PCBs (polychlorinated biphenyls) are found in river and lake sediments, soil near industrial sites, the atmosphere, ocean water, building materials from the mid-20th century, and the fatty tissue of animals throughout the food chain. Despite a U.S. manufacturing ban in 1979 and an international treaty in 2001, PCBs persist in virtually every environmental compartment because they resist natural breakdown. Some forms take more than seven years to degrade in soil, and heavily chlorinated varieties last even longer.
Why PCBs Are Still Everywhere
PCBs were manufactured between 1929 and 1977 for use as coolants and lubricants in transformers, capacitors, and other electrical equipment. They were prized because they don’t burn easily and are excellent insulators. But those same properties, chemical stability and resistance to heat, make them extraordinarily persistent once released. PCBs are also hydrophobic, meaning they repel water and cling to fats, organic matter, and sediment particles instead of dissolving and flushing away.
The U.S. banned PCB production in 1979 under the Toxic Substances Control Act. Internationally, the Stockholm Convention on Persistent Organic Pollutants, adopted in 2001 and in force since 2004, requires signatory countries to eliminate or reduce PCB releases. Yet the chemicals manufactured decades ago continue to cycle through air, water, soil, and living organisms.
River and Lake Sediments
Sediment at the bottom of rivers and lakes is one of the largest PCB reservoirs. PCBs bind tightly to fine particles and organic matter, settling into layers of mud where oxygen levels are low and degradation is slow. Concentrations vary enormously depending on proximity to former industrial discharge points. In moderately contaminated river sediment, levels around 255 micrograms per kilogram are common. At historically polluted sites, concentrations have been measured as high as 6,700 milligrams per kilogram, roughly 26,000 times greater.
The Hudson River in New York is one of the most well-known examples. A 200-mile stretch from Hudson Falls to New York City was placed on the EPA’s National Priorities List in 1984 after decades of PCB discharge from electrical manufacturing. Even after extensive dredging, contaminated sediment remains a source of ongoing exposure for aquatic life and, through the food chain, for people.
Soil Near Industrial and Urban Areas
PCBs reach soil through direct spills, leaching from landfills, runoff from contaminated sites, and fallout from the atmosphere. Once in soil, they bind to organic matter and stay put. The lighter PCB forms (those with fewer chlorine atoms) break down in aerobic soil with half-lives of roughly 7 to 17 months. Heavier, more chlorinated forms persist far longer: hexachlorinated varieties have soil half-lives of 3.4 to over 5 years, and the most heavily chlorinated types exceed 5 years with no firm upper bound established.
This means a single spill can leave detectable contamination in soil for decades. Urban soils near old industrial zones, electrical substations, and waste disposal sites tend to carry the highest burdens.
The Atmosphere and Long-Range Transport
PCBs evaporate slowly from contaminated soil, water, and building surfaces into the air, especially in warmer weather. Once airborne, lighter PCB forms can travel thousands of miles before redepositing. This process explains why PCBs are detected in Arctic ice, remote mountain lakes, and ocean regions far from any industrial source. Heavier forms deposit closer to their origin because they attach to particles and fall out of the atmosphere more readily.
Modeling research from MIT has shown that midlatitude processes, evaporation and transport from industrialized regions in temperate zones, are the primary driver of PCB contamination in the Arctic, rather than re-emission of PCBs already deposited there. In practical terms, this means the pollution generated decades ago in places like the U.S., Europe, and Russia is still redistributing itself across the globe.
Water
PCBs are poorly soluble in water, and that solubility drops sharply as the molecules get larger. The simplest biphenyl dissolves at roughly 130 millimoles per cubic meter, while the fully chlorinated form (with ten chlorine atoms) dissolves at just 0.007 millimoles per cubic meter. Because of this low solubility, PCBs in lakes and rivers are overwhelmingly attached to suspended particles or settled sediment rather than truly dissolved. Still, even trace dissolved concentrations matter because aquatic organisms absorb PCBs directly through their gills and skin, starting the process of bioaccumulation.
The Food Chain
PCBs concentrate dramatically as they move up the food chain, a process called biomagnification. Small organisms absorb PCBs from water and sediment. Small fish eat those organisms and store the chemicals in their fat. Larger predatory fish accumulate still higher levels, and top predators like tuna, dolphins, and seals carry the greatest burden.
Monitoring data from the Adriatic Sea illustrates this pattern clearly. Mussels and small fish reflect background pollution levels, but wild bluefin tuna sampled in the same waters had some of the highest PCB concentrations reported in the scientific literature. Dolphins carried even higher fat-adjusted levels than tuna or sea turtles. Two specific PCB forms, known as congeners 138 and 153, resist metabolic breakdown so effectively that they accumulate at the highest levels across nearly all marine species studied.
Buildings and Older Infrastructure
PCBs are embedded in millions of structures built or renovated before the late 1970s. The most common sources inside buildings are caulking compounds (the flexible sealant around windows and expansion joints), oil-based paints, and fluorescent light ballasts. On ships built before 1979, PCBs appear in cable insulation, rubber gaskets, electronic equipment, and specialty coatings.
These materials slowly release PCBs into indoor air and dust over time. NIOSH-funded research found PCB levels as high as 81 parts per million in dust inside school ventilation systems, well above the EPA’s 50 ppm limit for bulk materials. The same research uncovered an ongoing problem with old fluorescent light fixtures leaking PCBs, something regulators had assumed was resolved decades earlier. Schools, office buildings, and public housing from the 1950s through 1970s are the structures most likely to contain these materials.
How PCBs Reach People
For the general population, the two main routes of PCB exposure are diet and breathing indoor air, and they contribute roughly equal amounts. A coordinated study measuring both routes in Midwestern communities found dietary PCB exposure ranging from 1.2 to 120 micrograms per year, while inhalation exposure ranged from 0.2 to 160 micrograms per year.
Within the diet, meat is the single largest source, contributing about 50% of total dietary PCB intake. Dairy products account for roughly 25%, fish about 15%, and all other foods the remaining 10%. This surprises many people who associate PCB exposure primarily with fish. While certain fish species, particularly fatty predatory fish from contaminated waters, carry very high PCB levels per serving, meat and dairy make up a much larger share of most people’s overall diet, pushing their total contribution higher.
Inhalation exposure is particularly significant for people who live or work in older buildings with PCB-containing caulk, paint, or light fixtures. In schools with these materials, airborne PCB exposure alone ranged from 0.7 to 116 micrograms per year, comparable to the total dietary route for some individuals.