Healthy Milk, Healthy Baby
Chemical Pollution and Mother's Milk
BACK TO CHEMICAL OVERVIEW
Polychlorinated biphenyls (PCBs) are a group of common chemicals that are no longer manufactured but that remain in the environment. The term "PCBs" refers to a large group of 209 individual congeners -- members of the same structural group of chemicals with different configurations. PCBs generally occur as a complex mixture of some assortment of these congeners.
PCBs were used for numerous purposes including:
- stabilizers in polymers, paint, and adhesives;
- industrial lubricants and coolants in transformers, capacitors, and other electrical equipment; and
- insulating materials in electrical transformers, fluorescent lighting fixtures, and electrical appliances.
Health Effects of PCBs
PCBs are a complex and hazardous group of chemicals. While their acute toxicity is much lower than many of the organochlorine pesticides and the dioxins, they have serious long-term health effects at relatively low levels. High levels of exposure can cause effects in infants ranging from low birth weight; to abnormalities of the skin, hair, and nails; to hearing loss.
The most serious effects of PCBs, however, are on the brain. PCB exposures, particularly before birth, have been linked to lower IQ, hyperactivity, shortened attention span, and delayed acquisition of reading skills. PCBs interfere with thyroid hormone, and some researchers believe that this mechanism may explain some of the neurological effects of PCBs. Thyroid hormone is essential for normal growth and development of the brain before birth and throughout infancy. Some PCBs also mimic estrogen, leading to questions about possible associations with such diseases as breast cancer later in life. PCBs are also probable human carcinogens, based on animal studies and some studies of exposed workers.
PCBs in the Body
PCBs are extremely persistent and accumulate in the environment and in living organisms. The chemical properties of PCBs allow them to travel long distances on global air currents, resulting in contamination in such remote northern locations as the Arctic.
PCBs do not readily dissolve in water. When PCBs are released into water, most end up binding to sediments. PCBs released into the environment eventually enter the food chain and can build up in fish and other marine animals, so that they reach levels thousands of times higher than their original concentration in water.
Humans are exposed to PCBs in a number of ways, but eating PCB-contaminated food, especially fish, meat, and dairy products, is by far the most common exposure route. Other less common exposure scenarios include PCB leaks in old appliances or fluorescent lighting fixtures, living near leaking hazardous waste sites that contain PCBs, and repairing old PCB-containing transformers.
Historically, mass PCB poisonings have occurred as a result of food contamination. [1, 3-6] In Japan in 1968, poisonings occurred because of PCB-contaminated rice oil. A similar incident occurred in Taiwan, China in 1979 from PCB-contaminated cooking oil.
PCBs occur as an environmental contaminant around the world. Because of their pervasive nature, PCBs' contribution to the overall human body burden of chemicals is significant. Many researchers report that almost all samples of human blood, fat, or breast milk show some detectable level of PCBs. PCBs can probably be found in the blood of the entire U.S. population. The majority of breast-milk samples tested throughout the world show at least trace levels of PCBs.
Many studies looking at PCBs in women's bodies have found concentrations in breast milk that are four to ten times higher than in the mothers' blood. However, it is prenatal exposure (via trans-placental transfer) of PCBs that is believed to be more significant to the later health of the child. Many researchers have investigated the effect of PCB exposure on infants' neurobehavioral development, and a consensus is emerging that prenatal exposure to PCBs is much more important than exposure in breast milk.[7, 8]
Controlling Exposure: Bans and Restrictions
PCBs were recognized as hazardous in the 1970s, at least in part as a result of the poisoning incidents in Asia. As a result, international pressure to restrict the use of PCBs has grown.
Because of the evidence of widespread environmental damage, the manufacturing of PCBs in the United States was halted in 1977. Other countries that have banned the continued production of PCBs include Austria, Czech Republic, Finland, Germany, Liechtenstein, the Netherlands, Norway, and Switzerland. In many countries, the use of PCBs has been restricted to closed electrical systems.
Despite increased efforts to end the production of PCBs, they may still be manufactured in Russia and are still used in some countries. In many parts of the world, the main risks are associated with exposure from the destruction of materials containing PCBs. Thus, although production may have decreased, a vast number of products with high levels of PCBs remain in use. The degradation and disposal of these products should be the area of most concern.
Assessing the Extent of PCB Exposure: Limits and Benchmarks
Most chemicals that are either in widespread use or that have caused widespread contamination are subject to national and international benchmark levels, established to protect public health. But different agencies may have markedly different levels they consider "safe." In the case of PCBs, conclusive evidence demonstrating that any one of these benchmarks is protective or superior to the others does not exist.
The U.S. EPA has set a maximum contaminant level (MCL) of 0.5 parts per billion (ppb) for PCBs in water. The most relevant benchmark level was set by the U.S. Food and Drug Administration: a limit of 0.2 to 3 parts per million (ppm) for PCB residues in milk, eggs, other dairy products, poultry fat, fish, shellfish, and infant foods. Levels of PCBs in breast milk today in many countries exceed this benchmark level.
Breast-milk Monitoring Studies Measuring PCBs
PCBs have been measured in the breast milk of women from the following countries:
The extent of nation-specific information about PCBs in breast milk only reflects what has been examined thus far. Based on what we know about the pervasive nature of PCB contamination in the food chain and its ability to move great distances, PCBs are likely to occur throughout the world.
Limitations of Studies Measuring PCBs in Breast Milk
Unlike many other chemicals on the POPs list, it is extremely difficult to compare different measurements of PCBs in breast milk -- both within individual countries and between different countries and studies. The challenges arising from PCB data in breast milk include:
- Differences in analytical techniques. The last several decades have seen important changes in the techniques used to measure PCB levels in human tissue, making it difficult to compare recent data with older measurements. In addition, no accepted standardized method for analyzing breast milk for PCBs has emerged, so the problem of incomparable results continues even in new research.
- Number of congeners measured. Reported PCB levels in breast milk vary depending on how many congeners are measured. Some studies measure just one (Webb-McCall method) or only a few congeners considered to be the most prevalent or the most toxic. Others report a group of congeners thought to be an "indicator group" of total PCB contamination. Still others measure all congeners that analytical tools can detect. As a result, reported PCB levels in milk may differ greatly, depending on the choice of congeners sampled in each individual study, making comparisons virtually impossible.
- Differences in reporting. Because PCBs often are present in the environment along with other contaminants, their detection may be reported as a combined toxic equivalency factor that also includes dioxins and furans. In such cases, it is quite challenging to tease out just the component attributable to PCBs.
Some Important Examples of PCBs in Breast Milk
Because of the challenges presented by the data measuring PCBs in breast milk, it is difficult to assess trends. Some researchers have speculated that, over the last 25 years, levels may have decreased slightly. However, that conclusion is hardly definitive, and the question will most likely remain unanswered until data standardization issues are addressed.
In Sweden, where data have been collected following fairly consistent methods over time, evidence of a downward trend has emerged. Figure 35 shows this data.
Although most other data is difficult to interpret and compare, some other important issues are evident.
- Importance of diet. Many researchers have looked into the role of diet in the level of PCBs present in breast milk. In the United States, fish consumption in the Great Lakes area has been associated with a higher body burden of PCBs. In Canada, Inuit and fisherman populations have shown higher breast-milk levels than urban populations. This is attributable to higher fish consumption. Figure 36 shows the difference in breast-milk levels of Inuit and Caucasian women in Quebec, Canada. This study looks at just one group of PCBs (the di-ortho substituted PCB congeners), but it illustrates that the Inuit women with higher fish consumption have much higher breast-milk levels. This turns out not to be a universal trend -- a few studies have shown fish-eaters to have equivalent or lower PCB levels in their breast milk. Fish consumption is also not the only dietary exposure of concern. Other unique scenarios involving food contamination have resulted in increased breast-milk levels. For instance, in the Czech Republic, the use of a PCB-containing paint in grain silos led to breast-milk levels higher than those found in neighboring regions and countries.
- Importance of location. Location has also been shown to be an important indicator in breast-milk levels of PCBs. In general, the higher rates of industrial pollution associated with urban areas has resulted in higher levels of PCB residues in urban versus rural populations. Location may also affect the pattern of PCB pollution. People in different regions of the world may be exposed to different groups of congeners. For example, a study in the Kola Peninsula of Russia found distinctly different congener patterns in breast-milk samples from two different regions, the result of different types of industry and products.
Because of methodological challenges, it is hard to sustain final conclusions regarding the status of PCB levels in breast milk. Many researchers have looked into the risks posed to breastfeeding children by PCBs in breast milk,[4, 6-8, 15-17] and most have concluded that the small increased risk associated with breast-milk exposure to PCBs is outweighed by the benefits of breastfeeding. Other scientists have reinforced this finding by showing that breastfeeding will provide health benefits even if PCB residues are present in the breast milk.
Related Site on the Web
The International POPs Education Network hosts this website on PCBs, as part of the PCB Working Group.
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last revised 5.22.01