Polychlorinated biphenyls, or PCBs, are a class of compounds that share a similar structure -- two benzene rings linked by a single bond, with a varying number of chlorine atoms attached to the benzene rings. These compounds were widely produced and used in industry from the 1930s through the 1970s but were later found to have toxic effects on humans and the environment. PCBs that exhibit similar toxicity to dioxins are called dioxin-like PCBs.
Characteristics of PCBs
PCBs are colorless, odorless viscous liquids and largely insoluble in water. They have low vapor pressures at room temperature, which means that although they can evaporate, they are not very volatile. They are fairly nonreactive and degrade very slowly, so they can persist for a very long time in the environment. They have been found in many ecosystems, including sites far from where they were produced; they can also be taken up by plants, small animals and fish, accumulating up the food chain and potentially impacting people who consume the food.
Dioxin-Like PCBs
Some PCBs exhibit toxicity comparable to chemicals called polychlorinated dibenzo-p-dioxins -- usually dubbed dioxins in the popular press. These PCBs are called dioxin-like PCBs, and there are 12 of them. A simple way to imagine their structure is by picturing the benzene rings as two hexagons with a single line joining them. The carbons at either end of this single line are carbon 1 and 1', so the other members of the benzene rings are 2 and 2', 3 and 3', etc. Dioxin-like PCBs have at most one chlorine on carbon 2 & 2'; they never have a chlorine on carbon 6 or 6' and generally have chlorines on 3, 3', 4, 4' and sometimes 5 and 5'.
PCBs in the Environment
Dioxin-like PCBs are coplanar, meaning that all the carbons and chlorines lie in the same plane. Their primary distinguishing characteristic vis-a-vis other PCBs is their toxicity; dioxin-like PCBs are more toxic than other compounds in this same class. In the environment, dioxin-like PCBs and non-dioxin-like PCBs are found mingled together. Although they are no longer manufactured, they may still sometimes be released into the environment by burning wastes in incinerators, illegal dumping or poor handling of hazardous wastes. Once in the environment, they cycle among air, water and soil until they eventually break down.
Toxicity
According to the EPA, dioxin-like PCBs are probable human carcinogens, with clear evidence they cause cancer in animals. They damage the immune system and have reproductive toxicity, resulting in decreased birth weight. The immune system is one of the most sensitive targets of these chemicals. They also decrease thyroid hormone levels and cause learning deficits in animal models such as monkeys; according to the EPA, it is believed they cause similar effects in humans.
