Does Exposure To These PCBs Matter?
The chemical stability of PCBs that made them so useful as industrial products also makes them environmentally persistent and gives them properties that pose potentially serious health hazards. In addition to being slow to biodegrade, PCBs are lipophilic (fat soluble) so they can bioaccumulate and move up the food web. Certain PCBs have been identified as carcinogens.17
PCBs have also been identified as endocrine disruptors and shown to have adverse effects on the endocrine system, particularly on thyroid hormone function. They are also associated with skin and eye problems, liver toxicity, and adverse effects on the immune, nervous, and reproductive systems as well as on blood pressure and blood cholesterol levels. Prenatal and childhood PCB exposure has been associated with behavioral and cognitive problems. Among the PCB health effects now under investigation are their impacts on brain functions that control behavior, language, learning, and memory.17
Some nondioxin-like PCB congeners have been recognized as having what R. Thomas Zoeller, a University of Massachusetts professor of biology, calls “potent” effects on body systems regulated by the thyroid, at very low levels of exposure—those measured in micrograms per kilogram, or parts per billion. Thyroid hormones are involved with regulating a number of vital body systems, including metabolism and development, and they are essential for healthy cardiovascular and nervous system function. If PCBs can interfere with thyroid hormones, “no one can conclude that PCBs are safe,” says Zoeller.
Larry Robertson, a University of Iowa professor of environmental and occupational health, explains that since PCBs typically occur as mixtures of congeners, it is important to note that an individual congener’s health effects should not be excluded when considering PCB toxicity, even if that congener is not dominant in a particular sample or mixture. “Context is everything,” he says of health effects.
That PCBs have endocrine-disrupting effects is important to understanding exposure levels that may affect human health, as it is recognized that such chemicals can be biologically active at extremely low levels.18 It is also recognized that timing of exposure to endocrine disruptors is important, as exposure at one life stage can prompt effects different from those of exposure at another time. Early life, including prenatal development, is a time at which many body systems are particularly vulnerable to effects of endocrine disruptors.19
Zoeller says that thyroid hormone levels at age 2 have been correlated to adult IQ levels, and early-life PCB exposures have been associated with depressed IQ scores and impaired cognitive ability in children.20 He also says that studies have shown metabolites of PCB 105 and PCB 118 to be specifically related to cognitive function in animals.21,22 Furthermore, research by Isaac Pessah, chair of molecular biosciences at the University of California, Davis, School of Veterinary Medicine, has shown that neural network formation was adversely affected in rats exposed prenatally to PCB 95.23,24
The task of trying to assess all 209 PCB congeners and their metabolites is enormous, Pessah says, so he and his colleagues decided to look for certain cellular targets that appear to be extremely sensitive to PCBs. “The one we found that was most active was that activated by PCB 95,” he says. This congener has been found in sediment and in fish and human tissue, including children’s brain tissue.
Pessah says that the cellular receptor with which PCB 95 interacts—the ryanodine receptor—is similar to the aryl hydrocarbon receptor to which dioxin-like PCBs are known to bind. If a PCB or dioxin binds to the ryanodine receptor, it may cause the production of certain proteins that can interfere with normal cellular growth and differentiation, he explains. The interaction of PCB 95 with the ryanodine receptor, which is known to play a critical role in regulating the movement of calcium around the body,23,24,25 could be very important, as calcium is key to healthy neuron function.
“The brain is very organized,” Zoeller says, a characteristic integral to how it works. It appears from Pessah’s animal experiments that prenatal exposure to PCB 95 can interfere with this organization and disrupt the brain’s auditory perception, a function that is vital to speech and language abilities. Curiously, there were no overt signs that there was anything wrong with the exposed animals, Pessah says—they didn’t fail to thrive, and they could still hear—but their ability to interpret sound was impaired. These effects, he says, have potentially profound implications for PCB toxicology, for it may mean that a level of exposure that does not otherwise appear “toxic” could nevertheless change the course of neural development.
Another recent but very different type of study, an epidemiological study examining the relationship between environmental chemical exposure and fertility in couples, has found an association between exposure to certain PCB congeners—among them PCBs 118, 138, 167, and 209—and the reduced likelihood of becoming pregnant within a certain time.26 Study author Germaine Buck Louis, director and senior investigator at the Eunice Kennedy Shriver National Institute of Child Health and Human Development’s Division of Epidemiology and Prevention Research, cautions that “we cannot say if anything is causal.” But she says, these are “important signals that are being picked up,” particularly as this study indicates an association between environmental chemical exposure in both men and women and a delay in the time to successful conception and pregnancy.
In addition to the health effects of PCBs themselves, there is also evidence that some PCBs can, once in the body, break down into other compounds that can play a role in health effects. Among these compounds are hydroxylated PCB metabolites (OH-PCBs)—some of which have structures that resemble a thyroid hormone—and PCB sulfates, which can also be endocrine disruptors. Robertson says there could be as many as 15 or 20 different metabolites of a single parent PCB compound, depending on how that compound is metabolized. What appears to be emerging from the growing body of research on PCB health effects is an increasingly complex picture of the ways in which various PCB congeners and their metabolites can affect human health—potential effects that extend well beyond those of the dioxin-like and carcinogenic PCBs that have received the most research and remediation attention to date.