Healthy Milk, Healthy Baby
Chemical Pollution and Mother's Milk
Chemicals: Lead, Mercury, Cadmium, and Other Metals
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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-such as lead-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 bind to the fat, and so do not usually accumulate to 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, concentrations of 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.
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, and the brain. All of these contaminants are more likely to affect bottle-fed infants, because they are water contaminants or contaminants in infant formula.
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, and mining, as well as some natural sources. Inorganic mercury exposure primarily comes from dental amalgam fillings. The relative importance of this form of mercury and this type of exposure is highly controversial.
Exposure to cadmium often comes as a result of work or through hobbies, including metal plating, semiconductor manufacture, welding, soldering, ceramics, and painting. One other important source of cadmium is cigarette smoke; smokers typically have blood levels of cadmium approximately twice those of nonsmokers. Cadmium can also be a contaminant of drinking water, air, and food, particularly shellfish.
In general, the metals found in breast milk are usually at lower levels than are found in maternal blood. 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. Generally, infants fed formula made with tap water are at the highest risk from metals contaminating the water supply.
Bans and Restrictions
Heavy metals are not the subject of any broad bans, but lead has been banned from use in gasoline, paint, can linings, or water pipes in more than 50 countries. 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 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.
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.
Range of Metals Detected in Breast Milk Around the World
|Metal||Median (ppb)||Range (ppb)|
|Source - World Health Organization, 1993|
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 products. 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). 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. 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 37).
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. Fortunately, sufficient calcium intake during pregnancy and lactation significantly reduces the extraction of lead from the mother's bones. Thus women can significantly reduce their fetus' exposure to lead by getting adequate calcium during pregnancy and lactation, or by taking a calcium supplement.
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. Therefore, prenatal exposure is probably more important than lactational 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. 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.
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. 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.
Related Site on the Web
For more information on the problems with mercury in fish, visit Brain Food from the Environmental Working Group.
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 38)
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last revised 5.22.01