Polychlorinated biphenyls (PCBs) are man-made organic chemicals used in industrial machinery in the 20th century. Their unique chemical stability led to rapid commercial success and widespread adoption. The history of these compounds is a case study of a technological marvel that quickly became a global environmental disaster, prompting international regulatory action to address their long-term persistence.
Defining Polychlorinated Biphenyls
PCBs are characterized by a basic chemical structure consisting of two connected benzene rings, known as a biphenyl molecule. Chlorine atoms substitute for hydrogen atoms on this biphenyl structure, and the number and placement of these chlorine atoms can vary significantly. This chemical variation results in up to 209 distinct individual compounds, which are referred to as congeners.
In commercial production, PCBs were not manufactured as single compounds but as complex mixtures of various congeners. In the United States, these mixtures were primarily sold under the trade name Aroclor. Aroclor naming conventions, such as Aroclor 1254, used a four-digit code where the last two digits represented the approximate percentage of chlorine by weight in the mixture.
The Timeline of Invention and Early Production
The initial synthesis of a polychlorinated biphenyl was a laboratory event. PCBs were first synthesized in 1881 by German chemists, marking the chemical’s origin as a purely scientific endeavor. For decades, the compounds remained a chemical curiosity, lacking any immediate commercial application.
The transition from a laboratory compound to a mass-produced industrial product occurred in the late 1920s in the United States. Large-scale commercial manufacturing began in 1929 by the Swann Chemical Company, which was later acquired by Monsanto Chemical Company. Monsanto became the primary U.S. producer, manufacturing the chemicals under the Aroclor trade name. Between 1930 and 1975, the United States alone produced hundreds of thousands of tons of these chemical mixtures, making the country the world’s largest producer.
Industrial Adoption and Widespread Use
The widespread adoption of PCBs was driven by their unique combination of physical and chemical properties, which made them ideal for demanding industrial applications. They exhibit thermal and chemical stability, resisting degradation even under high heat and pressure. They are non-flammable and possess excellent electrical insulating capabilities, making them highly desirable in electrical equipment.
These attributes led to their extensive use as dielectric fluids in electrical transformers and capacitors, where their insulating and heat-transfer properties prevented equipment failure. Beyond the electrical industry, PCBs were incorporated as hydraulic fluids, plasticizers in paints, sealants, and plastics. They were also used in less confined applications, such as pigments and carbonless copy paper.
Scientific Confirmation of Environmental Danger
Despite their utility, early concerns about PCBs emerged in the 1930s when industrial workers exposed to the compounds reported health issues like chloracne and liver damage. The true scale of the environmental problem was not scientifically confirmed until the mid-1960s, a period marked by rising environmental awareness. In 1966, Swedish chemist Søren Jensen made a discovery while analyzing wildlife samples for the pesticide DDT.
Jensen’s instruments detected unknown chemical signals in the fat of animals, which he eventually identified as PCBs. This finding revealed that the compounds had spread far beyond their point of manufacture, demonstrating their capacity for long-range environmental transport. Scientists quickly established that PCBs do not easily break down and accumulate in the fatty tissues of living organisms, a process termed bioaccumulation.
The concentration of these persistent compounds increases as they move up the food chain, a mechanism known as biomagnification. This process resulted in high PCB levels in apex predators like seals and birds, leading to adverse effects such as reproductive failure and immune system suppression. In humans, exposure was linked to adverse health effects, including liver damage and neurological problems.
Global Action and the Manufacturing Ban
The scientific evidence of widespread environmental contamination and toxicity led to regulatory action in the United States and internationally. In the U.S., the Toxic Substances Control Act (TSCA) was enacted in 1976, specifically mandating the regulation of PCBs. Although Monsanto, the primary manufacturer, ceased production in 1977, the TSCA effectively banned the manufacture, processing, and distribution of PCBs in the United States in 1979.
The ban included limited exemptions for totally enclosed uses, such as existing electrical equipment. The global community later addressed the legacy of PCBs through the Stockholm Convention on Persistent Organic Pollutants (POPs), which was adopted in 2001. This international treaty listed PCBs as one of the original “dirty dozen” POPs and required signatory nations to eliminate their use in existing equipment by 2025.