Carolyn R. Klocke, Postdoctoral Scholar and Pamela J. Lein, Professor at University of California, Davis both argue that polychlorinated biphenyls (PCBs) are a persistent environmental health problem today
Polychlorinated biphenyls (PCBs) are a family of synthetic chemicals that were produced in large quantities for industrial and commercial applications beginning in the late 1920s through the late 1970s. PCB mixtures were synthesised globally and identified under several trade names, including Aroclor® (United States and United Kingdom), Clophen® (Germany), Phenclor® (France) and Kanechlor® (Japan). Chemically, PCBs are biphenyls with variable chlorine atoms substituted for the hydrogen atoms in the benzene rings. There are 209 possible PCB compounds – each of which is referred to as a congener – that are named according to the number and position of chlorine substitutions (i.e., lower-chlorinated congeners have lower number designations and higher-chlorinated congeners that have higher number designations).
While concern regarding adverse health outcomes associated with occupational exposures to PCBs arose as early as the 1930s, by the 1960s and 1970s there was significant alarm about the human health risks of PCBs in the environment. The manufacturing, use and disposal of PCBs had resulted in widespread PCB contamination of air, water and soil, and because PCBs are highly resistant to degradation, they had accumulated in the human food chain and were readily detected in human tissues, including breast milk.
These observations, coupled with emerging data linking environmental PCB levels to increased cancer risk in humans and animal models, impelled the United States Congress to institute a ban on PCB production in 1979. This was followed by a global ban on the production and use of PCBs by the Stockholm Convention on Persistent Organic Pollutants in 2001.
In the decades following the ban on PCB production, environmental PCB levels decreased significantly. During this time, basic research scientists identified the biological mechanisms by which PCBs cause cancer and regulatory scientists established “safe” exposure levels for PCBs in the environment and human food supplies based on attributable cancer risk. It was widely believed that the PCB problem was solved and that further research on PCBs was not warranted. However, emerging research on PCBs over the past decade has revealed a number of unexpected findings that suggest the mainstream understanding of PCB exposures and PCB toxicity may be too limited and that PCB regulations focused on cancer outcomes may not be protective of vulnerable populations.
One surprise from current research is that while environmental levels of PCBs are decreasing globally, levels have stabilised or may be increasing in some geographic regions. One explanation is the accelerated release of “legacy” PCBs from ageing products. For example, higher than expected levels of PCBs in the air over the city of Chicago are thought to be due in part to the release of PCBs from ageing paints and caulking materials used to construct municipal buildings during the era when PCBs were intentionally added to these construction materials. The release of legacy PCBs from paints and caulking materials may also explain why PCB levels in the indoor air of elementary schools in the United States exceed the 2009 public health guidelines set by the United States Environmental Protection Agency. Additionally, novel PCBs that were not part of the original industrial mixtures have been detected in the environment and in human tissues. The toxic potential of most of these contemporary PCBs, many of which are lower chlorinated congeners, is largely unknown.
Historically, consumption of contaminated food was thought to be the primary source of PCB exposure in humans, with fish, meat and dairy products comprising the main dietary sources of PCBs. However, recent studies documenting PCB contaminants in the air of major cities and indoor air of municipal buildings, including schools, suggest that inhalation may be a significant and underappreciated source of human exposure. While sources of airborne PCBs, which include both legacy PCBs as well as the lower chlorinated contemporary PCBs, are not yet completely understood, some studies have demonstrated that PCBs can be unintentionally produced during the synthesis of yellow and green paint pigments. Once dried, volatile PCBs can be released into the air (a phenomenon also referred to as “off-gassing”) to be inhaled by humans. Whether the toxic effects of PCB are different if they are inhaled from the air vs. ingested with food remains to be determined.
Another evolution in our understanding of the environmental health impacts of PCBs is the realisation that the developing brain is a vulnerable target of PCBs. PCBs interfere with the growth and maturation of neurons in the developing brain, which shifts the developmental trajectory of the brain in a manner that disrupts normal patterns of connections between brain regions. The magnitude of this effect differs depending on the specific PCB congener involved and whether it is a higher- or lower-chlorinated congener. Interestingly, a pathological change that is common to many neurodevelopmental disorders, including autism and attention deficit hyperactivity disorder (ADHD), is altered connectivity in the brain, and recent studies report that elevated maternal PCB levels are associated with increased risk of having a child with autism or ADHD.
The recent discovery of PCB contamination in the indoor and outdoor air has also raised concern regarding the effects of exposure to airborne PCBs on the developing lung. Lung development continues long after birth, so there is the possibility that inhalation of PCBs interferes with lung development and growth. Since PCBs are known to interfere with neuronal development, it is hypothesized that the inhalation of airborne PCBs may interfere with innervation of the lung, resulting in increased airway hyperreactivity, a hallmark characteristic of asthma. It has been hypothesized that airborne PCBs contribute to the unexplained and perplexing increase in childhood asthma since the 1960s.
Collectively, epidemiologic studies and experimental data from animal models suggest that further investigation of PCBs is warranted to understand how the changing patterns of PCB exposure are contributing to non-cancer outcomes, specifically neurodevelopmental disorders and potentially pediatric asthma. Such work is required to ensure that regulatory policies targeting PCBs are protective of the most vulnerable members of society.
Carolyn R. Klocke
Postdoctoral Scholar
University of California, Davis
www.linkedin.com/in/carolyn-klocke
Pamela J. Lein
Professor
University of California, Davis
Tel: +1 530 752 1970
www.vetmed.ucdavis.edu/lein-lab
