Polychlorinated biphenyls, commonly called PCBs, are a group of 209 synthetic chemicals that were manufactured for industrial use from 1929 until the United States banned their production in 1978. They were valued for their chemical stability, resistance to heat, and insulating properties, but those same qualities make them extraordinarily persistent in the environment. Decades after production stopped, PCBs remain a significant pollutant in soil, water, and the food chain.
Chemical Structure and Properties
PCBs are built on a backbone of two connected carbon rings (a biphenyl), with chlorine atoms attached at various positions. Depending on how many chlorine atoms are present and where they sit on the rings, there are 209 possible variations, called congeners. Commercial PCB products typically contained a mixture of 50 to 90 of these congeners.
The chlorine atoms make the molecules extremely stable. PCBs resist heat, don’t conduct electricity, and barely break down in the environment. They also dissolve readily in fats and oils rather than water, which is central to how they accumulate in living organisms.
Why PCBs Were Used
Their primary commercial role was as a dielectric fluid, the liquid inside electrical transformers and capacitors that prevents sparking. That application alone accounted for a large share of production. But manufacturers saw potential everywhere: PCBs were mixed into paints, varnishes, adhesives, inks, sealants, and even heat-transfer systems. A 1935 product guide from the original manufacturer listed uses ranging from fireproofing to moisture-proof paper to chewing gum.
Reports of workers getting sick from PCB exposure surfaced as early as the 1930s, and by the 1950s, illness among end users of PCB-based heat-exchange fluids was well documented. Despite these warnings, production continued for decades. The U.S. finally banned manufacturing and distribution in 1978 after mounting evidence of environmental contamination and health risks.
How PCBs Move Through the Environment
PCBs don’t stay where they’re released. They enter waterways through industrial discharge, landfill runoff, and improper disposal of old equipment. Once in an aquatic system, they’re absorbed by phytoplankton. Small fish and zooplankton eat enormous quantities of phytoplankton, concentrating the chemicals further in their bodies. Each step up the food chain multiplies the concentration, a process called biomagnification.
The results are dramatic. Top predators like lake trout, large salmon, and fish-eating birds can accumulate PCB concentrations in their fatty tissues that are millions of times higher than the concentration in surrounding water. In the Great Lakes, the highest PCB levels have been found in the eggs of herring gulls at the top of the food chain. In the Arctic, PCBs build up in the blubber of marine mammals, affecting polar bears and Indigenous communities that rely on those animals for nutrition.
How People Are Exposed Today
Even though PCBs haven’t been manufactured in the U.S. for over 45 years, exposure continues through two main routes: food and indoor air.
Eating fish from contaminated waterways is one of the most common sources of ongoing exposure. Because PCBs accumulate in fat, fatty fish from polluted lakes and rivers carry the highest concentrations. Many states issue fish consumption advisories for specific bodies of water.
The other significant source is older buildings. Caulking, paints, and fluorescent light ballasts manufactured before 1979 can contain PCBs that slowly release into indoor air. This is especially relevant in schools. An EPA study of six school buildings constructed between the late 1950s and early 1970s found a median indoor air PCB concentration of 318 nanograms per cubic meter, with some schools reaching over 800. The primary sources were caulking and fluorescent light ballasts. Studies of children and teachers in these buildings found elevated levels of certain PCB types in their blood compared to reference populations. Newer buildings, constructed after 1979, show much lower indoor air levels.
Health Effects of PCB Exposure
PCBs affect the body in multiple ways, and the type and severity of harm depends on the level and duration of exposure.
The most well-documented mass poisonings occurred in Japan (1968) and Taiwan (1979), where thousands of people consumed rice oil accidentally contaminated with heat-degraded PCBs. Victims developed severe skin eruptions called chloracne, liver damage, and a range of other symptoms. These incidents provided early, stark evidence of PCB toxicity in humans.
At lower, chronic exposure levels, PCBs act as endocrine disruptors, meaning they interfere with the body’s hormone systems. They can mimic or block hormones involved in thyroid function and reproduction. Some PCB congeners, called “dioxin-like” because of their flat molecular shape, activate the same cellular pathways as dioxins and are considered the most toxic. Others disrupt hormone signaling through different mechanisms. The net effect is interference with systems that regulate metabolism, growth, and fertility.
PCBs are classified as probable human carcinogens. The EPA has modeled excess cancer risk from PCB exposure across a range, with concern beginning at very low levels of chronic intake. Animal studies consistently show liver tumors and other cancers at sustained exposures. Kidney damage has also been observed in laboratory animals exposed to PCBs through inhalation and ingestion.
Effects on Children’s Development
Children exposed to PCBs before birth are particularly vulnerable. A comprehensive review of the research found consistent evidence linking prenatal PCB exposure to impaired cognitive development and attention problems that show up in middle childhood, roughly ages 6 to 12. There appeared to be a sex-specific pattern, with boys more affected than girls in terms of cognition and attention scores. Interestingly, the research found little or no link between prenatal PCB exposure and early childhood motor development or the risk of ADHD or autism spectrum disorder. The effects seem to be subtler, showing up as reduced performance on cognitive and attention tests rather than as diagnosed conditions.
Links to Diabetes
More recent research has connected PCB exposure to metabolic disease. In a study using national health survey data, one particular dioxin-like congener was the type most strongly associated with diabetes risk. People with higher blood levels of this congener had roughly double the odds of having diabetes compared to those with lower levels, even after adjusting for other risk factors like age, weight, and smoking. When combined with elevated levels of other specific congeners, the odds climbed even higher.
How PCB Levels Are Measured
PCB exposure is assessed through blood tests that measure the concentration of individual congeners in blood serum. Because PCBs dissolve in fat, results are typically reported as nanograms per gram of blood lipids. In the general U.S. population, the median total concentration of 40 commonly measured congeners is about 154 nanograms per gram of lipid. People with occupational exposure or heavy dietary exposure from contaminated fish can have levels several times higher.
For drinking water, the EPA sets a maximum contaminant level of 0.001 milligrams per liter, equivalent to 1 part per billion. That extremely low threshold reflects how toxic PCBs are at sustained low-dose exposure.
Global Efforts to Eliminate PCBs
The Stockholm Convention on Persistent Organic Pollutants, an international treaty, targets PCBs for complete elimination. PCBs are the only pollutant under the Convention with two binding deadlines: all PCB-containing equipment was to be removed from use by 2025, and all associated waste must be managed in an environmentally sound manner by 2028. Progress has been uneven, as vast quantities of PCB-containing equipment remain in use in developing countries, and contaminated sites around the world continue to leach these chemicals into soil and water.
Products containing PCBs, particularly old electrical equipment and building materials, are still widely present and commonly discarded or incinerated as waste. Each improper disposal creates another source of environmental contamination, extending the legacy of a chemical that was first sold nearly a century ago.