PCBs (polychlorinated biphenyls) are bad because they persist in the environment for years, accumulate in living tissue, and cause a range of serious health problems including cancer, hormone disruption, immune suppression, and developmental harm in children. Despite being banned from manufacturing in the late 1970s, PCBs remain a concern today because they break down extremely slowly and continue circulating through soil, water, air, and the food supply.
What PCBs Are and Why They Don’t Go Away
PCBs are synthetic chemicals made from benzene that’s been converted to biphenyl and then loaded with chlorine atoms, anywhere from 1 to 10 per molecule. They were widely used from the 1930s through the 1970s in electrical equipment, hydraulic fluids, paints, caulking, and plastics because of their chemical stability and resistance to heat. That same stability is exactly what makes them dangerous: the chemical bonds holding those chlorine atoms in place are incredibly difficult for natural processes to break apart.
PCBs are classified as persistent organic pollutants, meaning they have half-lives measured in years in soil and sediment. High-chlorine versions are the worst offenders. They resist metabolic breakdown in living organisms and linger in the environment far longer than their low-chlorine counterparts. Once released, PCBs don’t stay put. They cycle between soil, water, and air, spreading far from their original source. They’ve been detected in Arctic wildlife thousands of miles from any industrial site.
Cancer Risk
The International Agency for Research on Cancer classifies PCBs as carcinogenic to humans. The EPA’s own peer-reviewed cancer reassessment reached a similar conclusion, categorizing them as probable human carcinogens. Studies of workers with occupational PCB exposure found elevated rates of rare liver cancers and malignant melanoma, a serious form of skin cancer. The cancer risk isn’t limited to massive industrial exposures; it’s the long-term, low-level accumulation through food and environment that concerns researchers most, because PCBs build up in body fat over a lifetime.
Hormone Disruption
PCBs interfere with your thyroid hormones, which regulate metabolism, energy, and brain development. They do this through multiple pathways. Some PCB types trigger enzymes in the liver that break down and flush out thyroxine (T4), the main thyroid hormone, faster than your body can replace it. Others physically bump T4 off its transport proteins in the blood, leaving less available to reach your tissues. Still others act directly on the pituitary gland, disrupting the signaling loop that keeps thyroid levels balanced.
Research on older women (over age 60) found that higher PCB levels in the body were associated with lower T4 and higher TSH, the pituitary hormone that rises when it senses thyroid levels dropping. This pattern is consistent with the body trying to compensate for a real shortage. Thyroid disruption has downstream effects on weight, mood, heart function, and in pregnant women, fetal brain development.
Harm to Children’s Brain Development
Some of the most troubling evidence involves children exposed to PCBs before birth. A comprehensive systematic review of the research found a clear link between prenatal PCB exposure and problems with cognitive development and attention during middle childhood, roughly ages 6 to 12. Boys appeared to be more vulnerable, showing worse scores on cognition and attention tests compared to girls with similar exposure levels.
The review found little connection between prenatal PCB exposure and motor development problems or diagnoses like ADHD and autism in early childhood. But the cognitive and attention effects that emerge later suggest PCBs may cause subtle brain changes that only become apparent when children face more demanding academic and social tasks. These aren’t dramatic impairments in most cases. They’re shifts in the population average that translate to real disadvantages for individual kids at school.
Weakened Immune Response
PCB exposure suppresses the immune system’s ability to respond to vaccines, which is one of the most concrete and measurable ways these chemicals affect health. A study of children in the Faroe Islands, where the traditional diet includes whale blubber contaminated with PCBs, found striking results. For every doubling of PCB exposure, diphtheria antibody response dropped by 24% in 18-month-old children. In 7-year-olds, tetanus antibody response fell by 16% per doubling of exposure.
Both prenatal and postnatal exposure contributed to the effect. Among the 7-year-olds, 26 children had diphtheria antibody levels below what’s considered protective two years after receiving a booster shot. In practical terms, this means PCB exposure can leave vaccinated children without adequate protection against infectious diseases they were immunized against.
How People Are Still Exposed
Even though PCB production was banned decades ago, exposure continues through two main routes: food and old buildings.
Food is the primary source for most people. Fish and seafood account for 30 to 75% of total dietary PCB exposure across most population groups, with fatty fish being the single biggest contributor. Farmed salmon alone accounts for about 36% of PCB-related compounds found in all fish. Meat and dairy products add another 9 to 34% and 7 to 25%, respectively. PCBs dissolve in fat, so fattier foods carry higher concentrations. In 2018, the European Food Safety Authority cut its recommended safe weekly intake of dioxins and dioxin-like PCBs by a factor of seven, down to 2 picograms per kilogram of body weight per week, based on evidence of harm to reproductive health at levels previously considered safe.
Buildings constructed or renovated between 1950 and 1979 are the other significant source. The EPA has flagged potential widespread PCB contamination in schools and other structures from that era. Caulking from this period can contain up to 40% PCBs by weight and actively releases them into indoor air. PCBs from caulk also migrate into surrounding materials like brick and wood, making remediation complicated and expensive. Old fluorescent light ballasts from the same era are another risk. These have long exceeded their designed lifespan, and when they rupture, they can release PCBs directly into occupied spaces.
Global Efforts to Eliminate PCBs
PCBs are the only persistent organic pollutant under the Stockholm Convention with two binding global deadlines: all PCB-containing equipment must be disposed of by 2025, and all associated waste must be eliminated by 2028. Whether every country will meet those targets is another question. Millions of old transformers, capacitors, and building materials around the world still contain PCBs, and safe disposal requires specialized high-temperature incineration. In the meantime, PCBs already in soils, sediments, and ocean ecosystems will continue cycling through the food chain for generations.