PCBs: Cancer Linkage Found Decades after a Ban
Polychlorinated biphenyls (PCBs) are a rare success story in terms of U.S. regulation. This group of chemicals was banned from domestic manufacture in 1979, based on evidence of environmental persistence and bioaccumulation and concerns about toxic effects including liver cancer in laboratory animals. Concentrations of PCBs in people’s blood decreased significantly in the decades since the ban, and have continued to decline in recent years (Sjodin et al. 2014). Nevertheless, virtually every person still has some exposure based on the chemical’s continued persistence in the environment and food chain and its widespread presence in electrical equipment (e.g., transformers) and building materials (e.g., insulation, adhesives, caulking, oil-based paint).
There is recently mounting evidence that PCBs may cause hematopoietic cancers such as non-Hodgkin lymphoma (De Roos et al. 2005; Engel et al. 2007). These studies have mostly focused on adults, and have made links with disease based on PCB concentrations measured in blood. There is interest in studying the same question in children, given that acute lymphoblastic leukemia (ALL), the most common type of childhood cancer, is a lymphoid-cell malignancy that shares molecular and clinical similarities with certain adult lymphoid cancers.
In shifting the focus to examine the same question in children, the researcher encounters an issue. Parents are often reluctant to allow a blood draw from their child for research purposes; therefore, another way of estimating PCB exposure is needed. A child’s exposure to PCBs in early life generally results from a combination of what is passed along from the mother during pregnancy and breastfeeding, through dietary exposures in childhood, and through contact with PCBs in the environment. House dust, in particular, is proving to be an important source of PCB exposure in small children, due to the amount of time spent in the home (relative to older children and adults), time spent sitting or crawling on the ground, and hand-to-mouth behavior typical of babies and toddlers.
In a study of ALL conducted in California (Ward et al. 2009), PCBs concentrations were measured in carpet dust samples collected from the room where the child spent the most time when awake before diagnosis, and these PCBs were compared to those from homes of control children. In this study, the concentration of total PCBs was significantly higher in case homes than control homes, even after statistical adjustment for the child’s age, sex, and race, as well as breast feeding. The finding was particularly strong among non-Hispanic white children, where cases were over six times more likely than controls to have detectable levels of PCBs in house dust.
One might ask if this research currently relevant, since PCBs have already been strictly regulated for decades. The answer is a definitive ‘yes’. While it could be argued that the association with adult lymphoma seen in studies since the 1990s could be due to PCB exposure that accumulated in the distant past – during the time before and shortly after regulation when levels were higher – the same can’t be argued for the finding with childhood ALL. The children in the study were on average only 4 years old at the time of diagnosis. Therefore, if the finding between PCBs in carpet dust and childhood ALL truly reflects a causal association, this suggests that current environmental levels of PCBs, albeit relatively low, are still high enough to promote leukemogenesis. House dust is an ongoing source of exposure, as the chemicals are slow to degrade. Furthermore, there are new inputs of PCBs to household air and dust as PCB-containing equipment and building materials age and deteriorate. Research demonstrating links with health effects that haven’t been studied before will add to the knowledge of the total toxic legacy of these chemicals, and it is hoped, will provide further impetus for cleanup of contaminated sites and avoidance of sources of notably high exposure, such as fish consumption from contaminated lakes.
Finally, there are lessons to be learned from research on banned chemicals like PCBs about the exposure and toxicology of similar compounds. PCBs share some similarities in chemical structure and bioaccumulation patterns with other persistent organic pollutants including dioxins, organochlorine pesticides such as DDT and chlordane, and polybrominated diphenyl ethers (PBDEs). There is an urgent need for knowledge about possible health effects of PBDEs in particular, as these chemicals have accumulated rapidly in the environment more recently, through use as flame retardants in mattresses and other foam furniture. Ward et al. later considered PBDEs in their study of childhood ALL, reporting no association with total PBDEs, but increased risk associated with certain, specific congeners (Ward 2014). PBDEs were not significantly correlated with PCBs in their study; therefore, these findings may represent independent effects, suggesting the importance of studying chemical mixtures in carpet dust to characterize the total risk associated with this route of exposure. By following leads from what has been learned about PCBs, we hope to understand health risks from PBDEs sooner than decades after a ban.
De Roos AJ, Hartge P, Lubin JH, et al. Persistent organochlorine chemicals in plasma and risk of non-Hodgkin lymphoma. Cancer Res 2005;65:11214-6.
Engel LS, Lan Q, Rothman N. Polychlorinated biphenyls and non-Hodgkin lymphoma. Cancer Epidemiol Biomarkers prev 2007;16:373-6.
Sjödin A, Jones RS, Caudill SP, et al. Polybrominated Diphenyl Ethers, Polychlorinated Biphenyls, and Persistent Pesticides in Serum from the National Health and Nutrition Examination Survey: 2003−2008. Envir Sci Technol 2014;48:753-60.
Ward MH, Colt JS, Metayer C, et al. Residential exposure to polychlorinated biphenyls and organochlorine pesticides and risk of childhood leukemia. Environ Health Perspect 2009;117:1007-13.
Ward MH, Colt JS, Delziel NC, et al. Residential levels of polybrominated diphenyl ethers and risk of childhood acute lymphoblastic leukemia in California. Environ Health Perspect, 2014 (advance publication).
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