Healthy Milk, Healthy Baby
Chemical Pollution and Mother's Milk
Chemicals: Lead, Mercury, Cadmium and Other Metals
BACK TO CHEMICAL OVERVIEW
Metals play an important role in human biology, and trace amounts of some metals -- manganese, for example -- are essential to life. At higher concentrations, however, these same metals are toxic. In addition, some metals -- lead, for example -- do not occur naturally in the body, and their presence, usually as a result of occupational or pollution-related exposure, is detrimental to health. A number of potentially toxic metals have been reported in breast milk, including lead, mercury and cadmium. Unlike the persistent organic pollutants (POPs), metals do not accumulate in fat, and so do not usually achieve higher concentrations in breast milk than in blood. As a result, infants are likely to be exposed to higher levels before birth than during breastfeeding. Nonetheless, learning about metals in breast milk is important for two reasons: first, as a pathway of exposure, and second, as an indicator of likely prenatal exposures.
Many metals that have been reported in breast milk also contaminate drinking water and so can occur in infant formula at levels even higher than in breast milk. For example, an infant's exposure to cadmium from soy infant formula is about 20 times higher than the levels generally found in breast milk. Estimated cadmium intake from powdered formula has been estimated at six times higher than average levels in breast milk.1
Health Effects of Metals
Lead has often been called the leading environmental health threat to children. It is toxic to the developing brain, and at high levels results in numerous poisoning symptoms. In addition, at the low doses common today in many countries, lead has subtle effects on neurological functions, including learning, memory and attention span. Because the infant brain is developing rapidly both before birth and for several years after birth, lead exposures during this critical period are particularly detrimental to the future intellectual potential of children.
Mercury occurs in a number of forms in the environment. The most hazardous for children is methyl mercury, although inorganic mercury is also a potential concern. Methyl mercury, like lead, is toxic to the developing brain. While high-dose exposures can result in a cerebral palsy-like syndrome, low dose exposures may cause subtle deficits in learning and memory. Other metals, such as cadmium, arsenic and manganese, have not been as thoroughly studied in breast milk. Arsenic is known to cause cancer in humans, and high levels of manganese can cause a syndrome that resembles Parkinson's disease. Cadmium is toxic to the male reproductive system, the kidneys, bone and the brain. All of these contaminants are more likely to affect bottle-fed infants because they are water contaminants and are often found at higher concentrations in infant formula as compared with breast milk.
People can be exposed to metals in a number of ways, including at work in certain industries, from drinking contaminated water and eating contaminated food, or in hobbies that involve working with metals.
Lead exposure stems primarily from its use in gasoline, paint, water pipes and the lining of food cans. These uses have been banned in many countries, but still persist in many parts of the world. In addition, old, peeling paint and old water pipes can still cause exposures. Other common sources of lead include:
- painting or removing old paint;
- construction work;
- battery manufacturing or recycling;
- automobile repair;
- electronics work;
- ceramics and pottery glazed with lead;
- welding and soldering;
- firearm shooting and cleaning;
- jewelry making and repair;
- stained-glass-window making; and
- cosmetics, including certain hair dyes and kohl.
Most people are exposed to methyl mercury from fish, particularly such predator fish as swordfish, shark and tuna. Freshwater fish from contaminated lakes, rivers and estuaries can also bioaccumulate very high levels of methyl mercury, which are passed on to humans who eat the fish. Other sources of mercury include coal burning, incineration, chlorine manufacturing and mining, as well as some natural sources. Inorganic mercury exposure primarily comes from dental amalgam fillings.2 Exposure to cadmium often comes as a result of work or through hobbies, including metal plating, semiconductor manufacture, welding, soldering, ceramics and painting. In addition, it is a contaminant of drinking water, air and food, particularly shellfish. One other important source of cadmium is cigarette smoke; smokers typically have blood levels of cadmium approximately twice those of nonsmokers.3
Breast Milk Overview
In general, the metals found in breast milk are usually at lower levels than are found in maternal blood.4 Thus, breast milk is not the primary pathway of exposure for infants; prenatal trans-placental exposure is a much greater concern. That said, instances of high exposure through breast milk do occur, and are often important indicators of an infant's total exposure. One study found that longer duration of breastfeeding was associated with poorer infant growth in children whose mothers had higher levels of mercury in their bodies. Generally, infants fed formula made with tap water are at the highest risk from metals contaminating the water supply.
Bans and Restrictions
In recent years, some bans and restrictions regarding certain heavy metals have been implemented. For example, lead has been banned from use in gasoline, paint, can linings, or water pipes in more than 50 countries.5
The past few years have brought a great deal of state and local legislation restricting the use of mercury in products. For example, several states have passed laws or regulations governing the sale and use of mercury, requiring recycling and imposing notification requirements on the substance and many products containing it. There are many restrictions already in place relating to mercury containing thermometers, and many local communities have set up mercury fever thermometer exchange programs. Several states have adopted bans on offering for sale or use, or distributing for promotional purposes, mercury-added novelties. These are products intended mainly for personal or household enjoyment or for adornment, such as toys, games, ornaments, holiday decorations, apparel, jewelry, figurines and yard statues.6
Cadmium has also been the subject of legislation. The European Union banned the use of cadmium in materials and components of vehicles put on the market after July 1, 2003, and in new electrical and electronic equipment after July 1, 2006.7
Some countries have also had success reducing pollution from incinerators, power plants and factories, thereby reducing emissions of mercury and other metals.
Benchmarks and Exposure Limits
The U.S. EPA has set an action level for lead in water of 15 parts per billion (ppb) and an action level for inorganic mercury of 2 ppb. While the median level of lead in breast milk worldwide is only one-third as high as the U.S. drinking water limit, the most exposed populations have lead levels in breast milk that exceed this limit threefold. Average levels of mercury in breast milk are near the action level for water.
The World Health Organization (WHO) has set a daily permissible intake (DPI) level of 5 micrograms per kilogram per day (µg/kg/day) of lead for children, and the DPI for cadmium is 1 µg/kg/day (for an adult). The U.S. Agency for Toxic Substance and Disease Registry (ATSDR) has established a minimal risk level (MRL) for mercury of 2 µg/kg/day for inorganic mercury and 0.12 µg/kg/day for methyl mercury. On average, breastfeeding infants are unlikely to exceed these levels. However, in a few polluted communities around the world, infant exposures do exceed these levels.
In the United States, the Health Resources and Services Administration has published a blood lead action level for breastfeeding women of 40 micrograms per deciliter (µg/dL/day)of blood or above. Women with lead levels this high in their blood may be encouraged to choose alternatives to breastfeeding. Most women have blood lead levels far below 40 micrograms per deciliter. However, recent studies looking at children with blood lead levels of 10 micrograms per dL or less and pregnant women with blood lead levels less than 5 microg/dL have demonstrated significant effects on the children's memory and cognitive functioning. This suggests that the current lead standards may still be too high.8
Breast Milk Monitoring Studies
Metals have been detected in breast milk around the world. Countries that have conducted studies detecting one or more of the three major metals of concern (cadmium, lead and mercury) include:
Levels of lead, manganese and mercury vary widely in breast milk samples around the world, with very high levels detected in some places. Results of a WHO study on trace elements in breast milk are summarized below. These levels reflect both maternal-absorbed dose of these metals and infant exposure, and illustrate the large ranges of exposure across the population. Among women who eat a lot of fish, for example, levels of mercury in breast milk may exceed levels in unexposed women by 100 times.9
Range of Metals Detected in Breast Milk Around the World
|Metal||Median (ppb)||Range (ppb)|
|Source: World Health Organization, 199310|
ppb = parts per billion
Important Case Examples
Several specific examples of metals in the human body merit examination.
The elevated presence of lead in human blood samples has been an issue for decades, chiefly because of two products: lead-based gasoline and paint. Lead is ubiquitous in the environment as a result, and in many areas of the world, a significant level of lead turns up in breast milk (5 to 20 ppb).11 That said, lead does not concentrate in breast milk because it does not attach to fat; indeed, levels of lead are generally higher in a mother's blood than in her milk. Several studies have found higher blood lead levels in formula-fed infants than in breast-fed infants.12 This may be a result of contaminated formula cans or formula prepared using tap water with high lead levels. Lead levels in blood and breast milk correlate closely with areas where lead is still used in gasoline, with the highest levels in areas with heavy traffic. In addition, mothers in countries where lead is still used in gasoline, and mothers living near lead smelters, have higher levels of lead in their breast milk due to community contamination (see Figure 1 ).13
Much of the lead in breast milk does not come from the mothers' exposure during lactation. Instead, it comes from lead stored in the mothers' bones. Because lead mimics the beneficial mineral calcium, it is stored for decades along with calcium in the bones. During pregnancy and lactation, a woman's body extracts calcium from her own bones to provide calcium for her child's bone development. Calcium extraction from bone is greatest during lactation, and as a result, lead stored in the mother's bones also enters the blood and breast milk during pregnancy and lactation, posing an exposure risk to the fetus.14 A study in April 2003 confirmed that ensuring adequate dietary calcium intake or taking a calcium supplement before pregnancy, during pregnancy and during the entire lactation period decreases the blood lead level in lactating women. Supplemental dietary calcium most likely decreases the amount of calcium and lead that comes out of the mother's bones. Therefore, women can significantly reduce their baby's exposure to lead by getting adequate dietary calcium or taking a calcium supplement during pregnancy and lactation.15
Breast milk levels of mercury are usually lower than levels of lead. Mercury does not accumulate in breast milk; in fact, the levels in the mother's blood are generally about three times higher than the levels in milk.16 Therefore, prenatal exposure is probably more important than lactation exposure to mercury, in most cases. Two major forms of mercury can enter breast milk. The most hazardous, methyl mercury, does not enter breast milk at high rates because it is attached to red blood cells. But what little does get into breast milk is easily absorbed in the intestine of a nursing infant. The second form, inorganic mercury, enters breast milk easily but is not well absorbed in the infant's gastrointestinal system. One Swedish study found that the mercury in breast milk in the early months of breastfeeding was primarily inorganic mercury from dental amalgam fillings in the mother's mouth.17 However, after two months of lactation, mercury found in milk was primarily from methyl mercury associated with the mother's fish consumption, rather than dental amalgam fillings.18 In the past, mercury has been responsible for several mass poisonings -- in Minamata, Japan, and in Iraq. In both cases, food contaminated with methyl mercury led to illness and death. Some of those affected were breastfeeding children whose mothers had eaten the contaminated food. However, in both of these scenarios, the levels of mercury were extremely high.19 The average levels found in women's breast milk today are far lower than in those cases. In one recent study of mercury exposure, breast-fed infants tended to have higher residues of mercury detectable in their hair. The infants with higher hair mercury levels also had improved neurological development, including faster progression to sitting, creeping and standing. Because mercury is known to affect neurological development adversely, the faster development in infants with higher mercury levels was attributed to the benefits of breastfeeding. Thus any possible adverse effects of mercury in breast milk were overcome by the advantages of breastfeeding.20
A recent study on laboratory rats lends support to the merits of breastfeeding with respect to mercury. The study looked at the effects of diet that contained a moderate dose of mercury -- 5 parts per million -- fed to pregnant rats before and during pregnancy and during lactation. The newborn rats on the day of birth actually had 1.5 times more mercury in their blood than their mother. However, during the lactation period the babies' blood mercury levels dropped dramatically. Even though the breast milk contained some methyl-mercury, this study suggests that the exposure of the baby to mercury during breastfeeding is much lower than during fetal development. The diet used in this study simulated that of a high fish consuming-population. After weaning, the newborns were started on a mercury-containing diet and showed an increase in their blood mercury levels.21 However, a human study in the Faroe Islands, where people eat fish and whale meat, found that those children who were breastfed exclusively for six months ended up shorter and thinner than their peers at 18 months of age. When the researchers looked for an explanation for this finding, they discovered that the children who were exposed to the highest levels of mercury before birth and during breastfeeding were the ones with growth delays.22
A number of studies have examined the protective effects of Vitamin E on mercury, finding that vitamin E may decrease the toxic effects of mercury. In addition, the trace element selenium may enhance the ability to clear mercury from the body's cells. This data further supports the role of supplemental multivitamins during pregnancy.23
For more information on the problems with mercury in fish, visit NRDC's mercury pages.
Cadmium levels in breast milk are significantly associated with cigarette smoking. One German study showed a direct relationship between the number of cigarettes a mother smokes per day and the level of cadmium in her breast milk (Figure 2).24
A study in Japan investigated the interaction between some trace metals, including cadmium, and nutritional elements in breast milk. Researchers found a significant association between breast milk cadmium concentration and calcium secretion in breast milk. Increased cadmium in the breast milk appeared to decrease the amount of calcium secreted in the breast milk. In the study, the average concentration of cadmium in breast milk was 0.28 micrograms/liter. Studies have shown that cadmium toxicity targets the kidneys and bone, two crucial calcium metabolizing sites, thus decreasing the amount of calcium in blood and ultimately in breast milk. This data, in conjunction with previously noted data on blood lead levels and calcium, strongly supports previous conclusions about the role of calcium supplementation during pregnancy and lactation, and indicates that cadmium exposure may result in insufficient levels of calcium in the breast milk.25
Minerals: Beneficial and Toxic
Vitamins and trace minerals are necessary components of the human diet because they are either inadequately synthesized or not synthesized in the body. However, only small amounts of these elements are needed to carry out the necessary biological reactions. In larger doses, the trace minerals and metals can have toxic effects. In the past, the exact concentrations of many of these trace elements in breast milk has been unclear. One study in Germany determined the concentrations of numerous essential trace elements and toxic metals in human milk and selected infant formulas. Most of the concentrations in infant formulas (except in the case of Chromium) were approximately tenfold higher than in human milk. Therefore, the level of exposure through maternal breast milk is significantly lower than the environmental exposure through infant formula. Thus the data strongly supports the role of breastfeeding in limiting the concentration of exposure to certain potentially toxic metals.
|Element||Breast Milk Concentration (µg/dL)||Formula Concentration
|Selenium (Se)||17||Not Quantified|
|Arsenic (As)||6.7||Not Quantified|
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2. Concha, G., et al, "Low-level Arsenic Excretion in Breast Milk of Native Andean Women Exposed to High Levels of Arsenic in the Drinking Water," International Archives of Occupational and Environmental Health 71 (1998): pp. 42-46.
8. Canfield, R.L., C.R. Henderson, D.A. Cory-Slechta, C. Cox, T.A. Jusko, "Intellectual Impairment in Children with Blood Lead Concentrations below 10 micrograms per Deciliter," New England Journal of Medicine 348(16) (2003): pp. 1517-1526; Emory E., Z. Ansari, R. Pattillo, E. Archibold, J. Chevalier, "Maternal blood lead effects on infant intelligence at age 7 months," American Journal of Obstetrics and Gynecology 188(4) (2003): pp. s26-s32.
15. Hernandez-Avila M., T. Gonzalez-Cossio, J.E. Hernandez-Avila, I. Romieu, K.E. Peterson, A. Aro, E. Palazuelos, H. Hu, "Dietary calcium supplements to lower blood lead levels in lactating women: a randomized placebo-controlled trial," Epidemiology ;14(2) (2003): pp. 206-12.
17. Oskarsson, A., et al., "Total and Inorganic Mercury in Breast Milk and Blood in Relation to Fish Consumption and Amalgam Fillings in Lactating Women," Archives of Environmental Health 51(3) (1996): pp. 234-241.
21. Sakamoto M., A. Kakita, K. Wakabayashi, H. Takahashi, A. Nakano, H. Akagi, "Evaluation of changes in methylmercury accumulation in the developing rat brain and its effects: a study with consecutive and moderate dose exposure throughout gestation and lactation periods," Brain Res 949(1-2) (2002): pp. 51-9.
22. Grandjean P, E. Budtz-Jorgensen, U. Steuerwald, B. Heinzow, L.L. Needham, P.J. Jorgensen, P. Weihe, "Attenuated growth of breast-fed children exposed to increased concentrations of methylmercury and polychlorinated biphenyls," The FASEB Journal 17 (2003): pp. 699-701.
last revised 3.25.05