polychlorinated biphenyl

polychlorinated biphenyl is a class of synthetic organic chemicals characterized by the chlorination of a biphenyl molecule. These compounds were once widely manufactured for their desirable industrial properties, including chemical stability and electrical insulating capabilities. Their persistence in the environment and toxicity led to a global ban under the Stockholm Convention on Persistent Organic Pollutants.

Chemical properties and structure

The basic structure consists of two connected benzene rings, which can be chlorinated at any of ten available positions, leading to 209 possible congeners. The degree and pattern of chlorination significantly influence physical properties such as melting point and vapor pressure. These compounds exhibit high thermal stability and are generally inert to acidic and basic conditions, contributing to their environmental persistence. Their lipophilicity allows them to bioaccumulate readily in the fatty tissues of organisms.

Production and uses

Commercial production of these compounds began in 1929, primarily by the Monsanto Company in the United States and Bayer in Germany. They were marketed under trade names like Aroclor and Clophen. Major applications included use as dielectric fluids in transformers and capacitors, as plasticizers in paints and cement, and as hydraulic fluids. Their non-flammability made them particularly valuable in electrical equipment, a use championed by companies like General Electric.

Environmental and health effects

Widespread release occurred through industrial discharge, improper disposal, and incidents like the Yushō disease outbreak in Japan. These chemicals enter the food chain and biomagnify, posing significant risks to wildlife and humans. The World Health Organization classifies them as probable human carcinogens, linked to effects on the immune system, nervous system, and endocrine system. High-profile contamination events have been documented at sites like the Hudson River and the Great Lakes.

Regulation and remediation

Growing concern led to action by the United States Environmental Protection Agency, which banned manufacture under the Toxic Substances Control Act of 1976. Globally, the Stockholm Convention mandates the phase-out and destruction of existing stocks. Remediation strategies include incineration at high temperatures, chemical dechlorination, and bioremediation using microorganisms like Dehalococcoides. The cleanup of major sites, such as the Fox River in Wisconsin, has involved extensive dredging and sediment capping.

History and discovery

The compounds were first synthesized in 1881 by German chemists Hermann Schmidt and Gustav Schultz. Their industrial potential was recognized in the late 1920s, leading to rapid commercialization. The pivotal turning point was the work of ecologist Sören Jensen in 1966, who identified their pervasive environmental presence while studying DDT in Swedish wildlife. This discovery triggered decades of research by agencies like the National Institute of Environmental Health Sciences and ultimately led to international policy action.