Polychlorinated biphenyls or PCBs were used in transformers and other electrical equipment throughout the twentieth century because of their ideal electrical and thermal properties and the fact that acute toxicity of these chemical mixtures had not been observed.
In 1936, Monsanto asked the Harvard physician and researcher Cecil Drinker to investigate the health problems, primarily chloracne, observed in employees of the Halowax corporation, one of the early makers of PCBs. Most PCB mixtures contain small amounts of dioxin and the dioxin may actually have been the cause of the observed chloracne. PCBs and their health effects have been poorly understood for decades for three reasons—the complex chemistry of these mixtures, the different health effects caused by different types of PCBs, and the different susceptibility of humans and animals to these effects.
PCBs were manufactured by mixing chlorine with biphenyl to produce mixtures chlorinating between one and ten chlorine atoms per molecule of biphenyl. Individual PCB molecules are called congeners and 209 distinct congeners exist depending on the number and position of the chlorine atoms. Monsanto produced PCB mixtures at their plant in Anniston, AL. These mixtures were sold under the trade name Aroclor. The last two numbers of a designated Aroclor mixture represent the weight percent of chlorine in the mixture and thus, Aroclor 1254 contains 54% chlorine by weight.
Health Effects Associated with PCB Exposure
In 1968, Yusho disease broke out in northern Japan when people were poisoned from rice oil contaminated with PCBs. The contaminated rice oil was used for cooking and the heat produced polychlorinated quaterphenyls, substances with dioxin-like effects. In 1979, a similar poisoning incident called Yu-cheng disease occurred in Taiwan. The predominant symptom in both poisoning events was chloracne. Chloracne was very likely the result of the dioxin-like substances produced by heating and not the PCBs themselves.
In the late 1990s, General Electric conducted a two-year cancer bioassay and demonstrated a dose-related incidence of liver cancer in rats administered five different Aroclor mixtures. These mixtures contained small amounts of polychlorinated dioxins and furans, which also cause liver cancer in rats. Hence, these bioassay results are also equivocal as to whether PCBs are rodent carcinogens.
The Different Structures of PCBs Determine The Biological Effects
To understand the health effects of PCBs, the actual shape of individual congeners needs to be considered. The image below shows a diagram of a generic chlorinated biphenyl. Each ring structure many contain up to five chlorine atoms.
When chlorines are positioned at the ends of the molecule (positions 3,4,5 or 3’,4’,5’), the molecule generally takes on a coplanar or flat structure with both rings in the same plane. When the bulky chlorines are positioned at the locations next to the biphenyl bond, the two rings occur at an angle to accommodate the chlorine atoms and is a non-planar structure with each ring in a different plane.
Coplanar PCBs possess a similar shape to chlorinated dioxins and have similar biological effects mediated by activation of the aryl hydrocarbon receptor (AHR) and discussed in my previous blog. Non-coplanar PCBs may interact with calcium channels involved neural function and with targets of thyroid hormone function.
Do PCBs Produce Health Effects in Humans at Low Exposures?
The recognition of the potential for dioxin-like effects including cancer, neurobehavioral effects and endocrine disruption involving the thyroid system has engendered a large number of observational human studies of PCBs, conducted mostly on very small groups. In 2003, a critical synopsis of the literature on PCB effects concluded that “studies to determine whether PCBs cause cancer in humans, neurobehavioural effects, abnormal thyroid and immune function in children and low birth weight … are inconclusive and do not provide clinical evidence that PCBs at levels encountered with human exposure produce adverse health effects.”
PCBs continue to be linked with human health effects including type 1 and type 2 diabetes, asthma, birth size, neurodevelopment, prostate cancer and others. Most of these studies are of small size and thus lack statistical power. Methodological differences between studies complicate the ability to conduct meaningful meta-analyses. In consequence, the demonstration of whether or not PCBs are associated with various health effects in humans remains an elusive goal.
Ted W. Simon, PhD is an award winning toxicologist and scientist. Dr. Simon has 10+ years’ experience as a toxicologist employed by the Environmental Protection Agency (EPA). He served as EPA’s senior toxicologist in the waste management division working on risk and soil cleanup. He has expertise in toxicology, risk assessment, mathematical modeling, statistics, neuroscience, and environmental/ecological health issues as well as extensive national and international public speaking experience and 15+ years teaching experience. Dr. Simon is well published, including being author of textbook on toxicology and environmental risk assessment. He is an experienced expert witness on issues including environmental risk, drug and alcohol cases, DUI, accidental poisoning, and intellectual property. Dr. Simon has a special interest and expertise in dioxin-like chemicals and polychlorinated biphenyls (PCBs). He can be contacted at firstname.lastname@example.org or (770)359-9807.