The group of chemicals known as polychlorinated biphenyls (PCB) is one of the original twelve POPs covered by the Stockholm Convention. They possess properties including longevity, heat absorbance and form an oily liquid at room temperature that is useful for electrical utilities and in other industrial applications.

PCB are aromatic hydrocarbon compounds, consisting of two benzene rings linked by a carbon-carbon bond. The hydrogen atoms can be substituted by up to ten chlorine atoms. PCB exist as viscous liquids or resins and may be colourless or yellowish with a strong, characteristic smell. One of the most important characteristics of PCB is that they have excellent dielectric properties, are resistant to chemical and thermal degradation (they decompose at high temperatures above 1000 °C), are not affected by light and are not flammable.

Due to their physico-chemical properties, PCB were manufactured worldwide for use in a wide range of applications, most importantly as insulating fluids in transformers. PCB were also used in other types of closed and semi-closed applications, such as capacitors, as well as in so-called ‘open applications, such as paints, sealants and carbon paper.

PCB can cause serious health effects in humans and animals, including reproductive impairment and immune system dysfunctions. The International Agency for Research on Cancer (IARC) classified PCB as Group 1 “carcinogenic to humans”. PCB have been detected in human milk, and in some cases, observed levels for indicator PCB were several orders of magnitude higher than the WHO safety level . Once in the environment, PCB enter the food chain. More than 90% of human exposure to PCB is through food.

PCB are listed in Annex A to the Stockholm Convention. The production and new uses of PCB are banned, and Parties to the Stockholm Convention must eliminate the use of PCB in equipment by 2025 and to ensure the environmentally sound waste management of liquids containing PCB and equipment contaminated with PCB by 2028.