Polychlorinated biphenyls (PCBs) are chemical compounds with the formula C₁₂H₁₀−ₓClₓ. They were once used in making carbonless copy paper, as heat transfer fluids, and as cooling and insulating fluids in electrical equipment. PCBs are very harmful and can cause cancer. They were used in industrial and consumer products, but their production was banned worldwide by the Stockholm Convention on Persistent Organic Pollutants in 2001.
Although PCBs are no longer made in large amounts since the 1960s, they remain in the environment because they last a long time. In the United States, federal law banned most uses of PCBs on January 1, 1978, after studies showed their harmful effects on the environment. PCBs are still found in older electrical equipment, such as transformers, which are still in use.
The International Agency for Research on Cancer (IARC) says PCBs definitely cause cancer in humans. The U.S. Environmental Protection Agency (EPA) states that PCBs cause cancer in animals and are likely to cause cancer in humans.
Some PCBs have a similar structure and harmful effects to dioxins. They can also disrupt hormone systems, particularly the thyroid, and harm the nervous system. Chemicals similar to PCBs, called polybrominated biphenyls (PBBs), are used in similar ways and have similar environmental concerns.
An estimated 1.2 million tons of PCBs were produced worldwide. Even though the U.S. EPA banned PCBs in 1978, they continue to cause health problems because they remain in soil, water, and products made before 1979. In 1988, scientists in Japan estimated that 370,000 tons of PCBs were in the environment, and 780,000 tons were in products, landfills, dumps, or storage.
Physical and chemical properties
The compounds are pale-yellow, thick liquids. They do not mix well with water and have very low water solubility, ranging from 0.0027 to 0.42 nanograms per liter for Aroclors brand. However, they dissolve easily in most organic solvents, oils, and fats. They have very low vapor pressure at room temperature. Their dielectric constants range from 2.5 to 2.7, they conduct heat very well, and they have high flash points (170 to 380 degrees Celsius). The density is 1.44 grams per cubic centimeter at 30 degrees Celsius. Other physical and chemical properties vary greatly among the compounds. As the number of chlorine atoms increases, melting point and lipophilicity increase, while vapor pressure and water solubility decrease.
PCBs are difficult to break down or degrade, which made them useful for industrial applications. PCB mixtures are resistant to acids, bases, oxidation, hydrolysis, and changes in temperature. They can produce extremely toxic substances called dibenzodioxins and dibenzofurans when partially oxidized. Intentional breakdown of PCBs usually requires high heat or chemical catalysts (see Methods of destruction below).
PCBs can easily pass through skin, PVC (polyvinyl chloride), and natural rubber latex. Materials that resist PCBs include Viton, polyethylene, polyvinyl acetate (PVA), polytetrafluoroethylene (PTFE), butyl rubber, nitrile rubber, and Neoprene.
PCBs are made from biphenyl, which has the chemical formula C12H10, sometimes written as (C6H5)2. In PCBs, some hydrogen atoms in biphenyl are replaced by chlorine atoms. There are 209 different chemical compounds in which one to ten chlorine atoms can replace hydrogen atoms. PCBs are usually used as mixtures of these compounds and are identified by the single CAS number 1336-36-3. Approximately 130 individual PCBs are found in commercial products.
Toxic effects depend on the specific PCB. Based on structure and toxicity, PCBs are divided into two groups: coplanar or non-ortho-substituted arene substitution patterns and noncoplanar or ortho-substituted congeners.
Di-ortho-substituted, non-coplanar PCBs interfere with calcium-dependent intracellular signal transmission, which may cause nerve damage. Ortho-PCBs can disrupt thyroid hormone transport by binding to transthyretin.
Mixtures and trade names
Commercial PCB mixtures were marketed under the following names:
Czech Republic and Slovakia
The only North American producer, Monsanto Company, sold PCBs under the trade name Aroclor from 1930 to 1977. These products were named with a trade name followed by a four-digit number. The first two digits show the product series as defined by Monsanto (for example, 1200 or 1100 series). The last two digits indicate the percentage of chlorine by mass in the mixture. For example, Aroclor 1260 is part of the 1200 series and contains 60% chlorine by mass. It is a common misunderstanding that the first two digits refer to the number of carbon atoms in the product; the number of carbon atoms remains unchanged in PCBs. The 1100 series was a not fully refined material that was processed through distillation to create the 1200 series product.
An exception to the naming system is Aroclor 1016. This product was made by distilling Aroclor 1242 to remove highly chlorinated congeners, resulting in a more biodegradable material. The name "1016" was used during Monsanto’s research for tracking purposes but became the official name after the product was sold.
Different Aroclor mixtures were used at different times and for different purposes. In the United States, Aroclor 1260 and Aroclor 1254 were the main mixtures used in electrical equipment manufacturing before 1950. Aroclor 1242 was the primary mixture used during the 1950s and 1960s until it was stopped in 1971 and replaced by Aroclor 1016.
Production
In 2006, one estimate said that 1 million tons of PCBs were made. About 40% of this material was still being used. Another estimate said that the total amount of PCBs made worldwide was about 1.5 million tons. The United States was the biggest producer, making over 600,000 tons between 1930 and 1977. Europe made nearly 450,000 tons by 1984. It is unlikely that all PCB production worldwide can be fully counted because factories in Poland, East Germany, and Austria made unknown amounts. In 2002, there were still 21,500 tons of PCBs stored in eastern Slovakia.
Even though making PCBs on purpose is banned by international agreements, large amounts are still made accidentally. Studies show that 45,000 tons of PCBs are made each year in the US as a side product in some chemical processes. In the US, making PCBs for sale was banned in 1979 by the Toxic Substances Control Act (TSCA), a law that helps control harmful substances.
Applications
PCBs are useful because they do not burn easily and can block electricity well. When exposed to an electric arc, PCBs produce gases that do not catch fire.
PCBs are used in two main ways: closed systems and open systems. Closed systems include coolants and insulating fluids, such as transformer oil used in transformers and capacitors, like those found in old fluorescent light ballasts. Hydraulic fluids are also used in partially closed systems. In contrast, one major open use of PCBs was in carbonless copy paper (NCR paper), which still causes paper to become contaminated today.
Other open uses included lubricating and cutting oils, as well as plasticizers in paints and cements. PCBs were also added to flexible PVC coatings on electrical cables and electronic parts to keep them stable. They were used in pesticide extenders, flame retardants, and sealants for caulking, adhesives, and wood floor finishes, such as Fabulon and other products made by Halowax in the U.S. PCBs were also used as de-dusting agents, waterproofing compounds, and casting agents. They were added to paints and coal tars, which were widely used to coat water tanks, bridges, and other structures.
Today, PCBs can still be found in pigments used in inks for paper or plastic products. They are also present in old equipment such as capacitors, ballasts, X-ray machines, and other electronic waste.
Environmental transport and transformations
PCBs have entered the environment through their use and when they are disposed of. The way PCBs move through the environment is complicated and happens worldwide.
PCBs do not easily evaporate. This means they stay in water and soil instead of moving into the air. They are found in the water in the environment, in the organic part of soil, and inside living things, including humans. Water in the oceans holds a large amount of PCBs, even though PCBs do not mix well with water.
As ocean water becomes deeper and more pressurized, PCBs become heavier than water and sink to the deepest parts of the ocean.
A small amount of PCBs is found in the air around the world. The air is the main way PCBs travel globally, especially for types of PCBs with one to four chlorine atoms.
In the air, PCBs can be broken down by oxygen molecules or by sunlight breaking carbon-chlorine bonds, though this process is less common.
Air pollution with PCBs is usually lowest in rural areas, where levels are measured in picograms per cubic meter. Levels are higher in cities and can reach 1 nanogram per cubic meter or more. In Milwaukee, a level of 1.9 nanograms per cubic meter was measured, and this alone added about 120 kilograms of PCBs to Lake Michigan each year. In 2008, some homes in the U.S. had PCB levels as high as 35 nanograms per cubic meter, which is 10 times higher than the EPA’s safety limit of 3.4 nanograms per cubic meter.
At first, scientists thought PCBs in soil were the main source of PCBs in the air. However, recent research shows that releasing PCB-contaminated air from buildings is the main way PCBs enter the atmosphere.
In living things, PCBs can be broken down by sunlight, bacteria, or other living organisms. The speed of this breakdown depends on how many chlorine atoms are in the PCB molecule and where they are located. PCBs with fewer chlorine atoms or those with chlorine atoms in certain positions break down faster.
In bacteria, PCBs can be broken down by removing chlorine atoms or by using a special enzyme. In other living things, PCBs can be broken down by a type of enzyme called cytochrome P450.
Like other toxins that dissolve in fat, PCBs build up in living things over time. This happens because PCBs are easily stored in the bodies of organisms.
Plastic waste, especially tiny plastic pieces called microplastics, is a major source of PCBs entering the environment, especially the ocean. Freshwater systems, like rivers, help move plastic from land to the ocean, which carries PCBs into the sea. It is estimated that 88–95% of ocean plastic comes from just 10 major rivers.
Living things can take in PCBs by eating other living things that have already taken in PCBs from land, freshwater, or the ocean. Over time, the amount of PCBs in a living thing increases, a process called bioaccumulation. The amount of PCBs in a living thing also depends on its position in the food chain. Organisms at higher levels, like orcas or humans, have more PCBs than those at lower levels, like phytoplankton. If many organisms at a certain level in the food chain die from PCBs, it can cause a trophic cascade.
PCBs can harm human health or even cause death when eaten. Birds can carry PCBs from water sources to land through their droppings or dead bodies.
Biochemical metabolism
PCBs are changed in the body through a process called biotransformation, which helps make them more water-soluble so they can be removed more easily. This process depends on how many chlorine atoms are in the PCB molecule and where they are located on the rings. In Phase I reactions, an enzyme called Cytochrome P450 adds an oxygen atom to one of the benzene rings. The type of P450 enzyme involved determines where the oxygen is added. Enzymes activated by phenobarbital (PB) add oxygen to the meta-para positions of PCBs, while enzymes activated by 3-methylcholanthrene (3MC) add oxygen to the ortho-meta positions. PCBs with ortho-meta or meta-para protons can be broken down by either enzyme, making them easier to remove from the body. However, some metabolites with ortho-meta protons may become more stable due to increased space taken by oxygen atoms, which can lead to longer retention in the body.
The way PCBs are broken down also depends on the species of the organism. Studies on four sea turtle species (green, olive ridley, loggerhead, and hawksbill) found that green and hawksbill sea turtles break down PCB 52 more quickly than olive ridley or loggerhead sea turtles. This is because green and hawksbill turtles have higher levels of a protein called P450 2-like, which adds three hydroxyl groups to PCB 52, making it more water-soluble. Another protein, P450 3-like, is linked to breaking down PCB 77, but this was not measured in the study.
Temperature affects how aquatic species process PCBs. In yellow perch (Perca flavescens), the rate of PCB metabolism depends on water temperature. During colder months (fall and winter), only 11 of 72 PCB types were excreted, with half-lives exceeding 1,000 days. In warmer months (spring and summer), when water temperatures averaged above 20°C, persistent PCBs had half-lives of 67 days. PCBs were mainly removed through feces, growth dilution, and loss through respiratory surfaces. The excretion rate matched the perch’s natural activity patterns, with higher metabolism and lower PCB accumulation during warmer months. However, exposure to cold water combined with PCBs containing coplanar chlorine molecules can harm perch health.
Chiral compounds have mirror-image forms that behave similarly chemically but may be processed differently by the body. This was studied in bowhead whales (Balaena mysticetus) because they are large, slow-metabolizing animals (which allows PCBs to build up in fat) and few studies have examined chiral PCBs in whales. Researchers found PCB concentrations in blubber were about four times higher than in the liver, but this may depend on the whale’s age and sex. Female bowhead whales, which transfer PCBs and toxins to their offspring, had lower blubber concentrations than males of the same size (less than 13 meters).
Health effects
The toxicity of PCBs depends on the specific type of PCB. Some PCBs, called nonortho PCBs, do not have chemical groups attached in certain positions on their molecules (such as PCBs 77, 126, and 169). These PCBs often behave similarly to a highly toxic chemical called dioxin and are usually the most dangerous. Since PCBs are rarely found alone and are usually mixed with other chemicals, scientists use a system called toxic equivalency factors (TEFs) to compare their toxicity. TEFs assign a value between 0 and 1 to each PCB type, with more toxic PCBs receiving higher values. A very harmful chemical called 2,3,7,8-tetrachlorodibenzo[p]dioxin (a type of PCDD) has a TEF of 1. In June 2020, Pennsylvania’s State Impact reported that the U.S. Environmental Protection Agency (EPA) banned PCBs in 1979, but they remain in products made before that year. PCBs stay in the environment because they attach to soil and sediment. High exposure can cause birth defects, developmental delays, and liver changes.
People are most often exposed to PCBs through food, less often by breathing polluted air, and least through skin contact. After exposure, some PCBs change into other chemicals inside the body. These chemicals or unchanged PCBs can leave the body through feces or remain in the body for many years, with some estimates suggesting they stay for 10 to 15 years. PCBs build up in body fat and fat in breast milk. They increase in concentration as they move up the food chain and are found in fish and water from polluted areas. Human infants are exposed to PCBs through breast milk or through their mothers’ blood during pregnancy, placing them at the top of the food chain.
Workers who recycle old electronics may also be exposed to PCBs.
The most common health effects from very high PCB exposure include skin problems like chloracne (a severe acne-like condition) and rashes. These symptoms were first recorded in 1922 as signs of poisoning. Studies of workers exposed to PCBs have found changes in blood and urine that may indicate liver damage. In 1968, Japan experienced a mass poisoning called Yushō disease when 280 kilograms of PCB-contaminated rice bran oil was used as chicken feed, affecting over 1,800 people. Symptoms included skin and eye issues, irregular menstrual cycles, weakened immunity, fatigue, headaches, coughs, and unusual skin sores. Children exposed during this event showed poor cognitive development. Women exposed to PCBs before or during pregnancy may have children with lower cognitive abilities, weaker immune systems, and motor control issues.
There is evidence that people who crash diet after being exposed to PCBs may face greater health risks. Stored PCBs in body fat can enter the bloodstream when individuals lose weight rapidly.
PCBs harm the body by interfering with hormones. Depending on the specific PCB type, they can either block or mimic estradiol, a key female sex hormone. Mimicking estradiol may help estrogen-dependent breast cancer cells grow and could cause other cancers, such as those of the uterus or cervix. Blocking estradiol can lead to developmental problems in both males and females, including issues with sexual development, bones, and mental abilities. A study found that higher PCB levels in boys were linked to lower testosterone levels.
High PCB levels in adults have been shown to lower levels of the thyroid hormone triiodothyronine, which affects growth, metabolism, body temperature, and heart rate. It also weakens immunity and increases thyroid problems.
Animals that eat PCB-contaminated food, even briefly, may suffer liver damage and die. In 1968, 400,000 birds in Japan died after eating poultry feed contaminated with PCBs. Animals that consume smaller amounts of PCBs over weeks or months may develop anemia, acne-like skin conditions (chloracne), liver, stomach, and thyroid injuries (including liver cancer), and immune system changes. Other effects include behavioral changes, reproductive issues, and hearing loss in rats similar to symptoms of an underactive thyroid.
In 2013, the International Agency for Research on Cancer (IARC) classified dioxin-like PCBs as human carcinogens. The U.S. EPA states that PCBs cause cancer in animals and may do so in humans. Studies have found higher rates of melanoma (a type of skin cancer) and rare liver cancers in people exposed to PCBs.
In 2013, IARC noted that evidence linking PCBs to non-Hodgkin lymphoma was limited and inconsistent. Earlier research had suggested a connection between PCB levels and this type of cancer. PCBs may influence immune system cancers because high-dose exposure in lab animals affects their immune systems, and some human studies link PCBs to immune changes.
In the 1990s, Monsanto faced lawsuits over harm caused by PCBs used in electrical equipment by companies like Westinghouse. Monsanto and its customers also faced lawsuits from workers at scrap yards. Some cases were settled, while others were won by Monsanto, which argued it had warned customers about PCB dangers and required safety measures.
In 2003, Monsanto and Solutia Inc. (a Monsanto spin-off) paid $700 million to residents of West Anniston, Alabama, affected by PCB manufacturing and dumping. A six-week trial found Monsanto guilty of "outrageous behavior" and held the company responsible for negligence, nuisance, and other charges.
In 2014, a Los Angeles court ruled that Monsanto was not responsible for cancers linked to PCBs in the food supply of three plaintiffs with non-Hodgkin lymphoma. The jury found that PCBs sold between 1935 and 1977 were not a major cause of their cancers.
In 2015, cities like Spokane, San Diego, and San Jose sued Monsanto for cleanup costs at PCB-contaminated sites, claiming the company sold PCBs without proper warnings after learning of their dangers. Monsanto stated that third-party misuse or disposal of legally sold products was not its responsibility.
In July 2015, a Missouri court ruled that Monsanto, Solutia, Pharmacia, and Pfizer were not responsible for deaths and injuries linked to PCBs made before 1977. A jury found no evidence that PCB use caused the plaintiffs’ harm. Similar cases continue.
History
In 1865, the first "PCB-like" chemical was discovered. It was found to be a byproduct of coal tar. In 1876, a German chemist named Oscar Döbner synthesized the first PCB in a laboratory. After this, large amounts of PCBs were released into the environment. Today, measurable amounts of PCBs can still be found in the feathers of birds displayed in museums, even though PCB production peaked long ago.
In 1935, the Monsanto Chemical Company (later known as Solutia Inc.) began commercial production of PCBs. This followed earlier production by Swann Chemical Company, which started in 1929. PCBs, originally called "chlorinated diphenyls," were produced as mixtures of different chemical forms with varying amounts of chlorine. The electric industry used PCBs as a non-flammable replacement for mineral oil to cool and insulate transformers and capacitors. PCBs were also used as heat stabilizers in cables and electronic components to improve the heat and fire resistance of PVC.
In the 1930s, the harmful effects of PCBs and other chlorinated hydrocarbons, such as polychlorinated naphthalenes, were recognized due to industrial incidents. Between 1936 and 1937, several medical cases and reports linked PCBs to health problems. In 1936, a U.S. Public Health Service official described skin issues on the wife and child of a worker from the Monsanto Industrial Chemical Company. These symptoms were believed to result from contact with the worker’s clothing after he returned from work. In 1937, a conference on the dangers of PCBs was held at Harvard School of Public Health, and several publications about the toxicity of chlorinated hydrocarbons were released before 1940.
In 1947, Robert Brown warned chemists that Arochlors were "objectionably toxic." He stated that the maximum safe concentration of Arochlors in the air for an 8-hour workday was 1 milligram per cubic meter. He also noted that these chemicals could cause serious skin conditions.
In 1954, Kanegafuchi Chemical Co. Ltd. (later known as Kaneka Corporation) began producing PCBs, continuing until 1972.
Throughout the 1960s, Monsanto Chemical Company learned more about PCBs’ harmful effects on humans and the environment, according to internal documents leaked in 2002. However, PCB production and use continued with few restrictions until the 1970s.
In 1966, a Swedish chemist named Sören Jensen identified PCBs as an environmental contaminant. According to a 1994 article in Sierra, Jensen gave the name "PCBs" to chemicals previously called "phenols" or known by trade names like Aroclor, Kanechlor, Pyrenol, and Chlorinol. In 1972, PCB production plants operated in Austria, West Germany, France, the UK, Italy, Japan, Spain, the USSR, and the US.
In the early 1970s, Ward B. Stone of the New York State Department of Environmental Conservation published findings that PCBs were leaking from transformers and had contaminated soil near utility poles.
There have been claims that Industrial Bio-Test Laboratories falsified data related to PCB testing. In 2003, Monsanto and Solutia Inc., a spinoff of Monsanto, agreed to a $700 million settlement with residents of West Anniston, Alabama, who were affected by PCB manufacturing and dumping. A six-week trial concluded that Monsanto had acted "outrageously" and was held responsible for all six charges, including negligence, nuisance, and suppression of the truth.
Products containing PCBs that are "totally enclosed uses," such as insulating fluids in transformers and capacitors, vacuum pump fluids, and hydraulic fluids, are still allowed to be used in the US. Public, legal, and scientific concerns about PCBs arose from research showing they are likely carcinogens that can harm the environment. Despite over 50 years of research, strict regulations, and a ban on PCB production since the 1970s, PCBs remain in the environment and continue to be a topic of focus.
Pollution due to PCBs
In 1999, the Dioxin Affair happened when 50 kilograms of PCB transformer oil were mixed with recycled fat used to make 500 tonnes of animal feed. This caused problems for about 2,500 farms in several countries. The name "Dioxin Affair" was used because early tests incorrectly blamed dioxins for the contamination, but later tests showed dioxins were only a small part of the problem. The type of PCBs found suggested the contamination came from a mix of Aroclor 1260 and 1254. Over 9 million chickens and 60,000 pigs were destroyed due to the contamination. Scientists have debated the effects on human health because different methods were used to assess the risks. One group said exposure could increase cancer and neurological problems in newborns, while another said cancer was unlikely but developmental issues during pregnancy and infancy were a risk. Two businessmen who sold the contaminated feed were given two-year suspended sentences for their role in the crisis.
The Italian company Caffaro, based in Brescia, made PCBs from 1938 to 1984 after buying the patent rights from Monsanto. Pollution from this factory and the Anniston case in the U.S. are the largest known PCB contamination events in the world, based on the amount of toxic material released, the size of the polluted areas, the number of people affected, and the length of PCB production.
Since 1999, the local health authority in Brescia reported PCB levels in soil 5,000 times higher than allowed by law (0.001 mg/kg). Because of this and other findings, an environmental disaster complaint was filed with the Public Prosecutor’s Office in Brescia in June 2001. Studies showed that people in certain areas of Brescia, former factory workers, and those who ate contaminated food had PCB levels in their bodies 10–20 times higher than normal. PCBs entered the food supply when animals grazed on polluted pastures near the factory, especially local veal eaten by farmers’ families. People exposed to PCBs had a higher risk of Non-Hodgkin lymphoma but not other cancers.
In 1968, a mix of dioxins and PCBs entered rice bran oil produced in northern Kyushu, Japan. This caused Yushō disease, sickening more than 1,860 people.
In 1987, high levels of PCBs were found in soil at Kadena Air Base in Okinawa, reaching thousands of parts per million, some of the highest levels ever recorded.
In December 2008, Irish news sources reported that pork products had dioxin levels up to 200 times higher than the EU’s safe limit. The contamination came from PCB-contaminated feed used on 9 of Ireland’s 400 pig farms. Only one feed supplier was involved, and 38 beef farms also used the same feed, but no contaminated beef entered the food chain. The Irish government ordered the removal of all pork products made in Ireland since September 2008. The contamination was traced to PCBs in fuel oil burned at a feed processor, which created a toxic mix of PCBs, dioxins, and furans in the feed.
In Kenya, during the 2010s, thieves stole transformer oil from electric transformers and sold it to food stall operators for deep frying. Transformer oil lasts longer than regular cooking oil but poses health risks due to PCBs.
The Chemko chemical plant in Strážske, Slovakia, produced PCBs for the former communist bloc until 1984. This caused pollution in eastern Slovakia, especially in the Laborec River and Zemplínska Šírava reservoir.
Between 1962 and 1983, the Iskra Kondenzatorji company in Slovenia made capacitors using PCBs. Wastewater and improper waste disposal polluted the Krupa and Lahinja rivers. The pollution was discovered in 1983 when the Krupa River was planned as a water source. Cleanup efforts began, but the area remains highly polluted. PCBs were found in food like eggs, milk, and walnuts, and the Krupa River is still the most PCB-polluted river globally.
Several dolphin species, including striped dolphins, bottlenose dolphins, and orcas, have high PCB levels in their blubber that may harm their populations. The western Mediterranean Sea and the south-west Iberian Peninsula are major PCB pollution hotspots.
Monsanto produced PCBs at its plant in Newport, Wales, until the mid-1970s. Waste from the plant, including PCBs, was dumped at a quarry near Cardiff and a landfill, where it continues to pollute water.
Monsanto was the only U.S. company that made PCBs, and production stopped in 1977. In 2020, a $650 million settlement from Bayer (which bought Monsanto) was rejected, allowing PCB-related lawsuits to continue.
PCBs from Monsanto’s Anniston, Alabama, plant were dumped into Snow Creek, spreading to Choccolocco Creek and Logan Martin Lake. Class-action lawsuits were settled in the early 2000s, with compensation for residents near the polluted areas. Senator Donald Stewart helped lead these lawsuits after learning about community concerns.
In 2007, PCB levels remained highest in Snow and Choccolocco Creeks, though fish levels have declined over time. Disturbing sediment can release PCBs back into water and the food chain.
San Francisco Bay has been polluted by PCBs, a result of past PCB use in the area, which has mixed into the sediment.
Regulation
In 1972, the Japanese government stopped the making, using, and bringing into the country of PCBs.
In 1973, Sweden banned the use of PCBs in "open" or "dissipative" sources, such as materials in paints, cements, fire-retardant fabrics, and additives for electrical insulation, adhesives, paints, waterproofing, and railroad ties.
In 1981, the United Kingdom banned the use of PCBs in new equipment that kept them sealed inside. Almost all PCB production in the UK stopped by this time. However, the use of PCBs in older equipment with more than 5 liters of PCBs was not banned until December 2000.
In 1976, the United States Congress became worried about the harmful effects and long-lasting nature of PCBs in the environment. This led to a ban on making PCBs in the United States, which started on January 1, 1978, under the Toxic Substances Control Act. To follow the law, the Environmental Protection Agency (EPA) stopped new PCB production but allowed their continued use in electrical equipment for economic reasons. In 1979, the EPA began creating rules about how PCBs could be used and safely disposed of. The agency also provided guidelines for safely removing and getting rid of PCBs from older equipment.
The EPA set a goal of zero PCBs in public water systems. However, because of the limits of water treatment technology, the actual allowed level is 0.5 parts per billion, known as the maximum contaminant level.
Methods of destruction
PCBs were useful because they do not easily react with other substances, including their ability to resist burning. However, they can be destroyed by burning at 1000 °C. When burned at lower temperatures, some of them change into more dangerous pollutants, such as polychlorinated dibenzofurans and dibenzo-p-dioxins. When burned properly, the products are water, carbon dioxide, and hydrogen chloride. In some cases, PCBs are burned as a mixture with kerosene. PCBs have also been destroyed through a process called pyrolysis, which happens when they are heated with alkali metal carbonates.
Thermal desorption is a very effective method for removing PCBs from soil.
PCBs are not easily chemically reactive; they resist oxidation and hydrolysis, which are common in industrial processes. They also have high thermal stability, electrical resistance, low water solubility, and are not easily flammable. They do not react with acids or bases. These properties made them useful for industrial applications.
Many chemical compounds can break down or reduce PCBs. Often, PCBs are broken down using basic mixtures of glycols, which replace some or all of the chlorine. Other effective methods include using reductants like sodium or sodium naphthalene. Vitamin B12 has also shown potential in this process.
Using microorganisms to break down PCBs from polluted areas, relying on the combined action of multiple microorganisms, is called bioremediation of polychlorinated biphenyl. Some microorganisms break down PCBs by breaking the C-Cl bonds. Microbial dechlorination is usually slower than other methods. Enzymes taken from microbes can also break down PCBs. In 2005, a type of bacteria called Shewanella oneidensis broke down a large amount of PCBs in soil samples. A low voltage current can help speed up the breakdown of PCBs by microbes.
Research has shown that some ligninolytic fungi can break down PCBs.
Bioremediation
Removing PCBs from sediments in estuaries and coastal rivers is very challenging because of the water above the sediment and the risk that chemicals may be stirred up during cleanup. The most common way to remove PCBs is to dig up the sediment and place it in a landfill. This method can cause problems, such as stirring up chemicals when the sediment is disturbed and harming ecosystems.
A safer and less expensive option is bioremediation, which uses living organisms to clean up polluted areas. Phytoremediation, a type of bioremediation that uses plants, has been shown to remove many types of pollutants, including PCBs, from soil. A study in New Bedford Harbor found that a type of seaweed called Ulva rigida can effectively remove PCBs from sediment. During a bloom, U. rigida grows into a thick layer on top of the sediment, allowing it to absorb large amounts of PCBs. In one test, PCB levels in U. rigida reached 1,580 micrograms per kilogram of seaweed within 24 hours. Live seaweed absorbed more PCBs than dead seaweed, but dead seaweed still absorbed significant amounts.
Homologs
For a full list of the 209 types of PCBs, see the PCB congener list. Note that biphenyl is often included in scientific writings even though it is not technically a PCB congener because it does not have chlorine parts.