What are they?
Polychlorinated Biphenyls (PCBs) are human-made chemicals that are highly toxic and persistent in the environment. They were once widely used in electrical, heat transfer and hydraulic equipment, and in some commercial products like paint, plastics, adhesives and surface coatings. In the 1970s and 1980s, their use was severely restricted due to their harmful environmental impacts, and in 1980 Canada banned their import.
Because PCBs are very stable and do not break down easily, they are still present in the environment today. PCBs can still be found in industrial equipment and electrical installations, and may be released (along with other toxic chemicals) through fires. PCBs can enter aquatic ecosystems through leaks and spills, industrial effluent, and leaching from uncontained landfills. PCBs can also become airborne and travel long distances on wind currents before they are deposited back to land and water in rain and snow.
PCBs are organic molecules that contain chlorine atoms. There are 209 numbered congeners or subtypes of PCBs. These can be organized into groups according to the number of chlorine atoms in each molecule (e.g., monochlorobipenyls have one chlorine atom per molecule, trichlorobiphenyls have three chlorine atoms per molecule).
Three different trichlorobiphenyl molecules, each containing three chlorine atoms.
Why do they matter?
PCBs are toxic to fish at low concentrations and are probable human carcinogens (cancer-causing substances). Certain congeners are more toxic than others, but PCBs are almost always found as complex mixtures of multiple congeners. In rivers and lakes, PCBs tend to accumulate primarily in sediments. Because of this, national guidelines for PCBs apply to how much is present in the sediment, rather than in the water.
PCBs bioaccumulate in organisms, especially those organisms that live in sediments or feed on the bottom of lakes and rivers. PCB concentrations also progressively increase, or biomagnify, up the food chain.
PCB concentrations increase over time in fish and when larger predatory fish eat smaller fish that have PCBs in their tissues.
How are they measured?
In aquatic ecosystems, PCBs can be measured in water, in sediments and in the tissues of organisms like fish. These samples must be submitted to a lab for analysis. PCB concentrations may be reported by labs as total PCBs, or individually by congener or congener groupings.
Related to PCBs: The greater the organic matter content of sediments, the more likely they are to accumulate PCBs if exposed.