Polychlorinated biphenyls, known as PCBs, are a family of synthetic chlorinated compounds that were widely used as insulating fluids in electrical transformers from the late 1920s through the 1970s. They were prized for their chemical stability, resistance to fire, and excellent electrical insulating properties. That same chemical stability, however, is precisely what makes them so dangerous: PCBs barely break down in the environment, they accumulate in living tissue, and they are now classified as known human carcinogens. Despite bans stretching back decades, millions of tonnes of PCB-containing equipment remain in use or in storage around the world, and the health and environmental consequences continue to unfold.
What PCBs Actually Are and Why They Ended Up in Transformers
PCBs are a group of 209 related chemical compounds, each built on a two-ring carbon structure (biphenyl) with varying numbers of chlorine atoms attached. The number and position of those chlorine atoms determine each compound’s specific properties, but as a class, PCBs share several traits that made them attractive for industrial use: they resist heat, do not conduct electricity, are not easily flammable, and remain chemically stable under conditions that would degrade most organic fluids.
Transformers need an insulating fluid that can also carry heat away from the coils inside. Mineral oil has traditionally served that role, but PCBs offered a significant safety advantage because they would not catch fire the way mineral oil could. In the United States, PCBs were manufactured from 1929 through the mid-1970s and were used primarily as electrical insulating fluids in capacitors and transformers, though they also found their way into hydraulic fluids, heat transfer systems, lubricants, plasticizers, fire retardants, caulks, adhesives, plastics, and even carbonless copy paper.1PubMed. Applications of polychlorinated biphenyls Many PCB-filled transformers and capacitors continued operating for decades after manufacturing stopped, because the equipment itself was still functional and there was no easy way to dispose of the fluid inside.
Why PCBs Refuse to Go Away
The core danger of PCBs starts with persistence. The chlorine atoms bonded to the biphenyl backbone make these molecules extraordinarily resistant to biological and chemical breakdown. In soil, sediment, and water, PCBs can persist for decades to centuries depending on the specific compound. Highly chlorinated variants are especially stubborn, resisting microbial degradation that would break down most organic pollutants within months or years.
When a transformer leaks, is improperly disposed of, or is damaged in a fire or flood, the PCBs in the oil can spread into surrounding soil and groundwater. From there, they enter aquatic systems. Because PCBs dissolve readily in fats rather than water, they are taken up by organisms and stored in fatty tissue rather than being excreted. This means that as you move up a food chain, PCB concentrations increase dramatically at each step, a process called biomagnification. In a marine food web, certain PCB compounds showed trophic magnification factors between roughly 1.5 and 4.2, meaning concentrations multiplied by that amount at each trophic level.2PubMed. Biomagnification of persistent organic pollutants along a high-altitude aquatic food chain in the Tibetan Plateau: Processes and mechanisms Top predators accumulate the highest burdens. In San Francisco Bay, harbor seals had the highest concentrations of PCBs of any species studied, exceeding thresholds for potential harm, because of their position at the top of the food web.3PubMed. Food web bioaccumulation model for polychlorinated biphenyls in San Francisco Bay, California, USA
How PCBs Harm Human Health
The evidence for PCB toxicity in humans comes from occupational studies, accidental mass poisoning events, and decades of environmental monitoring. The effects touch nearly every organ system, though the skin, liver, nervous system, and endocrine system bear the heaviest impact.
Dermatological effects are among the earliest and most visible signs of PCB exposure. Chloracne, a severe, persistent form of acne caused by chlorinated compounds, is the hallmark skin condition. Workers with moderate-to-high PCB and dioxin exposure have shown higher rates of skin hyperpigmentation, and the probability of developing it was statistically higher in those with elevated blood levels of PCBs and dioxin-like compounds.4PubMed. Hyperpigmentation and higher incidence of cutaneous malignancies in moderate-high PCB- and dioxin exposed individuals Beyond skin problems, exposed populations have reported higher rates of headaches, gum pigmentation, goiter, anemia, and joint and spine diseases. In the Taiwan Yucheng cohort, women exposed to PCBs reported anemia more than twice as often as unexposed controls, and exposed men reported arthritis and herniated discs at roughly four and three times the rate of controls, respectively.5PubMed Central. Chloracne, goiter, arthritis, and anemia after polychlorinated biphenyl poisoning: 14-year follow-Up of the Taiwan Yucheng cohort
Neurological and respiratory symptoms round out the clinical picture. Reviews of human exposure cases have identified impairment of liver function, respiratory findings, and neurological symptoms as consistent features of significant PCB uptake. Skin manifestations have even been observed in newborns whose mothers were exposed during pregnancy.6PubMed Central. Symptoms, signs and findings in humans exposed to PCBs and their derivatives
Cancer Risk and the IARC Classification
In 2013, the International Agency for Research on Cancer upgraded PCBs to Group 1, meaning “carcinogenic to humans.” Dioxin-like PCBs received the same classification.7PubMed. Use of mechanistic data in the IARC evaluations of the carcinogenicity of polychlorinated biphenyls and related compounds That is the agency’s highest category and puts PCBs alongside substances like asbestos and tobacco smoke.
The specific cancer types most linked to PCB exposure are melanoma and non-Hodgkin lymphoma. A systematic review and meta-analysis of epidemiological studies found a pooled risk increase of about 30% for melanoma among occupationally exposed workers, and population-based studies of non-Hodgkin lymphoma showed roughly 50% higher odds for people in the highest versus lowest category of blood PCB levels.8PubMed. Do polychlorinated biphenyls cause cancer? A systematic review and meta-analysis of epidemiological studies on risk of cutaneous melanoma and non-Hodgkin lymphoma Those same authors noted, however, that the evidence is not as iron-clad as the Group 1 label might imply for these specific cancers, with relatively few studies available for melanoma. The IARC classification rested heavily on mechanistic evidence alongside the epidemiological data.
A large Danish cohort study of people living in PCB-contaminated buildings added further texture. Overall cancer risk was not elevated for residents, but those in the highest exposure category had roughly a threefold higher risk of liver cancer and meningiomas compared to those with the lowest exposures, with signs of a dose-response relationship.9PubMed Central. Cancer Risk following Residential Exposure to Airborne Polychlorinated Biphenyls: A Danish Register-Based Cohort Study These findings suggest that the cancer risk from PCBs may be concentrated in specific organ sites rather than being a blanket increase across all cancers.
Endocrine Disruption and Effects on the Developing Brain
PCBs are among the most thoroughly studied endocrine-disrupting chemicals. They can interfere with thyroid hormone signaling, estrogen activity, and androgen pathways. This matters most during fetal development and early childhood, when hormones orchestrate the wiring of the brain.
Thyroid hormones play a crucial role in coordinating the timing of brain development, including the proliferation of neurons, their migration to the right locations, and the formation of connections between them. PCBs can alter the availability of thyroid hormone inside cells and interact directly with thyroid hormone receptors, leading to effects that look sometimes like too little thyroid hormone and sometimes like too much. Either direction can produce permanent changes in how neural networks are structured, potentially causing lifelong deficits in cognitive performance, motor function, and behavior.10PubMed. Endocrine disrupting polyhalogenated organic pollutants interfere with thyroid hormone signalling in the developing brain In some studies, these neurobehavioral outcomes differed between males and females, likely because PCBs also interfere with estrogen- and androgen-dependent brain development.
Beyond cognitive effects, animal research has shown that PCBs can suppress reproductive function by disrupting the brain circuits that regulate it, and may reduce motivation for social behavior and induce depressive-like symptoms through changes to dopamine and glutamate signaling in the brain’s limbic system.11PubMed Central. Endocrine-disrupting actions of PCBs on brain development and social and reproductive behaviors
The Yusho and Yu-Cheng Poisoning Disasters
The two most devastating episodes of mass PCB poisoning happened through contaminated cooking oil. In 1968 in western Japan, an event known as Yusho (“oil disease”) sickened thousands of people. In 1979, a nearly identical disaster struck central Taiwan, known as Yu-Cheng.12PubMed Central. PCB and PCDF congeners in the blood and tissues of yusho and yu-cheng patients In both cases, PCBs used as heat exchange fluids in the rice bran oil manufacturing process leaked into the cooking oil during deodorization.
Analysis of the contaminated Yu-Cheng oil found it was heavily contaminated with PCBs at an average concentration of 62 parts per million, along with polychlorinated quaterphenyls and polychlorinated dibenzofurans. These additional toxic byproducts were generated when the PCBs were heated above 200°C for prolonged periods during industrial use.13PubMed Central. PCBs, PCQs and PCDFs in Tissues of Yusho and Yu-Cheng Patients This detail matters for transformer oil as well: when PCB-containing transformer oil overheats or is involved in a fire, it can generate dioxin-like compounds that are even more toxic than the PCBs themselves. Much of what we know about the long-term health effects of PCBs, including the skin, joint, and blood disorders described earlier, comes from the decades-long follow-up of these two cohorts.
Damage to Wildlife and Marine Ecosystems
PCBs have not just harmed humans. Their persistence and biomagnification have turned them into a global ecological problem, especially in marine environments where long-lived top predators accumulate extreme body burdens.
A large European study of over a thousand stranded or biopsied marine mammals found that three out of four cetacean species studied, including striped dolphins, bottlenose dolphins, and killer whales, had mean blubber PCB concentrations that far exceeded all known marine mammal toxicity thresholds. Some small or declining populations of bottlenose dolphins and killer whales in the northeast Atlantic showed low recruitment rates consistent with PCB-induced reproductive toxicity. Despite the mid-1980s EU ban, blubber PCB concentrations initially dropped but then stabilized, meaning the problem has effectively stalled rather than continuing to improve.14Scientific Reports. PCB pollution continues to impact populations of orcas and other dolphins in European waters
Reproductive harm is a particularly insidious effect. UK-stranded harbour porpoises collected over more than 25 years showed that PCB exposure was associated with reduced testes weight in adult males in good body condition, suggesting the chemicals are directly impairing reproductive capacity in the wild.15PubMed. Polychlorinated biphenyls are associated with reduced testes weights in harbour porpoises (Phocoena phocoena) For populations that are already small or geographically isolated, even a modest drop in reproductive success can push a species toward decline.
Where the World Stands on Cleanup
Regulating PCBs started nationally in the 1970s. The United States banned their manufacture in 1979 under the Toxic Substances Control Act, and the European Union followed with its own ban in the mid-1980s. Globally, the Stockholm Convention on Persistent Organic Pollutants, which entered force in 2004, requires parties to achieve environmentally sound management of PCBs by 2028.
Progress has been uneven. At most 30% of countries that signed the Stockholm Convention are on track to meet the 2028 deadline. Over 10 million tonnes of PCB-containing materials remain worldwide, with the largest stockpiles in countries that lack the infrastructure to manage them. A few countries have made real headway: Canada (specifically Ontario) and Czechia have reduced their stocks of pure PCBs by 99% over the past decade. The United States, which is not a party to the Stockholm Convention, has seen only about a 3% decrease in its PCB mass since 2006 and maintains a large but poorly inventoried stock.16PubMed Central. Persistent Problem: Global Challenges to Managing PCBs
The gap between regulatory ambition and on-the-ground reality is one of the defining features of the PCB problem. Even in countries with strict laws, old transformers sitting on utility poles or in building basements may still contain PCB-laden oil because replacing them is expensive and logistically complex.
Testing Transformer Oil for PCBs
If you manage or encounter an older transformer, the oil inside may or may not contain PCBs. Transformers manufactured before the late 1970s are the main concern, but cross-contamination during servicing means that even some newer units can test positive. Statistical analysis of transformer fleets does show a trend: newer transformers tend to have lower PCB content, though the relationship is weak enough that age alone is not a reliable predictor.17IOP Publishing. Analysing the correlation between polychlorinated biphenyl (PCB) content in transformer oil to ensure compliance with environmental regulations
The gold standard for PCB detection in transformer oil is gas chromatography coupled with mass spectrometry, which can identify and quantify individual PCB compounds with high accuracy. For field screening, simpler methods have been developed, including immunoassay-based tests that use a two-step liquid-liquid extraction process. These can provide rapid results and have been validated against laboratory analysis of real transformer oil samples.18PubMed. Simple immunoassay for detection of PCBs in transformer oil In many jurisdictions, transformer oil must be tested before equipment is decommissioned, serviced, or moved, and there are specific concentration thresholds (often 50 parts per million) that determine whether the oil must be handled as hazardous waste.
Destroying PCBs in Contaminated Oil
Getting rid of PCBs is not as simple as burning the oil. Incomplete combustion can generate dioxins and furans that are even more toxic. High-temperature incineration in specialized facilities (typically above 1,200°C) has been the traditional disposal route, but it is expensive and not available everywhere.
Chemical dechlorination offers an alternative. The idea is to strip the chlorine atoms off the biphenyl backbone using reactive chemicals, converting PCBs into less toxic or non-toxic products. Several approaches have been developed. Using nanometric sodium hydride combined with a titanium catalyst achieved 99.9% destruction of PCBs under relatively mild conditions.19PubMed. A practical approach to the degradation of polychlorinated biphenyls in transformer oil Another method using polyethylene glycol with potassium hydroxide and aluminum at 150°C for four hours achieved better than 99.99% removal of the most toxic PCB compounds, including the dioxin-like congeners.20Bulletin of the Korean Chemical Society. Destruction and Removal of PCBs in Waste Transformer Oil by a Chemical Dechlorination Process
One particularly attractive approach treats the oil directly with metallic calcium in an alcohol mixture, which dechlorinates the PCBs while leaving the transformer oil itself clean enough to be recirculated for industrial use. This avoids the need to destroy perfectly good insulating oil along with the contaminant.21Separation and Purification Technology. Direct and complete cleansing of transformer oil contaminated by PCBs In practice, the choice of method depends on the concentration of PCBs, the volume of oil, local regulations, and the availability of specialized facilities.
Protecting Workers Who Handle PCB Equipment
People who work with old transformers, electrical equipment, or PCB waste face a direct exposure risk. The highest contamination levels have been detected at e-waste recycling sites, and a clear correlation exists between working in PCB-contaminated environments and elevated blood PCB concentrations.22PubMed Central. Polychlorinated Biphenyls (PCBs) in the Environment: Occupational and Exposure Events, Effects on Human Health and Fertility PCBs can enter the body through skin contact, inhalation of vapors or contaminated dust, and accidental ingestion.
The good news is that proper safety measures work. A study monitoring PCB transport workers found that during initial handling without special precautions, their urinary levels of PCB metabolites rose several-fold after work. After recommended exposure-reduction measures were implemented, workers handling the same type of PCB equipment showed no increase in their PCB levels.23PubMed. Monitoring OH-PCBs in PCB transport worker’s urine as a non-invasive exposure assessment tool Practical protections include chemical-resistant gloves and coveralls, respiratory protection when cutting or heating equipment that may contain PCBs, containment of spills and drips, and decontamination procedures before leaving the work area.
What Replaced PCBs in Modern Transformers
After PCBs were banned, the electrical industry shifted primarily back to mineral oil, which remains the most common transformer insulating fluid. Mineral oil is cheaper and performs well, though it carries a fire risk that PCBs did not. For applications where fire safety or environmental protection is paramount, silicone-based fluids and synthetic esters have been used.
More recently, plant-based insulating fluids have gained significant traction. Made from vegetable oils like soybean or rapeseed, these fluids are non-toxic and ultimately biodegradable. After more than 20 years of field experience, plant-based oils are now used in over 600,000 transformers worldwide. They have proven especially useful in applications where mineral oil fails to meet fire-safety or environmental standards, such as transformers installed inside buildings or in environmentally sensitive locations.24Renewable and Sustainable Energy Reviews. A critical review of plant-based insulating fluids for transformer: 30-year development The shift represents a genuine improvement: if a plant-based oil spills, it biodegrades in weeks to months rather than persisting for decades. The era of replacing one persistent pollutant problem with another appears, at least in the transformer fluid world, to be over.