers Milk. 17th Edition. New York, NY: Springer Publishing; 2017, p. 611-613.

Lawrence R, Lawrence R. Breastfeeding: A Guide for the Medical Profession, 8th ed. Philadelphia, Pennsylvania: Mosby, 2016, p. 399.

WHO Global Advisory Committee on Vaccine Safety Statement on thiomersal. July 2006.

Jensen TK, Grandjean P, Jorgensen EB, White RF, Debes F, Weihe P. Effects of breast feeding on neuropsychological development in a community with methylmercury exposure from seafood. J Expo Anal Environ Epidemiol. 2005 Jan 26.

Dorea JG. Mercury and lead during breast-feeding. Br J Nutr. 2004 Jul;92(1):21-40.

Sakamoto M, Kubota M, Matsumoto S, Nakano A, Akagi H. Declining risk of methylmercury exposure to infants during lactation. Environ Res. 2002 Nov;90(3):185-9.

Gundacker C, Pietschnig B, Wittmann KJ, Lischka A, Salzer H, Hohenauer L, Schuster E. Lead and mercury in breast milk. Pediatrics. 2002 Nov;110(5):873-8.

Sandborgh-Englund G, Ask K, Belfrage E, Ekstrand J. Mercury exposure in utero and during infancy. J Toxicol Environ Health A. 2001 Jul 6;63(5):317-20.

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Drexler H, Schaller KH. The mercury concentration in breast milk resulting from amalgam fillings and dietary habits. Environ Res. 1998 May;77(2):124-9.

Drasch G, Aigner S, Roider G, Staiger F, Lipowsky G. Mercury in human colostrum and early breast milk. Its dependence on dental amalgam and other factors. J Trace Elem Med Biol. 1998 Mar;12(1):23-7.

Berglund A, Molin M. Mercury levels in plasma and urine after removal of all amalgam restorations: the effect of using rubber dams. Dent Mater. 1997 Sep;13(5):297-304.

Yang J, Jiang Z, Wang Y, Qureshi IA, Wu XD. Maternal-fetal transfer of metallic mercury via the placenta and milk. Ann Clin Lab Sci. 1997 Mar-Apr;27(2):135-41.

Oskarsson A, Schultz A, Skerfving S, Hallen IP, Ohlin B, Lagerkvist BJ. Total and inorganic mercury in breast milk in relation to fish consumption and amalgam in lactating women. Arch Environ Health. 1996 May-Jun;51(3):234-41.

Marsh DO, Clarkson TW, Myers GJ, Davidson PW, Cox C, Cernichiari E, Tanner MA, Lednar W, Shamlaye C, Choisy O, et al. The Seychelles study of fetal methylmercury exposure and child development: introduction. Neurotoxicology. 1995 Winter;16(4):583-96.

Grandjean P, Weihe P, White RF. Milestone development in infants exposed to methylmercury from human milk. Neurotoxicology. 1995 Spring;16(1):27-33.

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Klemann D, Weinhold J, Strubelt O, Pentz R, Jungblut JR, Klink F. Effects of amalgam fillings on the mercury concentrations in amniotic fluid and breast milk. Dtsch Zahnarztl Z. 1990 Mar;45(3):142-5.

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Marsh DO, Clarkson TW, Cox C, Myers GJ, Amin-Zaki L, Al-Tikriti S. Fetal methylmercury poisoning. Relationship between concentration in single strands of maternal hair and child effects. Arch Neurol. 1987 Oct;44(10):1017-22.

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