Polychlorinated biphenyls (PCBs) are chemical compounds once widely used in industrial manufacturing, but now banned due to their harmful effects on human and animal health. PCBs have a diverse structure and were commonly used in electrical transformers, industrial cooling systems, paints, caulking products, and plastic coatings. Exposure to large amounts of PCBs can cause severe skin disorders, liver cancer, developmental problems in children, and affect the thyroid and immune system. PCBs are persistent organic pollutants that can bioaccumulate in the environment and remain for long periods of time. Despite bans on new production, PCBs still persist in the environment and can be found in air, soil, water, and even remote areas such as the Arctic.
Polychlorinated biphenyls, also known in more casual conversation as PCBs, are a group of chemical compounds that were once widely used in industrial manufacturing. The compounds are natural to the extent that they are usually byproducts of burning tar or other related processes, but scientists have also found ways to produce them artificially in large quantities, which is the format in which they have been most popular in the past. They were often used in things like industrial coolants and power transformer stabilizers, as well as some paints and sealants. In the mid-1970s, scholars began to question their safety and tests have amply demonstrated that these compounds can be very harmful to both humans and animals. They have been banned from use in most parts of the world, but still persist in the environment in many places.
Basic composition
Structurally, PCBs are a diverse group of compounds. The basic structure is a pair of benzene rings, which consist of a circle of six carbon atoms with attached hydrogen atoms. The rings have double bonds and each ring has one or more chlorine atoms. The number of chlorine atoms per ring can range from one to ten, resulting in 209 possible different compounds. As a group they have the chemical formula C12H10-xClx.
How and why they were used
There have been a couple of notable uses for these compounds over the years, but they’ve traditionally been most popular in the manufacture of things like electrical transformers and industrial cooling systems. Adding the chemical to cooling and insulation fluids helped keep them stable and made them more efficient.
The chemical also has a number of adhesive properties and, as a result, was a common addition to paints, caulking products and plastic coatings for things like appliances and structures exposed to the outdoor elements. Most of the uses were industrial, but some things like waterproof coatings and floor finishes sold for home use also contained high levels of the compound.
International ban
Modern researchers today generally agree that this class of chemicals, while effective, carry a number of serious risks to human health. As a result, PCBs were banned in the United States by an act of Congress in 1979 and in most other countries by the Stockholm Convention on Persistent Organic Pollutants, an international regulatory body, in 2001. These bans basically forbade the continued manufacture and sale of chemicals; they generally had no effect on appliances or products already in use. Therefore, there are still PCB-containing fluids and coatings at play in households and businesses. As a result, people carrying out repairs or renovations to older structures often need to be specially trained in how to handle the chemical if they come in contact with it.
Effects of exposure
Most people are exposed to only low doses of PCBs, which the human body is generally capable of breaking down. These toxins are, however, cumulative. This means they build up in the body over time.
The effects of exposure to large amounts of the compound in any of its forms can be quite profound. Acute toxicity can manifest as severe skin disorders. Exposed animals can develop liver cancer, and polychlorinated biphenyls are also considered probable carcinogens in people. These substances have been shown to cause developmental problems in children who have been exposed to them. The thyroid and immune system can also be affected by PCBs.
Role of bioaccumulation
The effects of polychlorinated biphenyls are in part due to their ability to bioaccumulate. When they are attached to sediment in the water, they are usually eaten by small animals and fish. This can increase the concentration of polychlorinated biphenyl in these creatures. Any other organism that then eats the contaminated fish is subject to a much higher dose of the chemical. People who eat a lot of fish from contaminated water can be exposed this way.
Persistence in the environment
Despite bans on new production, these compounds still find their way into the environment. This most often happens due to chemical leaks or fires; Improperly dumped industrial waste can also cause contamination. Some consumer products that contain polychlorinated biphenyls can also leach the chemical if they are disposed of in landfills not designed to process hazardous waste. Incineration can also be a contributing cause.
Their general resistance to degradation means PCBs can remain in the environment for long periods of time and are known as persistent organic pollutants. The chemical properties can vary, usually depending on the number and location of these chlorine atoms. In general, the more chlorine atoms a compound has, the more resistant it is to decay.
PCBs can be found in the air, soil and water and have even been found in remote areas such as the Arctic. Polychlorinated biphenyl molecules with few chlorine atoms are light and are often found in the air. Molecules with more chlorine atoms are typically heavier and more likely to be toxic and carcinogenic. These heavy compounds are frequently found in sediments in water.
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