Polychlorinated biphenyl
Overview
Polychlorinated biphenyls (PCBs; CAS number
CAS registry number
CAS Registry Numbersare unique numerical identifiers assigned by the "Chemical Abstracts Service" toevery chemical described in the...

 ) are a class of organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s (specifically organochloride
Organochloride
An organochloride, organochlorine, chlorocarbon, chlorinated hydrocarbon, or chlorinated solvent is an organic compound containing at least one covalently bonded chlorine atom. Their wide structural variety and divergent chemical properties lead to a broad range of applications...

s) with 2 to 10 chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 atoms attached to biphenyl
Biphenyl
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...

, which is a molecule composed of two benzene rings. The chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 for PCBs is C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

12H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

10-xCl
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

x. There are 209 different variable of these arrangements and orientations that make up the PCB family, but only 130 are commercially used.

PCBs were widely used as dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 and coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

 fluids, for example in transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s, capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s, and electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

s.
Encyclopedia
Polychlorinated biphenyls (PCBs; CAS number
CAS registry number
CAS Registry Numbersare unique numerical identifiers assigned by the "Chemical Abstracts Service" toevery chemical described in the...

 ) are a class of organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s (specifically organochloride
Organochloride
An organochloride, organochlorine, chlorocarbon, chlorinated hydrocarbon, or chlorinated solvent is an organic compound containing at least one covalently bonded chlorine atom. Their wide structural variety and divergent chemical properties lead to a broad range of applications...

s) with 2 to 10 chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 atoms attached to biphenyl
Biphenyl
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...

, which is a molecule composed of two benzene rings. The chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 for PCBs is C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

12H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

10-xCl
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

x. There are 209 different variable of these arrangements and orientations that make up the PCB family, but only 130 are commercially used.

PCBs were widely used as dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 and coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

 fluids, for example in transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s, capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s, and electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

s. Due to PCBs' toxicity
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

 and classification as a persistent organic pollutant
Persistent organic pollutant
thumb|right|275px|State parties to the Stockholm Convention on Persistent Organic PollutantsPersistent organic pollutants are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes...

, PCB production was banned by the United States Congress in 1979 and by the Stockholm Convention on Persistent Organic Pollutants
Stockholm Convention on Persistent Organic Pollutants
Stockholm Convention on Persistent Organic Pollutants is an international environmental treaty, signed in 2001 and effective from May 2004, that aims to eliminate or restrict the production and use of persistent organic pollutants .- History :...

 in 2001. Concerns about the toxicity of PCBs are largely based on compounds within this group that share a structural similarity and toxic mode of action
Mode of action
Historically, pesticides have often been classified according to their chemical groups and this is useful for understanding the properties of a given compound. However, it is the Mode of Action group which possibly represents the most useful pesticide classification for biologists...

 with dioxin. Toxic effects such as endocrine disruption and neurotoxicity
Neurotoxicity
Neurotoxicity occurs when the exposure to natural or artificial toxic substances, which are called neurotoxins, alters the normal activity of the nervous system in such a way as to cause damage to nervous tissue. This can eventually disrupt or even kill neurons, key cells that transmit and process...

 are also associated with other compounds within the group. Therefore, the current maximum containment levels as stated by the EPA for PCBs in drinking water systems is 0.5 ppb

Physical and chemical properties

PCB congener
Congener
Congener has several different meanings depending on the field in which it is used. Colloquially, it is used to mean a person or thing like another, in character or action.-Biology:In biology, congeners are organisms within the same genus...

s are odor
Odor
An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...

less, taste
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

less, clear to pale-yellow, viscous
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

s (highly chlorinated mixtures are more viscous and deeper yellow). They are formed by electrophilic chlorination
Electrophilic halogenation
In organic chemistry, an electrophilic aromatic halogenation is a type of electrophilic aromatic substitution. This organic reaction is typical of aromatic compounds and a very useful method for adding substituents to an aromatic system....

 of biphenyl
Biphenyl
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...

 with chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 gas. PCBs have low water solubilities
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

 — 0.0027-0.42 ng/L for Aroclors, and low vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

s at room temperature, but they have high solubilities in most organic solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

s, oils, and fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

s. They have high dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...

s, very high thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

, high flash point
Flash point
The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a flash point requires an ignition source...

s (from 170 to 380 °C) and are chemically fairly inert, being extremely resistant to oxidation, reduction, addition
Addition reaction
An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one....

, elimination
Elimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism...

, and electrophilic substitution
Electrophilic substitution
Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a group in a compound, typically but not always hydrogen. Electrophilic aromatic substitution is characteristic of aromatic compounds and is an important way of introducing functional groups onto benzene...

. The density varies from 1.182 to 1.566 kg/L. Other physical and chemical properties vary widely across the class. As the degree of chlorination increases, melting point and lipophilicity increase, and vapour pressure and water solubility decrease.

All PCBs are man-made and chemically engineered. This emphasizes the point that natural environmental and animal systems cannot breakdown PCBs once they have entered.

PCBs readily penetrate skin
Absorption (skin)
Skin absorption is a route by which substances can enter the body through the skin. Along with inhalation, ingestion and injection, dermal absorption is a route of exposure for toxic substances and route of administration for medication...

, PVC
PVC
Polyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...

 (polyvinyl chloride), and latex
Latex
Latex is the stable dispersion of polymer microparticles in an aqueous medium. Latexes may be natural or synthetic.Latex as found in nature is a milky fluid found in 10% of all flowering plants . It is a complex emulsion consisting of proteins, alkaloids, starches, sugars, oils, tannins, resins,...

 (natural rubber). PCB-resistant materials include Viton
Viton
Viton is a brand of synthetic rubber and fluoropolymer elastomer commonly used in O-rings and other molded or extruded goods. The name is a registered trademark of DuPont Performance Elastomers L.L.C.....

, polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

, polyvinyl acetate
Polyvinyl acetate
Polyvinyl acetate, PVA, PVAc, poly, is a rubbery synthetic polymer with the formula n. It belongs to the polyvinyl esters family with the general formula -[RCOOCHCH2]-...

 (PVA), polytetrafluoroethylene
Polytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....

 (PTFE), butyl rubber
Butyl rubber
Butyl rubber is a synthetic rubber, a copolymer of isobutylene with isoprene. The abbreviation IIR stands for Isobutylene Isoprene Rubber. Polyisobutylene, also known as "PIB" or polyisobutene, n, is the homopolymer of isobutylene, or 2-methyl-1-propene, on which butyl rubber is based...

, nitrile rubber
Nitrile rubber
Nitrile rubber, also known as Buna-N, Perbunan, or NBR, is a synthetic rubber copolymer of acrylonitrile and butadiene. Trade names include Nipol, Krynac and Europrene....

, and Neoprene
Neoprene
Neoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...

.

In terms of structural relationship to toxicity, PCBs fall into 2 distinct categories, referred to as coplanar or non-ortho-substituted arene substitution patterns
Arene substitution patterns
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...

 and noncoplanar or ortho-substituted congeners. The coplanar group members have a fairly rigid structure, with the 2 phenyl rings in the same plane. This gives the molecule a structure similar to polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans
Polychlorinated dibenzofurans
Polychlorinated dibenzofurans are a group of halogenated organic compounds which are toxic environmental pollutants. They are known teratogens, mutagens, and suspected human carcinogens. PCDFs tend to co-occur with polychlorinated dibenzodioxins...

, and allows it to act in the same way as these molecules as an agonist of the aryl hydrocarbon receptor
Aryl hydrocarbon receptor
The Aryl hydrocarbon receptor is a member of the family of basic-helix-loop-helix transcription factors. AhR is a cytosolic transcription factor that is normally inactive, bound to several co-chaperones...

 (AhR) in organisms. These type of PCBs are considered as contributors to overall dioxin toxicity, and the term dioxin
Dioxin
Polychlorinated dibenzodioxins , or simply dioxins, are a group of organic polyhalogenated compounds that are significant environmental pollutants. They are commonly but inaccurately referred to as dioxins for simplicity, because every PCDD molecule contains a dioxin skeletal structure as the...

 is often used interchangeably when the environmental and toxic impact of these compounds is considered. Noncoplanar PCBs, with chlorine atoms at the ortho positions, have not been found to activate the AhR, and are not considered part of the dioxin group; however, studies have indicated some neurotoxic and immunotoxic effects, but at levels much higher than normally associated with dioxins, and thus of much less concern to regulatory bodies.

PCBs are very stable compounds and do not decompose
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...

 readily. This is due to their chemical inability to oxidize and reduce in the natural environment. Furthermore, PCBs have a long half life (8 to 10 years) and are insoluble in water, which contributes to their stability. Their destruction by chemical, thermal, and biochemical processes is extremely difficult, and presents the risk of generating extremely toxic dibenzodioxins and dibenzofurans through partial oxidation. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 (see Methods of destruction below).

Alternative names

Commercial PCB mixtures were marketed under the following names.:
Brazil
  • Ascarel


Former Czechoslovakia
  • Delor


France
  • Phenoclor
  • Pyralène (both used by Prodolec)


Germany
  • Clophen (used by Bayer
    Bayer
    Bayer AG is a chemical and pharmaceutical company founded in Barmen , Germany in 1863. It is headquartered in Leverkusen, North Rhine-Westphalia, Germany and well known for its original brand of aspirin.-History:...

    )


Italy
  • Apirolio
  • Fenclor


Japan
  • Kanechlor (used by Kanegafuchi)
  • Santotherm (used by Mitsubishi
    Mitsubishi
    The Mitsubishi Group , Mitsubishi Group of Companies, or Mitsubishi Companies is a Japanese multinational conglomerate company that consists of a range of autonomous businesses which share the Mitsubishi brand, trademark and legacy...

    )
  • Pyroclor


Former USSR
  • Sovol
  • Sovtol


United Kingdom
  • Aroclor xxxx (used by Monsanto Company)
  • Askarel


United States
  • Aroclor xxxx (used by Monsanto Company)
  • Asbestol
  • Askarel
  • Bakola131
  • Chlorextol - Allis-Chalmers trade name
  • Hydol
  • Inerteen (used by Westinghouse)
  • Noflamol
  • Pyranol/Pyrenol (used by General Electric
    General Electric
    General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...

    )
  • Saf-T-Kuhl
  • Therminol


Applications

PCBs were used as coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

s and insulating fluids (transformer oil
Transformer oil
Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage...

) for transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s and capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s, such as those used in old fluorescent light ballasts. . PCBs were also used as plasticizer
Plasticizer
Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...

s in paints and cements, stabilizing additives in flexible PVC
PVC
Polyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...

 coatings of electrical wiring and electronic components, pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...

 extenders, cutting oils
Cutting fluid
Cutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols , and air or other gases. They may be made from petroleum distillates, animal...

, reactive flame retardant
Flame retardant
Flame retardants are chemicals used in thermoplastics, thermosets, textiles and coatings that inhibit or resist the spread of fire. These can be separated into several different classes of chemicals:...

s, lubricating oils
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...

, hydraulic fluid
Hydraulic fluid
Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...

s, and sealant
Sealant
A sealant may be viscous material that has little or no flow characteristics and stay where they are applied or thin and runny so as to allow it to penetrate the substrate by means of capillary reaction...

s (for caulking
Caulking
Caulking is one of several different processes to seal joints or seams in various structures and certain types of piping. The oldest form of caulking is used to make the seams in wooden boats or ships watertight, by driving fibrous materials into the wedge-shaped seams between planks...

 in schools and commercial buildings), adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

s, wood floor finishes (such as Fabulon and other products of Halowax
Halowax
Halowax is a New York-based company that was later owned by Union Carbide. It was subsequently taken over by Pittsburgh, Pennsylvania-based Koppers, which was later renamed Beazer East. It is the largest US producer of polychlorinated naphthalenes and polychlorinated biphenyls for floor finishing...

 in the U.S.), paint
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...

s, de-dusting agents, water-proofing compounds, casting agents, vacuum pump
Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...

 fluids, fixative
Fixative
A fixative is a stabilizing or preservative agent:*Fixative , a liquid usually sprayed over a finished piece of artwork to better preserve it and prevent smudging...

s in microscopy, surgical implants, and in carbonless copy ("NCR") paper
Carbonless copy paper
Carbonless copy paper , non-carbon copy paper, or NCR paper is an alternative to carbon paper, used to make a copy of an original, handwritten document without the use of any electronics...

.

History

In 1865 the first "PCB-like" chemical was discovered, and was found to be a byproduct of coal tar. Years later in 1881, Germany chemists synthesized the first PCB in a laboratory. From the initial synthesis of PCB until 1914, large amounts of PCB were released into the environment. So much so that there are still measurable amounts of PCB in feathers of birds currently held in museums.

PCBs, originally termed "chlorinated diphenyls," were commercially produced as complex mixtures containing multiple isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s at different degrees of chlorination. In the United States, commercial production of PCBs was taken over in 1929 by Monsanto Company from Swann Chemical Company. Manufacturing levels increased in response to the electrical industry's need for a "safer" (than flammable mineral oil
Mineral oil
A mineral oil is any of various colorless, odorless, light mixtures of alkanes in the C15 to C40 range from a non-vegetable source, particularly a distillate of petroleum....

) cooling and insulating fluid for industrial transformers and capacitors. PCBs were also commonly used as stabilizing additives in the manufacture of flexible PVC coatings for electrical wiring and electronic components to enhance the heat and fire resistance of the PVC.

The toxicity associated with PCBs and other chlorinated hydrocarbons, including polychlorinated naphthalene
Polychlorinated naphthalene
Polychlorinated naphthalene products are made by chemically reacting chlorine with naphthalene, a soft, pungent solid made from coal or petroleum and often used for mothproofing. The generic chemical formula is C10H10-nCln. Commercial PCNs are mixtures of up to 75 chlorinated naphthalene...

s, was recognized very early due to a variety of industrial incidents. Between 1936 and 1937, there were several medical cases and papers released on the possible link between PCBs and its detrimental health effects. One of the earlier cases in 1936 described an incident where a U.S. Public Health Service official described a wife and child of a worker from the Monsanto Industrial Chemical Company who exhibited blackheads and pustules on their skin. The official attributed these symptoms to contact with the worker's clothing after he returned from work. A conference about the hazards was organized at Harvard School of Public Health
Harvard School of Public Health
The Harvard School of Public Health is one of the professional graduate schools of Harvard University, located in the Longwood Area of the Boston, Massachusetts neighborhood of Mission Hill, which is next to Harvard Medical School. HSPH is considered a significant school focusing on health in the...

 in 1937, and a number of publications referring to the toxicity of various chlorinated hydrocarbons were published before 1940. Robert Brown reminded chemists in 1947 that Arochlors were "objectionably toxic. Thus the maximum permissible concentration for an 8-hr. day is 1 mg/m3 of air. They also produce a serious and disfiguring dermatitis
Dermatitis
-Etymology:Dermatitis derives from Greek derma "skin" + -itis "inflammation" and genetic disorder.-Terminology:There are several different types of dermatitis. The different kinds usually have in common an allergic reaction to specific allergens. The term may describe eczema, which is also called...

". However, PCB manufacture and use continued with few restraints until the 1970s.

PCBs are persistent organic pollutant
Persistent organic pollutant
thumb|right|275px|State parties to the Stockholm Convention on Persistent Organic PollutantsPersistent organic pollutants are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes...

s and have entered the environment through both use and disposal. The environmental transport of PCBs is complex and nearly global in scale. The public, legal, and scientific concerns about PCBs arose from research indicating they are likely carcinogens having the potential to adversely impact the environment and, therefore, undesirable as commercial products. Despite active research spanning five decades, extensive regulatory actions, and an effective ban on their production since the 1970s, PCBs still persist in the environment and remain a focus of attention.

The only North American producer, Monsanto Company, marketed PCBs under the trade name Aroclor from 1930 to 1977. These were sold under trade names followed by a 4-digit number. In general, the first two digits refer to the number of carbon atoms in the biphenyl skeleton (for PCBs this is 12); the second two numbers indicate the percentage of chlorine by mass in the mixture. Thus, Aroclor 1260 has 12 carbon atoms and contains 60% chlorine by mass. An exception is Aroclor 1016, which also has 12 carbon atoms, but has 42% chlorine by mass. Different Aroclors were used at different times and for different applications. In electrical equipment manufacturing in the USA, Aroclor 1260 and Aroclor 1254 were the main mixtures used before 1950; Aroclor 1242 was the main mixture used in the 1950s and 1960s until it was phased out in 1971 and replaced by Aroclor 1016.

Manufacture peaked in the 1960s, by which time the electrical industry had lobbied the U.S. Congress to make them mandatory safety equipment. In 1966, they were determined by Swedish chemist Dr. Soren Jensen to be an environmental contaminant, and it was Dr. Jensen, according to a 1994 article in Sierra
Sierra
Sierra can refer to:- Mountains and mountain ranges :* Sierra mountains * Sierra de Juárez, mountain range in Baja California, Mexico...

, who named them PCBs. Previously, they had simply been called "phenols" or referred to by various trade names, such as Aroclor, Kennechlor, Pyrenol, Chlorinol and others.

Their commercial utility was based largely on their chemical stability, including low flammability
Fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....

, and desirable physical properties, including electrical insulating properties. Their chemical and physical stability has also been responsible for their continuing persistence in the environment, and the lingering interest decades after regulations were imposed to control environmental contamination.

In 1972, PCB production plants existed in Austria, the then Federal Republic of Germany
West Germany
West Germany is the common English, but not official, name for the Federal Republic of Germany or FRG in the period between its creation in May 1949 to German reunification on 3 October 1990....

, France, Great Britain, Italy, Japan, Spain, USSR, and USA.

In 1973, the use of PCBs was banned in "open" or "dissipative" sources, such as:
  • plasticisers in paints and cements
  • casting agents
  • fire retardant
    Fire retardant
    A fire retardant is a substance other than water that reduces flammability of fuels or delays their combustion. This typically refers to chemical retardants but may also include substances that work by physical action, such as cooling the fuels; examples of these include fire-fighting foams and...

     fabric treatments and heat stabilizing additives for PVC electrical insulation
  • adhesives
  • paints and water-proofing
  • railway sleepers


However, they continued to be allowed in "totally enclosed uses" such as transformers and capacitors, which, in certain failure modes or out-of-specification conditions, can leak, catch fire, or explode. It was Ward B. Stone of the New York State Department of Environmental Conservation
New York State Department of Environmental Conservation
The New York State Department of Environmental Conservation is responsible for the conservation, improvement, and protection of natural resources within the U.S. state of New York. It was founded in 1970, replacing the previous Conservation Department...

 (NYSDEC) who first published his findings in the early 1970s that PCBs were leaking from transformers and had contaminated the soil at the bottom of utility poles. Concern over the toxicity and persistence (chemical stability) of PCBs in the environment led the United States Congress to ban their domestic production in 1979, although some use continues in closed systems such as capacitors and transformers.

"Enclosed uses" of PCBs include:
  • capacitor
    Capacitor
    A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

    s
  • insulating fluids in transformers
  • vacuum pump
    Vacuum pump
    A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...

     fluids
  • hydraulic fluid
    Hydraulic fluid
    Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...

    s

In the UK, closed uses of PCBs in new equipment were banned in 1981, when nearly all UK PCB synthesis ceased, but closed uses in existing equipment containing in excess of 5 litres of PCBs were not stopped until December 2000.

In Japan, PCBs were first produced by Kanegafuchi Chemical Co. Ltd. (Kaneka) in 1954 and production continued until 1972 when the Japanese government banned the production, use, and import of PCBs.

Estimates have put the total global production of PCBs on the order of 1.5 million tons. The United States was the single largest producer with over 600,000 tons produced between 1930 and 1977. The European region follows with nearly 450,000 tons through 1984. It is unlikely that a full inventory of global PCB production will ever be accurately tallied, as there were factories in Poland, East Germany, and Austria that produced unknown amounts of PCBs.

Massachusetts

Pittsfield, in western Massachusetts, was home to the General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...

 (GE) transformer and capacitor divisions, and electrical generating equipment built and repaired in Pittsfield powered the electrical utility grid throughout the nation. PCB-contaminated oil routinely migrated from GE’s 254 acres (1 km²) industrial plant located in the very center of the city to the surrounding groundwater, nearby Silver Lake, and to the Housatonic River
Housatonic River
The Housatonic River is a river, approximately long, in western Massachusetts and western Connecticut in the United States. It flows south to southeast, and drains about of southwestern New England into Long Island Sound...

, which flows through Massachusetts, Connecticut, and down to Long Island Sound.

PCB-containing solid material was widely used as fill, including oxbows
Meander
A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...

 of the Housatonic River
Housatonic River
The Housatonic River is a river, approximately long, in western Massachusetts and western Connecticut in the United States. It flows south to southeast, and drains about of southwestern New England into Long Island Sound...

.
In the 1940s, GE launched a giveaway program in which locals could receive PCB-contaminated material to use as fill. The recipients were required to sign a letter stating they were receiving clean fill and releasing GE for any subsequent problems. Beginning in 1997, the Massachusetts Department of Environmental Protection began evaluating over 700 suspect properties. As of 2010, over 250 properties had been found to have dangerous concentrations of PCBs in soil, and 180 properties had had contaminated soil removed.

New Bedford Harbor, which is a listed Superfund
Superfund
Superfund is the common name for the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 , a United States federal law designed to clean up sites contaminated with hazardous substances...

 site, contains some of the highest sediment concentrations in the marine environment.

Current Massachusetts water levels for PCBs are 0.0005mg/L.

New York State

Between approximately 1947 and 1977, GE released between 500,000 lbs and 1,500,000 lbs of PCBs into the Hudson River
Hudson River
The Hudson is a river that flows from north to south through eastern New York. The highest official source is at Lake Tear of the Clouds, on the slopes of Mount Marcy in the Adirondack Mountains. The river itself officially begins in Henderson Lake in Newcomb, New York...

. The PCBs came from the company's two capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 manufacturing plants at Hudson Falls and Fort Edward
Fort Edward (town), New York
Fort Edward is a town in and the county seat of Washington County, New York, United States. It is part of the Glens Falls Metropolitan Statistical Area. The town population was 5,892 at the 2000 census....

 in New York State.

In 1976, because of concern over continuing high levels of PCBs in local fish and other aquatic organisms, and the unacceptable risk to the health of consumers of such fish, the NYSDEC banned all fishing in the upper Hudson River, as well as commercial fishing of striped bass
Striped bass
The striped bass is the state fish of Maryland, Rhode Island, South Carolina, and the state saltwater fish of New York, Virginia, and New Hampshire...

 and several other species in the lower Hudson River, and also issued advisories restricting the consumption of fish caught within a 20 miles (32.2 km) long segment of the Hudson River from Hudson Falls to Troy
Troy, New York
Troy is a city in the US State of New York and the seat of Rensselaer County. Troy is located on the western edge of Rensselaer County and on the eastern bank of the Hudson River. Troy has close ties to the nearby cities of Albany and Schenectady, forming a region popularly called the Capital...

.

There have been many programs of remediation work to reduce the PCB pollution. In 1984, approximately 200 miles (321.9 km) of the Hudson River was designated a Superfund
Superfund
Superfund is the common name for the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 , a United States federal law designed to clean up sites contaminated with hazardous substances...

 site, and attempts to clean up the upper Hudson River began, including the removal in 1977-8 of 180000 cubic yards (137,619.9 m³) of contaminated river sediments near Fort Edward. In 1991, further PCB pollution was found at Bakers Falls, near the former GE Hudson Falls factory, and a program of remediation was started. In August 1995, a 40 miles (64.4 km) reach of the upper Hudson was reopened to fishing, but only on a catch-and-release basis. Removal of contaminated soil from Rogers Island
Rogers Island (New York)
Rogers Island is an island on the Hudson River, in Washington County, New York, that once formed part of the third largest city in colonial North America, and is considered the "spiritual home" of the United States Special Operations Forces, particularly the United States Army...

 was completed in December, 1999. In 2002, the United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

 (EPA) announced a further 2650000 cubic yards (2,026,070.4 m³) of contaminated sediments in the upper Hudson River would be removed.

North Carolina

One of the largest PCB "spills" in American history occurred in the summer of 1978 when 31,000 gallons of PCB-contaminated oil were criminally and deliberately sprayed in 3 foot (0.9144 m) swaths along the roadsides of some 240 miles (386.2 km) of North Carolina highway shoulders in 14 counties and at the Fort Bragg Army Base. The crime, known as "the midnight dumpings," lasted nearly 2 weeks, as drivers of a black-painted tanker truck drove down one side of rural Piedmont highways spraying their noxious liquid and then up the other side the following night.

Under the direction of Governor James B. Hunt
Jim Hunt
James Baxter Hunt Jr. is an American politician who was the 69th and 71st Governor of the state of North Carolina . He is the longest-serving governor in the state's history.-Early life:...

, Jr., state officials then erected large, yellow warning signs along the contaminated highways that read: "CAUTION: PCB Chemical Spills Along Highway Shoulders."

The timing of the PCB crime was important. Toxic waste had become a national concern, and as the PCBs were literally being dumped along the roadsides in North Carolina, the Environmental Protection Agency's Toxic Substance Control Act, (TSCA), which specifically addressed the disposal of PCBs became effective (August 2, 1978). Also, simultaneously, residents of the contaminated Love Canal
Love Canal
Love Canal was a neighborhood in Niagara Falls, New York, located in the white collar LaSalle section of the city. It officially covers 36 square blocks in the far southeastern corner of the city, along 99th Street and Read Avenue...

 community were being evacuated from their homes because of an old toxic landfill. The pressure to build new, safer landfills was enormous. The rationale for the midnight dumpings, according to the state's logic, was that new EPA regulations were going to make disposing of PCBs more expensive, and companies might try to save money by disposing of toxic chemicals illegally; therefore, there was a pressing need to build convenient and affordable toxic waste landfills in North Carolina.

Within a couple of weeks of the crime, Robert Burns and his sons, Timothy and Randall, were arrested for dumping the PCBs along the roadsides. Burns was a business partner of Robert "Buck" Ward, Jr., of the Ward PCB Transformer Company, in Raleigh. Ward and Burns eventually did minimal time in jail for their crime because of federal charges; however, state charges were dropped.

While the Governor's plan was to pick up the roadside PCBs and to bury them in a landfill in rural Warren County
Warren County, North Carolina
-Demographics:As of the census of 2000, there were 19,972 people, 7,708 households, and 5,449 families residing in the county. The population density was 47 people per square mile . There were 10,548 housing units at an average density of 25 per square mile...

, one of the most highly contaminated PCB counties, he faced a fierce opposition from residents who didn't want persistent, toxic PCBs to be buried in a landfill, which - according to new TSCA regulations - would be built only a few feet above their water table. For four years, residents of Warren County mounted an opposition to the PCB landfill that would eventuate in a six-week, non-violent direct-action campaign that would launch the environmental justice movement and that would, according to Johns Hopkins Professor Dr. Eileen McGurty, "transform environmentalism." see:Transforming Environmentalism: Warren County, PCBs, and the Origin of Environmental Justice.

Indiana

From the late 1950s through 1977, Westinghouse Electric used PCBs in the manufacture of capacitors in its Bloomington, Indiana
Bloomington, Indiana
Bloomington is a city in and the county seat of Monroe County in the southern region of the U.S. state of Indiana. The population was 80,405 at the 2010 census....

 plant. Reject capacitors were hauled and dumped in area salvage yards and landfills, including Bennett's Dump, Neal's Landfill and Lemon Lane Landfill. Workers also dumped PCB oil down factory drains, which contaminated the city sewage treatment plant. The City of Bloomington gave away the sludge to area farmers and gardeners, creating anywhere from 200 to 2000 sites, which remain unaddressed. Over 2 million pounds of PCBs were estimated to have been dumped in Monroe and Owen counties. Although federal and state authorities have been working on the sites' environmental remediation, many areas remain contaminated. Concerns have been raised regarding the removal of PCBs from the karst
KARST
Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...

 limestone topography, and regarding the possible disposal options. To date, the Westinghouse Bloomington PCB Superfund site case does not have a Remedial Investigation/Feasibility Study (RI/FS) and Record of Decision (ROD), although Westinghouse signed a US Department of Justice Consent Decree in 1985. The 1985 consent decree required Westinghouse to construct an incinerator that would incinerate PCB-contaminated materials. Due to public opposition to the incinerator, however, the State of Indiana passed a number of laws that delayed and blocked its construction. The parties to the consent decree began to explore alternative remedies in 1994 for six of the main PCB contaminated sites.

On 15 February 2008, Monroe County approved a plan to clean up the three remaining contaminated sites in the City of Bloomington, at a cost of $9.6 million to CBS Corp.
CBS Corporation
CBS Corporation is an American media conglomerate focused on commercial broadcasting, publishing, billboards and television production, with most of its operations in the United States. The President and CEO of the company is Leslie Moonves. Sumner Redstone, owner of National Amusements, is CBS's...

, the successor of Westinghouse.

The Great Lakes

In 1976 environmentalists found PCBs in the sludge at Waukegan Harbor, the southwest end of Lake Michigan. They were able to trace the source of the PCBs back to the Outboard Marine Corporation that was producing boat motors next to the harbor. By 1982, the Outboard Marine Corporation was court ordered to released quantitative data referring to their PCB waste released. The data stated that from 1954 they released 100,000 tons of PCB into the environment, and that the sludge contained PCBs in concentrations as high as 500,000 mg/kg.

Much of the Great Lakes
Great Lakes
The Great Lakes are a collection of freshwater lakes located in northeastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth by total surface, coming in second by volume...

 area is still heavily polluted with PCBs, despite extensive remediation work. Locally-caught fresh water fish and shellfish are contaminated with PCBs, and their consumption is restricted.

Alabama

PCBs (manufactured through most of the 20th century) originating from Monsanto Chemical Company in Anniston, Alabama
Anniston, Alabama
Anniston is a city in Calhoun County in the state of Alabama, United States.As of the 2000 census, the population of the city is 24,276. According to the 2005 U.S. Census estimates, the city had a population of 23,741...

 were dumped into Snow Creek, which then spread to Choccolocco Creek, then Logan Martin Lake
Logan Martin Lake
Logan Martin Lake is located in east central Alabama on the Coosa River approximately east of Birmingham, Alabama. This Coosa River reservoir was built in 1965 by Alabama Power Company. The lake, nicknamed Lake of a Thousand Coves by locals, has of shoreline along its length sandwiched between...

. In the early 2000s, class action lawsuits (led, in at least one case, by the late Johnnie Cochran
Johnnie Cochran
Johnnie L. Cochran, Jr. was an American lawyer best known for his leadership role in the defense and criminal acquittal of O. J...

) were settled by local land owners, including those on Logan Martin Lake
Logan Martin Lake
Logan Martin Lake is located in east central Alabama on the Coosa River approximately east of Birmingham, Alabama. This Coosa River reservoir was built in 1965 by Alabama Power Company. The lake, nicknamed Lake of a Thousand Coves by locals, has of shoreline along its length sandwiched between...

, and Lay Reservoir (downstream on the Coosa River
Coosa River
The Coosa River is a tributary of the Alabama River in the U.S. states of Alabama and Georgia. The river is about long altogether.The Coosa River is one of Alabama's most developed rivers...

), for the PCB pollution.

Today, the highest pollution levels remain concentrated in Snow and Choccolocco Creeks. Concentrations in fish have declined and continue to decline over time; sediment disturbance, however, can resuspend the PCBs from the sediment back into the water column and food web.

South Carolina

From 1955 until 1977, the Sangamo Weston plant in Pickens, SC, used PCBs to manufacture capacitors, and dumped 400,000 pounds of PCB contaminated wastewater into the Twelve Mile Creek. In 1990, the EPA declared the 228 acre (0.92268408 km²) site of the capacitor plant, its landfills and the polluted watershed, which stretches nearly 1000 acres (4 km²) downstream to Lake Hartwell as a Superfund
Superfund
Superfund is the common name for the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 , a United States federal law designed to clean up sites contaminated with hazardous substances...

 site. Two dams on the Twelve Mile Creek are to be removed and on Feb. 22, 2011 the first of two dams began to be dismantled. Some contaminated sediment is being removed from the site and hauled away, while other sediment is pumped into a series of settling ponds.

Belgium

In 1999, the Dioxine affair
Dioxine affair
The Dioxin Affair was a political crisis that struck in Belgium during the spring of 1999.Contamination of feedstock with polychlorinated biphenyls was detected in animal food products, mainly eggs and chickens....

 caused serious trouble for the Belgian government, when PCBs were found in chicken and eggs.

Slovakia

The chemical plant Chemko in Strážske
Strážske
Strážske ; ) is a small town and municipality in Michalovce District in the Kosice Region of eastern Slovakia. It is located in the most northern part of Michalovce District.-Geography:...

 (east Slovakia
Slovakia
The Slovak Republic is a landlocked state in Central Europe. It has a population of over five million and an area of about . Slovakia is bordered by the Czech Republic and Austria to the west, Poland to the north, Ukraine to the east and Hungary to the south...

) was an important producer of polychlorinated biphenyls for the former communist block (Comecon
Comecon
The Council for Mutual Economic Assistance , 1949–1991, was an economic organisation under hegemony of Soviet Union comprising the countries of the Eastern Bloc along with a number of communist states elsewhere in the world...

) until 1984. Chemko contaminated a large part of east Slovakia, especially the sediments of the Laborec river and reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...

 Zemplínska šírava
Zemplínska šírava
Zemplínska šírava is a dam and lake in eastern Slovakia, near the town of Michalovce, wholly belonging to the Michalovce District. The name recalls the historical Zemplín region. The dam was built in 1961–1965, covers an area of , has an average depth of , with a maximum depth of...

.

Republic of Ireland

In December 2008, a number of Irish news sources reported testing had revealed "extremely high" levels of dioxins, by toxic equivalent
Toxic equivalent
Toxic equivalence factor expresses the toxicity of dioxins, furans and PCBs in terms of the most toxic form of dioxin, 2,3,7,8-TCDD. The toxicity of the individual congeners may vary by orders of magnitude....

, in pork products, ranging from 80 to 200 times the EU's
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

 upper safe limit of 1.5 pg WHO-TEQDFP/μg i.e. 0.12 to 0.3 parts per billion.

Brendan Smith
Brendan Smith (politician)
Brendan Smith is an Irish Fianna Fáil politician. He has been a Teachta Dála for the Cavan–Monaghan constituency since 1992. He previously served as the Minister for Agriculture, Fisheries and Food and Minister for Justice and Law Reform .Born in Cavan in 1956, Smith was educated at Bawnboy...

, the Minister for Agriculture, Fisheries and Food, stated the pork contamination was caused by PCB-contaminated feed that was used on 9 of Ireland's 400 pig farms, and only one feed supplier was involved. Smith added that 38 beef farms also used the same contaminated feed, but those farms were quickly isolated and no contaminated beef entered the food chain. While the contamination was limited to just 9 pig farms, the Irish government requested the immediate withdrawal and disposal of all pork-containing products produced in Ireland and purchased since 1 September 2008. This request for withdrawal of pork products was confirmed in a press release by the Food Safety Authority of Ireland on December 6.

It is thought that the incident resulted from the contamination of fuel oil used in a drying burner at a single feed processor, with PCBs. The resulting combustion produced a highly toxic mixture of PCBs, dioxins and furans, which was included in the feed produced and subsequently fed to a large number of pigs. http://www.agriculture.gov.ie/media/migration/publications/2010/DioxinReport211209revised190110.pdf

Environmental transport and transformations

Due to their low vapour pressure, PCBs accumulate primarily in the hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

, in the organic fraction of soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

, and in organisms. Despite their hydrophobicity, the immense volume of water in the oceans is still capable of dissolving a significant quantity of PCBs.

However, a small volume of PCBs has been detected throughout the atmosphere, from the most urbanized areas that are the centers for PCB pollution, to regions north of the Arctic Circle. While the hydrosphere is the main reservoir, the atmosphere serves as the primary route for global transport of PCBs, particularly for those congeners with one to four chlorine atoms.

Atmospheric concentrations of PCBs tend to be lowest in rural areas, where they are typically in the picogram per cubic meter range, higher in suburban and urban areas, and highest in city centres, where they can reach 1 ng/m³ or more. In Milwaukee, an atmospheric concentration of 1.9 ng/m³ has been measured, and this source alone was estimated to account for 120 kg/year of PCBs entering Lake Michigan
Lake Michigan
Lake Michigan is one of the five Great Lakes of North America and the only one located entirely within the United States. It is the second largest of the Great Lakes by volume and the third largest by surface area, after Lake Superior and Lake Huron...

. Concentrations as high as 35 ng/m³, 10 times higher than the EPA guideline limit of 3.4 ng/m³, have been found inside some houses in the U.S.

Volatilization of PCBs in soil was thought to be the primary source of PCBs in the atmosphere, but recent research suggests ventilation of PCB-contaminated indoor air from buildings is the primary source of PCB contamination in the atmosphere.

In the atmosphere, PCBs may be degraded by hydroxyl radical
Hydroxyl radical
The hydroxyl radical, •OH, is the neutral form of the hydroxide ion . Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides or, in...

s, or directly by photolysis of carbon-chlorine bonds (even if this is a less important process).

In biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...

, PCBs can be degraded by either bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 or eukariotes, but the speed of the reaction depends on both the number and the disposition of chlorine atoms in the molecule: less substituted, meta- or para- substituted PCBs undergo biodegradation faster than more substituted congeners.

In bacteria, PCBs may be dechlorinated through reductive dechlorination
Reductive dechlorination
Reductive dechlorination is a term that is used to describe certain types of degradation of chlorinated organic compounds by chemical reduction with release of inorganic chloride ions.- Biological :...

, or oxidized by dioxygenase
Dioxygenase
In molecular biology, a dioxygenase is an enzyme which catalyses the incorporation of both atoms of molecular oxygen into substrates using a variety of reaction mechanisms. Cleavage of aromatic rings is one of the most important functions of dioxygenases, which play key roles in the degradation of...

 enzyme.

In eukariotes, PCBs may be oxidized by the cytochrome P450 enzyme.

Health effects

The toxicity of PCBs had been known since before its first commercial production through research done by producing companies themselves in the 1930s; however, these conclusions were dismissed as negligible.

The toxicity
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

 of PCBs to animals was first noticed in the 1970s, when emaciated seabird corpses with very high PCB body burdens washed up on beaches. Since seabirds may die far out at sea and still wash ashore, the true sources of the PCBs were unknown. Where they were found was not a reliable indicator of where they had died.

The toxicity of PCBs varies considerably among congeners. The coplanar PCBs, known as nonortho PCBs because they are not substituted at the ring positions ortho
Arene substitution patterns
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...

 to (next to) the other ring, (i.e. PCBs 77, 126, 169, etc.), tend to have dioxin-like properties, and generally are among the most toxic congeners. Because PCBs are almost invariably found in complex mixtures, the concept of toxic equivalency factors (TEF
TEF
The Test d'évaluation du français is a test of fluency in French for non-native speakers. It is awarded by the CCIP. It is often required to be admitted into universities and is recognized by the Federal government of Canada as a proof of fluency in immigration procedures.The test is made up of...

s) has been developed to facilitate risk assessment and regulatory control, where more toxic PCB congeners are assigned higher TEF values on a scale from 0 to 1. One of the most toxic compounds known, 2,3,7,8-tetrachlorodibenzo[p]dioxin
2,3,7,8-Tetrachlorodibenzodioxin
2,3,7,8-Tetrachlorodibenzo-p-dioxin is a polychlorinated dibenzo-p-dioxin . It is the most potent compound of the series and became known as a contaminant in Agent Orange, a herbicide used in the Vietnam War, as well as the Seveso disaster...

, is assigned a TEF of 1.

PCBs also have shown toxic and mutagenic effects by interfering with hormones in the body. PCBs, depending on the specific congener, have been shown to both inhibit and imitate estradiol
Estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

, the main sex hormone in females. Imitation of the estrogen compound can feed estrogen-dependent breast cancer cells, and possibly cause other cancers, such as uterine or cervical. Inhibition of estradiol can lead to serious developmental problems for both males and females, including sexual, skeletal, and mental development issues.

Exposure and excretion

Individuals can be exposed to PCBs through breathing in contaminated air, consuming contaminated food, and by skin contact with old electrical equipment that contain PCBs. Once exposed, some PCBs may change to other chemicals inside the body. These chemicals or unchanged PCBs can be excreted in feces or may remain in a person's body fat or other organs for months. PCBs may also collect in milk fat and be transmitted to infants through breast-feeding.

Signs and symptoms

  • Humans
The most commonly observed health effects in people exposed to extremely high levels of PCBs are skin conditions, such as chloracne
Chloracne
Chloracne is an acne-like eruption of blackheads, cysts, and pustules associated with over-exposure to certain halogenated aromatic compounds, such as chlorinated dioxins and dibenzofurans...

 and rash
Rash
A rash is a change of the skin which affects its color, appearance or texture. A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cracked or blistered, swell and may be painful. The causes, and...

es, but these were known to be symptoms of acute systemic poisoning dating back to 1922. Studies in workers exposed to PCBs have shown changes in blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 and urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

 that may indicate liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

 damage. In Japan in 1968, 280 kg of PCB-contaminated rice bran oil
Rice bran oil
Rice bran oil is the oil extracted from the germ and inner husk of rice. It is notable for its very high smoke point of and its mild flavor, making it suitable for high-temperature cooking methods such as stir frying and deep frying...

 was used as chicken feed, resulting in a mass poisoning, known as Yushō disease
Yusho Disease
was a mass poisoning by PCBs which occurred in northern Kyūshū, Japan in 1968. In January 1968, rice bran oil produced by Kanemi Company in Kyushu started to become contaminated with polychlorinated biphenyls and polychlorinated dibenzofurans...

, in over 14,000 people. Common symptoms included dermal and ocular lesion
Lesion
A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...

s, irregular menstrual cycle
Menstrual cycle
The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....

s and lowered immune responses. Other symptoms included fatigue, headaches, coughs, and unusual skin sores. Additionally, in children, there were reports of poor cognitive development.

There has been evidence to show crash dieters that have been exposed to PCBs have an elevated risk of health complications. Stored PCBs in the adipose tissue becomes mobilized into the blood when individuals begin to crash diet.

  • Animals
Animals that eat PCB-contaminated food even for short periods of time suffer liver damage and may die. In 1968 in Japan, 400,000 birds died after eating poultry feed that was contaminated with PCBs. Animals that ingest smaller amounts of PCBs in food over several weeks or months develop various health effects, including anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...

; acne-like skin conditions (chloracne
Chloracne
Chloracne is an acne-like eruption of blackheads, cysts, and pustules associated with over-exposure to certain halogenated aromatic compounds, such as chlorinated dioxins and dibenzofurans...

); and liver, stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

, and thyroid
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...

 gland injuries (including hepatocarcinoma). Other effects of PCBs in animals include changes in the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

, behavioral alterations, and impaired reproduction. PCBs are not known to cause birth defects in humans, although those that have dioxin-like activity are known to cause a variety of teratogenic effects in animals.

  • Effects during pregnancy/breastfeeding
Women who were exposed to relatively high levels of PCBs in the workplace or ate large amounts of fish contaminated with PCBs had babies that weighed slightly less than babies from women who did not have these exposures. Babies born to women who ate PCB-contaminated fish also showed abnormal responses in tests of infant behavior. Some of these behaviors, such as problems with motor skills and a decrease in short-term memory, lasted for several years. Other studies suggest that the immune system was affected in children born to and nursed by mothers exposed to increased levels of PCBs. The most likely way infants will be exposed to PCBs is from breast milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...

. Transplacental transfers of PCBs were also reported.

Since PCBs accumulate in adipose tissues, mothers exposed to PCBs can pass their exposure to newborn infants through the lipid-rich breast milk produced.
Studies have shown PCBs alter estrogen levels in the body and contribute to reproduction problems. In the womb, males can be feminised or the baby may be intersex, neither a male nor a female. Also, both sets of reproductive organs may develop. More instances of this are being reported. Biological magnification of PCBs has also led to polar bears and whales that have both male and female sex organs and males that cannot reproduce. This effect is known as endocrine disruption
Endocrine disruptor
Endocrine disruptors are chemicals that interfere with endocrine in animals, including humans. These disruptions can cause cancerous tumors, birth defects, and other developmental disorders...

. Endocrine disrupting chemicals (EDCs) pose a serious threat to reproduction in top-level predators.

Cancer link

A few studies of workers indicate PCBs were associated with specific kinds of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

 in humans, such as cancer of the liver and biliary tract. PCBs also have been shown to mimic the action of estrogen in breast cancer cells and can enhance breast carcinogenesis. Rats that ate food containing high levels of PCBs for two years developed liver cancer. The Department of Health and Human Services (DHHS) has concluded PCBs may reasonably be anticipated to be carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

s. The EPA and the International Agency for Research on Cancer
International Agency for Research on Cancer
The International Agency for Research on Cancer is an intergovernmental agency forming part of the World Health Organisation of the United Nations....

 (IARC) have determined that PCBs are probably carcinogenic to humans. PCBs are also classified as probable human carcinogens by the National Cancer Institute
National Cancer Institute
The National Cancer Institute is part of the National Institutes of Health , which is one of 11 agencies that are part of the U.S. Department of Health and Human Services. The NCI coordinates the U.S...

, World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

, and the Agency for Toxic Substances and Disease Registry
Agency for Toxic Substances and Disease Registry
The Agency for Toxic Substances and Disease Registry is a federal public health agency within the United States Department of Health and Human Services. The agency focuses on minimizing human health risks associated with exposure to hazardous substances...

. Recent research by the National Toxicology Program
National Toxicology Program
The National Toxicology Program is an inter-agency program run by the United States Department of Health and Human Services to coordinate, evaluate, and report on toxicology within public agencies....

 has confirmed PCB126 (Technical Report 520) and a binary mixture of PCB126 and PCB153 (Technical Report 531) are carcinogens.

Mechanism of action

As discussed, PCBs exhibit a wide range of toxic effects. These effects may vary depending on the specific PCB. Similar to dioxin, toxicity of coplanar PCBs and mono-ortho-PCBs are thought to be primarily mediated via binding to aryl hydrocarbon receptor
Aryl hydrocarbon receptor
The Aryl hydrocarbon receptor is a member of the family of basic-helix-loop-helix transcription factors. AhR is a cytosolic transcription factor that is normally inactive, bound to several co-chaperones...

 (AhR). Because AhR is a transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...

, abnormal activation may disrupt cell function by altering the transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

 of gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s. The concept of toxic equivalency factors (TEF) is based on the ability of a PCB to activate AhR.

However, not all effects may be mediated by the AhR receptor, and PCBs do not alter estrogen concentrations to the same degree as other ligands of the AhR receptor, such as PCDD and PCDF, possibly reflecting the reduced potency of PCBs to induce CYP1A1
CYP1A1
Cytochrome P450, family 1, subfamily A, polypeptide 1 is a protein that in humans is encoded by the CYP1A1 gene. The protein is a member of the cytochrome P450 superfamily of enzymes.-Function:...

 and CYP1B1
CYP1B1
Cytochrome P450 1B1 is an enzyme that in humans is encoded by the CYP1B1 gene.- Function :CYP1B1 belongs to the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids...

. Examples of other actions of PCBs include di-ortho-substituted non-coplanar PCBs interfering with intracellular signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...

 dependent on calcium
Calcium in biology
Calcium plays a pivotal role in the physiology and biochemistry of organisms and the cell. It plays an important role in signal transduction pathways, where it acts as a second messenger, in neurotransmitter release from neurons, contraction of all muscle cell types, and fertilization...

; this may lead to neurotoxicity
Neurotoxicity
Neurotoxicity occurs when the exposure to natural or artificial toxic substances, which are called neurotoxins, alters the normal activity of the nervous system in such a way as to cause damage to nervous tissue. This can eventually disrupt or even kill neurons, key cells that transmit and process...

. Ortho-PCBs may disrupt thyroid hormone
Thyroid hormone
The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...

 transport by binding to transthyretin
Transthyretin
Transthyretin is a serum and cerebrospinal fluid carrier of the thyroid hormone thyroxine and retinol binding protein bound to retinol. This is how transthyretin gained its name, transports thyroxine and retinol...

.

Congeners that show estrogenic effects bind to the 17 β estradiol receptor. The resulting complex is allowed to enter the nucleus and bind to specific segments of DNA, where it regulates transcription of certain segments of DNA into mRNA. This transcription may not be regulated properly, since it is being controlled by PCBs and not estradiol.

Containment

Because of its difficult containment, many buildings (at least in the U.S.A.) with known high PCB dangers have been evacuated and shut down. In many states, including California, laws require any building with such dangers to be sealed and locked, with large warning signs on every entrance point indicating a PCB presence and also a notice to indicate the presence of chemicals known to cause cancer, health problems or reproductive harm. Until a safe solution can be well established, many of these buildings remain undemolished and sealed. Some forms of containment other than building closure and lockdown are below.

Landfill
Landfill
A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...

 – Large quantities of PCBs have been placed in landfill sites, mainly in the form of transformers and capacitors. Many municipal sites are not designed to contain these pollutants and PCBs are able to escape into the atmosphere or ground water. No emissions above background are seen if the landfill is designed correctly.

Methods of destruction

These can be separated into three distinct categories: physical, microbial, and chemical destruction.

Physical

Incineration
Incineration
Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials. Incineration and other high temperature waste treatment systems are described as "thermal treatment". Incineration of waste materials converts the waste into ash, flue gas, and...

 – Although PCBs do not ignite themselves, they can be combusted under extreme and carefully controlled conditions. The current regulations require that PCBs are burnt at a temperature of 1200 °C for at least two seconds, in the presence of fuel oil and excess oxygen. A lack of oxygen can result in the formation of PCDDs, PCDFs and dioxins, or the incomplete destruction of the PCBs. Such specific conditions mean that it is extremely expensive to destroy PCBs on a tonnage scale, and it can only be used on PCB-containing equipment and contaminated liquid. This method is not suitable for the decontamination of affected soils.

Ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 – In a similar process to combustion, high power ultrasonic waves are applied to water, generating cavitation
Cavitation
Cavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...

 bubbles. These then implode or fragment, creating microregions of extreme pressures and temperatures where the PCBs are destroyed. Water is thought to undergo thermolysis, oxidising the PCBs to CO, CO2 and hydrocarbons such as biphenyl, and releasing chlorine. The scope of this method is limited to those congeners that are the most water-soluble, those isomers with the least chlorine substitution.

Irradiation
Irradiation
Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...

 – If a deoxygenated mixture of PCBs in isopropanol or mineral oil
Mineral oil
A mineral oil is any of various colorless, odorless, light mixtures of alkanes in the C15 to C40 range from a non-vegetable source, particularly a distillate of petroleum....

 is subjected to irradiation with gamma rays then the PCBs will be dechlorinated to form biphenyl
Biphenyl
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...

 and inorganic chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

. The reaction works best in isopropanol if potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...

 (caustic potash) is added. Solvated electron
Solvated electron
A solvated electron is a free electron in a solution. Solvated electrons occur widely although they are often not observed directly. The deep colour of solutions of alkali metals in ammonia arises form the presence of solvated electrons: blue when dilute and copper-colored when more concentrated...

s are thought to be responsible for the reaction. If oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...

, sulfur hexafluoride
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...

, or nitrobenzene
Nitrobenzene
Nitrobenzene is an organic compound with the chemical formula C6H5NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale as a precursor to aniline. Although occasionally used as a flavoring or perfume...

 is present in the mixture then the reaction rate is reduced. This work has been done recently in the US often with used nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

 as the radiation source.

Pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

 – Destruction of PCBs with pyrolysis using plasma arc processes, like incineration, uses heat. However, unlike incineration, there is no combustion. The long-chain molecules are broken with extreme temperature provided by an electric arc in an inert environment. Adequate post-pyrolysis treatment of the resultant products is required in order to prevent the risk of back reactions.

Microbial

Much recent work has centered on the study of micro-organisms that are able to decompose
Biodegradation
Biodegradation or biotic degradation or biotic decomposition is the chemical dissolution of materials by bacteria or other biological means...

 PCBs. In general, these organisms work in one of two ways: Either they use the PCB as a carbon source or destruction takes place through reductive dechlorination, with the replacement of chlorine with hydrogen on the biphenyl skeleton. However, there are significant problems with this approach. First, these microbes tend to be highly selective in their dechlorination, with lower chlorinated biphenyls being readily transformed, and with preference to dechlorination in the para and meta positions. Second, microbial dechlorination tends to be rather slow-acting on PCB as a soil contaminant in comparison to other methods. Last, while microbes work well in laboratory conditions, there is often a problem in transferring a successful laboratory strain to a natural system. This is because the microbes can access other sources of carbon, which they decompose in preference to PCBs.

Further recent developments have focused on testing enzymes and vitamins extracted from microbes that show PCB activity. Especially promising seems to be the use of vitamin B12, in which a cobalt ion is in oxidation state (III) under normal redox conditions. Using titanium (III) citrate as a strong reductant converts the cobalt from Co(III) to Co(I), giving a new vitamin known as B12s, which is a powerful nucleophile and reducing catalyst. This can then be used on PCBs, which it dechlorinates in a rapid and selective manner.

Chemical

Many chemical methods are available to destroy or reduce the toxicity of PCBs.

Nucleophilic aromatic substitution
Nucleophilic aromatic substitution
right|300px|Aromatic nucleophilic substitutionA nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring...

 is a method of destroying low-concentration PCB mixtures in oils, such as transformer oil. Substitution of chlorine by polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...

s occurs in under two hours under a blanket of nitrogen, to prevent oxidation of the oil, to produce aryl
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....

 polyglycols, which are insoluble in the oil and precipitate out.

Between 700 and 925 °C, H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

2 cleaves the carbon-chlorine bond, and cleaves the biphenyl nucleus into benzene, yielding HCl
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

 without a catalyst. This can be performed at lower temperatures with a copper catalyst, and to yield biphenyl. However, since both of these routes require an atmosphere of hydrogen gas and relatively high temperatures, they are prohibitively expensive.

Reaction with highly electropositive metals, or strong reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

s such as sodium naphthalide, in aprotic solvents results in a transfer of electrons to the PCB, the expulsion of a chloride ion, and a coupling of the PCBs. This is analogous to the Wurtz reaction
Wurtz reaction
The Wurtz reaction, named after Charles-Adolphe Wurtz, is a coupling reaction in organic chemistry, organometallic chemistry and recently inorganic main group polymers, whereby two alkyl halides are reacted with sodium to form a new carbon-carbon bond:...

 for coupling halogenoalkanes. The effect is to polymerise many molecules, thereby reducing the volatility, solubility, and toxicity of the mixture. This methodology is most successful on low-strength PCB mixtures and can also be performed electrochemically in a partly aqueous bicontinuous microemulsion
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...

.

The solution photochemistry
Photochemistry
Photochemistry, a sub-discipline of chemistry, is the study of chemical reactions that proceed with the absorption of light by atoms or molecules.. Everyday examples include photosynthesis, the degradation of plastics and the formation of vitamin D with sunlight.-Principles:Light is a type of...

 of PCBs is based on the transfer of an electron to a photochemically excited PCB from a species such as an amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

, to give a radical anion. This either expels a chloride ion and the resulting aryl radical
Aryl radical
An Aryl radical in organic chemistry is an reactive intermediate and an arene compound incorporating one free radical carbon atom as part of the ring structure. As such it is the radical counterpart of the Arenium ion. The parent compound is the phenyl radical C6H5....

 extracts a hydrogen atom from the solvent or immediately becomes protonated
Protonation
In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...

, leading to the loss of a chlorine atom. It is useful only for water-soluble PCBs.

The major pathway for atmospheric destruction of PCBs is via attack by OH
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

 radicals. Direct photolysis can occur in the upper atmosphere, but the ultraviolet wavelengths necessary to excite PCBs are shielded from the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 by the ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...

. It has, however, been shown that higher wavelengths of light (> 300 nm) can degrade PCBs in the presence of a photosensitizer, such as acetone.

The Schwartz reaction is the subject of much study, and has significant benefits over other routes. It is advantageous since it proceeds via a reductive process, and thus yields no dioxins through oxidation. The proposed reaction scheme involves the electron transfer from a titanium (III) organometallic species to form a radical anion on the PCB molecule which expels chlorine to eventually form the relatively non-toxic biphenyl.

PCB info table

For a complete list of PCB congeners, see PCB Congener List
PCB Congener List
A complete list of polychlorinated biphenyl congeners.-Explanation of PCB "Descriptors":Congener descriptors give a shorthand notation for geometry and substituent positions...

. Note that biphenyl, while not technically a PCB congener due to its lack of chlorine substituents, is still typically included in the literature.

PCB Homolog CASRN Cl Substituents Number of Congeners
Biphenyl
Biphenyl
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...

92-52-4 0 1
Monochlorobiphenyl 27323-18-8 1 3
Dichlorobiphenyl 25512-42-9 2 12
Trichlorobiphenyl 25323-68-6 3 24
Tetrachlorobiphenyl 26914-33-0 4 42
Pentachlorobiphenyl 25429-29-2 5 46
Hexachlorobiphenyl 26601-64-9 6 42
Heptachlorobiphenyl 28655-71-2 7 24
Octachlorobiphenyl 55722-26-4 8 12
Nonachlorobiphenyl 53742-07-7 9 3
Decachlorobiphenyl 2051-24-3 10 1

See also

  • Bay mud
    Bay mud
    Bay mud consists of thick deposits of soft, unconsolidated silty clay, which is saturated with water; these soil layers are situated at the bottom of certain estuaries, which are normally in temperate regions that have experienced cyclical glacial cycles...

  • GE
    Gê are the people who spoke Ge languages of the northern South American Caribbean coast and Brazil. In Brazil the Gê were found in Rio de Janeiro, Minas Gerais, Bahia, Piaui, Mato Grosso, Goias, Tocantins, Maranhão, and as far south as Paraguay....

  • Monsanto
    Monsanto
    The Monsanto Company is a US-based multinational agricultural biotechnology corporation. It is the world's leading producer of the herbicide glyphosate, marketed in the "Roundup" brand of herbicides, and in other brands...

  • Organochlorine compound
  • Polybrominated biphenyls
  • Zodiac
    Zodiac (novel)
    Zodiac: An Eco-Thriller is Neal Stephenson's second novel, which tells the story of an environmentalist, Sangamon Taylor, uncovering a conspiracy involving industrialist polluters in Boston Harbor. The "Zodiac" of the title refers to the brand of inflatable motor boats the hero uses to get around...

    , a novel by Neal Stephenson
    Neal Stephenson
    Neal Town Stephenson is an American writer known for his works of speculative fiction.Difficult to categorize, his novels have been variously referred to as science fiction, historical fiction, cyberpunk, and postcyberpunk...

     which involves PCBs and their impact on the environment.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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