Polychlorinated biphenyls (PCBs) are chemicals with chlorine in their structure. They were once used to make carbonless copy paper, as liquids to transfer heat, and as materials to help electrical equipment work and stay cool. PCBs are very harmful and can cause cancer. Their use in industrial and consumer products was banned worldwide by the Stockholm Convention on Persistent Organic Pollutants in 2001.
Although production of PCBs dropped greatly after the 1960s, when many problems were discovered, they are still found in many places. Scientists found that PCBs are harmful to the environment and classified them as long-lasting pollutants. Because of this, the United States banned most uses of PCBs in 1978. PCBs remain in the environment because they were widely used in electric transformers.
The International Agency for Research on Cancer (IARC) says PCBs definitely cause cancer in humans. The U.S. Environmental Protection Agency (EPA) says PCBs cause cancer in animals and are likely to cause cancer in people.
Some PCBs are similar in structure and harmful effects to dioxins. They can also harm hormone systems, especially the thyroid, and damage the nervous system. The bromine versions of PCBs are called polybrominated biphenyls (PBBs), which are used in similar ways and have similar environmental issues.
About 1.2 million tons of PCBs were made worldwide. Even though the U.S. banned PCBs in 1978, they continued to cause health problems in later years because they remain in soil, sediment, and products made before 1979. In 1988, Japanese scientists 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 ng/L for Aroclors brand. However, they dissolve easily in most organic solvents, oils, and fats. They have low vapor pressure at room temperature. Their dielectric constants range from 2.5 to 2.7, and they have high thermal conductivity and high flash points (170 to 380 °C). The density is 1.44 g/cm³ at 30 °C. Other physical and chemical properties vary widely among the group. As the number of chlorine atoms increases, melting point and lipophilicity increase, while vapor pressure and water solubility decrease.
PCBs do not break down easily, which made them useful in industrial applications. PCB mixtures are resistant to acids, bases, oxidation, hydrolysis, and changes in temperature. They can form extremely toxic substances called dibenzodioxins and dibenzofurans through partial oxidation. Intentional breakdown of PCBs usually requires high heat or chemical catalysts.
PCBs can easily pass through skin, PVC (polyvinyl chloride), and latex (natural rubber). 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 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 where one to ten chlorine atoms can replace hydrogen atoms. PCBs are usually used as mixtures and are identified by the single CAS number 1336-36-3. About 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 categories: coplanar or non-ortho-substituted arene substitution patterns and noncoplanar or ortho-substituted congeners.
Di-ortho-substituted, non-coplanar PCBs interfere with cell signaling that relies on calcium, which may cause nerve damage. Ortho-PCBs can disrupt thyroid hormone transport by attaching to a protein called transthyretin.
Mixtures and trade names
Commercial PCB mixtures were sold under the following names:
In the Czech Republic and Slovakia, PCBs were marketed under different trade names.
In North America, the only producer of PCBs was the Monsanto Company. From 1930 to 1977, Monsanto sold PCBs under the trade name Aroclor. Each product had a name followed by a four-digit number. The first two digits showed the product series as defined by Monsanto, such as the 1200 series or 1100 series. The last two digits showed 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 incorrect to believe that the first two digits represented the number of carbon atoms in the product. The number of carbon atoms does not change in PCBs. The 1100 series was a roughly processed material that was later refined to create the 1200 series.
An exception to the naming system was Aroclor 1016. This product was made by refining Aroclor 1242 to remove highly chlorinated parts, making it easier for the environment to break down. The name "1016" was used during Monsanto’s research to track the product, but it remained as the official name after production began.
Different Aroclor mixtures were used at different times and for different purposes. Before 1950, Aroclor 1260 and Aroclor 1254 were mainly used in electrical equipment manufacturing in the United States. From the 1950s to the 1960s, Aroclor 1242 was the primary mixture used until it was replaced in 1971 by Aroclor 1016.
Production
An estimate from 2006 suggested that 1 million tonnes of PCBs had been produced. About 40% of this material was still in use at that time. Another estimate placed the total global production of PCBs at around 1.5 million tonnes. The United States was the largest producer, with over 600,000 tonnes made between 1930 and 1977. The European region produced nearly 450,000 tonnes by 1984. It is unlikely that a complete count of all PCB production worldwide will ever be accurate because factories in Poland, East Germany, and Austria made unknown amounts of PCBs. As of 2002, 21,500 tons of PCBs were still stored in the eastern regions of Slovakia.
Although the deliberate production of PCBs is banned by international agreements, significant amounts are still unintentionally produced. Research shows that 45,000 tons of PCBs are legally created each year in the United States as by-products of certain chemical and product formulations.
Commercial production of PCBs was banned in the United States in 1979 with the passage of the Toxic Substances Control Act (TSCA).
Applications
PCBs are useful because they do not burn easily and can block electricity. When electricity arcs, they produce gases that do not burn.
The use of PCBs is often divided into two types: closed and open. Closed uses include coolants and insulating fluids, such as transformer oil in transformers and capacitors, like those found in old fluorescent light ballasts. Hydraulic fluids are considered a semi-closed use. In contrast, a major open use of PCBs was in carbonless copy ("NCR") paper, which still causes paper contamination today.
Other open uses included lubricating and cutting oils, and as plasticizers in paints and cements. PCBs were also used as stabilizing additives in flexible PVC coatings for electrical cables and electronic parts, as pesticide extenders, and as reactive flame retardants. They were added to 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 in de-dusting agents, waterproofing compounds, and casting agents. Additionally, they were used as plasticizers in paints and in "coal tars" that coated water tanks, bridges, and other infrastructure.
Today, PCBs are still found in pigments used in inks for paper or plastic products. They remain in old equipment, such as capacitors, ballasts, X-ray machines, and other electronic waste.
Environmental transport and transformations
PCBs enter the environment through their use and disposal. Their movement and long-term presence in the environment are complex and affect the entire planet.
PCBs have low vapor pressure, which means they do not easily turn into gas. Because of this, they mainly build up in water systems, even though they do not mix well with water. They also collect in the organic parts of soil and in living organisms, including humans. The largest storage place for PCBs is in the water, especially in the oceans. The vast amount of water in the oceans can still hold a large amount of PCBs.
As ocean water becomes deeper and more pressurized, PCBs become heavier than water and sink to the deepest parts of the ocean. These areas hold the highest concentrations of PCBs.
A small amount of PCBs has been found in the Earth's atmosphere. The atmosphere is the main way PCBs travel around the world, especially for types of PCBs with one to four chlorine atoms.
In the atmosphere, PCBs can be broken down by chemicals called hydroxyl radicals or by sunlight breaking apart carbon-chlorine bonds, though this process is less common.
PCB levels in the air are usually lowest in rural areas, where they are measured in picograms per cubic meter. Levels are higher in suburban and urban areas and highest in city centers, where they can reach 1 nanogram per cubic meter or more. In a city called Milwaukee, a concentration of 1.9 nanograms per cubic meter was recorded. This source alone was estimated to add 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.
It was once thought that PCBs in soil were the main source of PCBs in the air. However, recent research suggests that the release of PCB-contaminated indoor air from buildings is the primary way PCBs enter the atmosphere.
In the biosphere, PCBs can be broken down by sunlight, bacteria, or other living organisms. The speed of this process 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 more quickly.
In bacteria, PCBs may be broken down by removing chlorine atoms or by using an enzyme called dioxygenase. In other living organisms, PCBs are broken down by an enzyme called cytochrome P450.
Like many toxins that dissolve in fat, PCBs build up in the bodies of living things over time. This happens because PCBs are easily stored in organisms.
Plastic pollution, especially tiny plastic pieces called microplastics, is a major source of PCBs entering the biosphere and marine environments. PCBs collect in oceans because rivers carry plastic pollution from land to 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 organisms that have already absorbed PCBs from land, freshwater, or ocean environments. Over time, PCB levels in an organism increase, a process called bioaccumulation. The amount of PCBs in an organism also depends on its position in the food chain. Organisms at higher levels, such as orcas or humans, accumulate more PCBs than those at lower levels, like phytoplankton. If enough organisms at a certain level in the food chain are harmed by toxins like PCBs, a trophic cascade can occur, disrupting the ecosystem.
PCBs can harm human health or even cause death when consumed. Birds can carry PCBs from water sources to land through their feces or dead bodies.
Biochemical metabolism
Polychlorinated biphenyls (PCBs) are changed by the body’s process to make harmful, fat-soluble toxins more water-soluble so they can be removed more easily. This process depends on how many chlorine atoms are in the PCBs and where they are located on the molecule. In the first stage of metabolism, enzymes called Cytochrome P450 add oxygen to one of the benzene rings. The type of P450 enzyme determines where the oxygen is added. Enzymes from phenobarbital (PB) add oxygen to the meta-para positions of PCBs, while enzymes from 3-methylcholanthrene (3MC) add oxygen to the ortho-meta positions. PCBs with ortho-meta or meta-para protons can be broken down by either type of enzyme, making them more likely to leave the body. However, some broken-down PCBs with ortho-meta protons have more difficulty being removed because the added oxygen makes them more stable.
The way PCBs are broken down also depends on the type of organism. Different animals have slightly different P450 enzymes that process certain PCBs more effectively. In the livers of four sea turtle species (green, olive ridley, loggerhead, and hawksbill), green and hawksbill turtles break down PCB 52 faster than olive ridley or loggerhead turtles. This is because green and hawksbill turtles have more 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 studied in this research.
Temperature affects how aquatic animals process PCBs. In yellow perch (Perca flavescens), the rate of PCB metabolism depends on water temperature. In fall and winter, only 11 out of 72 types of PCBs were removed from the body, with half-lives longer than 1,000 days. During spring and summer, when water temperatures were above 20 °C, persistent PCBs had half-lives of 67 days. The main ways PCBs are removed are through feces, growth, and loss through breathing. The speed of PCB removal matches the perch’s natural energy use, which is highest in spring and summer. During warmer months, the perch’s faster metabolism reduces PCB buildup. However, cold periods combined with toxic PCBs that have coplanar chlorine molecules can harm the perch’s health.
Chiral compounds have two mirror-image forms that share similar chemical and physical properties but can be broken down differently by the body. This was studied in bowhead whales (Balaena mysticetus) because they are large animals with slow metabolisms (so PCBs build up in fat) and few studies have looked at chiral PCBs in whales. Researchers found PCB levels in whale blubber were about four times higher than in the liver. However, this result likely depends on the age and sex of the whales. Female whales that are pregnant transfer PCBs and other harmful substances to their fetuses, so blubber PCB levels in females were much lower than in males of the same size (less than 13 meters).
Health effects
The toxicity of PCBs depends on the type of PCB. Coplanar PCBs, which are not substituted at certain positions on their molecules (such as PCBs 77, 126, and 169), often behave similarly to dioxins and are typically the most toxic. Because PCBs are rarely found alone and are usually mixed with other chemicals, scientists use toxic equivalency factors (TEFs) to compare their toxicity. TEFs assign higher values to more toxic PCBs on a scale from 0 to 1. For example, 2,3,7,8-tetrachlorodibenzo[p]dioxin, a type of dioxin, has a TEF of 1. In 1979, the U.S. Environmental Protection Agency (EPA) banned PCBs, but they remain in older products and the environment because they stick to soil and sediment. High exposure can cause birth defects, developmental delays, and liver changes.
People are mostly exposed to PCBs through food, less often by breathing contaminated air, and rarely through skin contact. After exposure, some PCBs change into other chemicals in the body. These chemicals or unchanged PCBs can leave the body through feces or stay in the body for many years, with an estimated half-life of 10–15 years. PCBs build up in body fat and milk fat. They also increase in concentration as they move up the food chain and are found in fish and water from polluted sources. Infants are exposed through breast milk or while in the womb.
Workers who recycle old electronics may also be exposed to PCBs.
High levels of PCB exposure in humans often cause skin issues, such as chloracne and rashes, which were first recognized in 1922 as signs of severe poisoning. Studies on workers exposed to PCBs have found changes in blood and urine that may signal liver damage. In 1968, Japan experienced a poisoning event called Yushō disease, where over 1,800 people became ill after eating chicken feed contaminated with PCBs. Symptoms included skin and eye problems, irregular menstrual cycles, weakened immunity, and cognitive delays in children. Women exposed to PCBs during or before pregnancy may have children with lower cognitive abilities, weaker immune systems, and motor issues.
Crash dieters who have been exposed to PCBs may face health risks because stored PCBs in body fat can enter the bloodstream when fat is lost quickly.
PCBs can harm health by interfering with hormones. Depending on the type, PCBs may block or mimic estradiol, a key female hormone. Mimicking estradiol may support the growth of certain cancers, such as breast, uterine, or cervical cancers. Blocking estradiol can cause developmental problems in both males and females, including issues with sexual, skeletal, and mental development. Studies have also linked PCBs to lower testosterone levels in adolescent boys.
High PCB levels in adults are associated with reduced levels of 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 or die. In 1968, 400,000 birds died after eating PCB-contaminated poultry feed. Animals exposed to smaller amounts of PCBs over weeks or months may develop anemia, skin conditions similar to acne (chloracne), liver, stomach, and thyroid injuries, and immune system changes. PCBs with dioxin-like activity can cause birth defects in animals and lead to hearing loss and symptoms like hypothyroidism in rats.
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, with evidence linking them to melanoma and rare liver cancers in workers. However, IARC found limited and inconsistent evidence for PCBs causing non-Hodgkin lymphoma, though earlier studies suggested a connection. PCBs may influence immune system cancers because high-dose experiments on animals and some human studies have shown immune system effects.
In the 1990s, Monsanto faced lawsuits over PCB-related harm from workers at companies like Westinghouse. Monsanto and its partners also faced legal action from third parties, such as scrap yard workers. Some cases were settled, while others were won by Monsanto, which argued it had warned customers about PCB dangers.
In 2003, Monsanto and Solutia Inc. agreed to a $700 million settlement with residents of West Anniston, Alabama, affected by PCB manufacturing and dumping. A trial found Monsanto guilty of "outrageous behavior" and held it responsible for multiple charges, including negligence.
In 2014, a Los Angeles court ruled that Monsanto was not responsible for non-Hodgkin lymphoma in three plaintiffs who claimed their cancers were caused by PCBs in the food supply. The jury found that Monsanto’s PCB sales before 1977 were not a major cause of the illnesses.
In 2015, cities like Spokane, San Diego, and San Jose sued Monsanto for cleanup costs at PCB-contaminated sites, claiming the company failed to warn about PCB dangers. Monsanto denied responsibility, stating that improper use or disposal by others was not its fault.
In 2015, a Missouri court ruled that Monsanto, Solutia, Pharmacia, and Pfizer were not liable for deaths and injuries linked to PCBs made before 1977. The jury found no evidence that past PCB use caused the plaintiffs’ harm. Similar cases continue.
History
In 1865, the first chemical similar to PCBs was discovered. It was found to be a byproduct of coal tar. In 1876, a German chemist named Oscar Döbner created the first PCB in a laboratory. After this, large amounts of PCBs were released into the environment. Today, scientists can still measure PCBs in the feathers of birds that lived before PCB production reached its highest level.
In 1935, the Monsanto Chemical Company (later known as Solutia Inc.) began producing PCBs commercially. This followed earlier work by Swann Chemical Company, which started in 1929. PCBs were originally called "chlorinated diphenyls" and were made as mixtures of different chemical structures with varying levels 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 parts 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 several industrial incidents. Between 1936 and 1937, medical cases and reports were published about the possible health risks of PCBs. In 1936, a U.S. Public Health Service official described a worker’s wife and child who had blackheads and pustules on their skin. The official believed these symptoms were caused by contact with the worker’s clothing after he returned from work. In 1937, a meeting about the dangers of PCBs was held at Harvard School of Public Health. Before 1940, several scientific papers were published about the toxicity of various chlorinated hydrocarbons.
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 1.0 milligrams per cubic meter. He also noted that Arochlors could cause serious and disfiguring skin conditions.
In 1954, Kanegafuchi Chemical Co. Ltd. (later known as Kaneka Corporation) began producing PCBs, and continued until 1972.
During the 1960s, Monsanto Chemical Company learned more about the harmful effects of PCBs on humans and the environment, as revealed by leaked internal documents 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 PCBs their name, which had previously been called "phenols" or referred to by trade names such as Aroclor, Kanechlor, Pyrenol, and Chlorinol. In 1972, PCB production plants were operating 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 the soil at the bottom of utility poles.
There have been claims that Industrial Bio-Test Laboratories altered test results related to PCBs. 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. During a six-week trial, a jury found that Monsanto had acted outrageously and held the companies and their successors responsible for 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 may cause cancer and 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 study and concern.
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 led to contamination that affected about 2,500 farms in several countries. The name "Dioxin Affair" came from a mistake in early testing, which incorrectly identified dioxins as the main problem. In reality, dioxins were only a small part of the contamination caused by reactions involving PCBs. Tests showed the contamination came from a mix of two types of PCBs called Aroclor 1260 and 1254. Over 9 million chickens and 60,000 pigs were destroyed because of the contamination. Scientists have debated the effects on human health because different methods were used to assess the risks. One group predicted higher cancer rates and more neurological problems in people exposed as babies. Another study said cancer risks were unlikely, but developmental issues during pregnancy or infancy might be a bigger concern. Two businessmen who sold the contaminated feed were given two-year suspended sentences for their role in the crisis.
An Italian company called Caffaro, based in Brescia, made PCBs from 1938 to 1984 after getting the rights to use a patent from Monsanto. Pollution from this factory and a similar case in Anniston, USA, are the largest known examples of PCB contamination in water and soil. These cases involved large amounts of toxic chemicals, wide areas affected, many people involved, and long production times.
Since 1999, the local health authority in Brescia reported PCB levels in soil 5,000 times higher than allowed by law. Because of this and other investigations, in June 2001, an environmental disaster complaint was filed with the Public Prosecutor's Office in Brescia. Studies on adults in Brescia found that people living in certain areas, former workers at the plant, and those who ate contaminated food had PCB levels in their bodies up to 10–20 times higher than normal. PCBs entered the food supply through animals that grazed on polluted pastures near the factory, especially local veal eaten by farming families. People exposed to PCBs had a higher risk of a type of cancer called Non-Hodgkin lymphoma, but not other cancers.
In 1968, a mix of dioxins and PCBs got into rice bran oil made in northern Kyushu, Japan. This oil made more than 1,860 people sick, and the illness was called Yushō disease.
In 1987, high levels of PCBs were found in soil at Kadena Air Base in Okinawa, Japan, with levels reaching thousands of parts per million, some of the highest ever recorded.
In December 2008, Irish news sources reported that pork products had dioxin levels 80 to 200 times higher than the EU's safety limit. The contamination came from PCB-contaminated feed used on 9 of Ireland’s 400 pig farms. Only one feed supplier was involved. While 38 beef farms also used the same feed, they were quickly isolated, and no contaminated beef entered the food supply. The Irish government ordered the removal of all pork products made in Ireland since September 1, 2008. This was confirmed by the Food Safety Authority of Ireland on December 6.
The contamination likely happened when PCBs in fuel oil burned at a feed processor created a toxic mix of PCBs, dioxins, and furans. This mix was added to pig feed, leading to contamination in many pigs.
In the 2010s, thieves in Kenya stole transformer oil from electric transformers and sold it to food stall operators for deep frying. Transformer oil lasts longer than regular oil, but it contains PCBs, which harm people who eat the food.
In Slovakia, the Chemko chemical plant made PCBs for the former communist bloc until 1984. This caused pollution in parts of eastern Slovakia, especially in the sediments of the Laborec River and the Zemplínska šírava reservoir.
Between 1962 and 1983, a company in Slovenia called Iskra Kondenzatorji made capacitors using PCBs. Wastewater and improper waste disposal led to PCB contamination in rivers like Krupa and Lahinja. The pollution was discovered in 1983 when the Krupa River was planned as a water source. Cleanup efforts began, but the soil and water remain highly polluted. PCBs were found in eggs, cow milk, and walnuts, and the Krupa River is still the most PCB-polluted river in the world.
Several dolphin species in the Mediterranean Sea and southwest Iberian Peninsula have high levels of PCBs in their blubber. These levels may harm their populations and slow recovery. These areas are considered "hotspots" for PCB pollution.
Monsanto made PCBs at its plant in Newport, South Wales, until the mid-1970s. Waste from this plant, including PCBs, was dumped in a quarry near Cardiff and a landfill, where it continues to pollute water.
Monsanto was the only U.S. company that made PCBs, and it stopped production in 1977. In 2020, a court rejected a $650 million settlement from Bayer, which bought Monsanto, allowing PCB-related lawsuits to continue.
PCBs from Monsanto’s plant in Anniston, Alabama, were dumped into Snow Creek, which spread to Choccolocco Creek and Logan Martin Lake. In the early 2000s, lawsuits were settled with landowners affected by the pollution. Senator Donald Stewart, who later led legal cases against Monsanto, first learned about the pollution after helping a church in Anniston. Other lawyers joined to represent people outside the main polluted area, including Johnnie Cochran.
In 2007, the highest PCB levels remained in Snow and Choccolocco Creeks. PCB levels in fish have decreased over time, but disturbing sediment can release PCBs back into water and the food chain.
San Francisco Bay has been contaminated by PCBs, a result of past PCB use across the watershed and deep mixing into sediments.
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 materials that release them into the environment, such as plasticizers in paints and cements, casting agents, fire retardant fabric treatments, heat stabilizers in PVC electrical insulation, adhesives, paints, waterproofing materials, and railroad ties.
In 1981, the United Kingdom banned the use of PCBs in new equipment, and most PCB production in the UK ended. However, the use of PCBs in older equipment that contained more than 5 liters of PCBs was not stopped until December 2000.
In 1976, concerns about the harmful effects and long-lasting nature of PCBs in the environment led the United States Congress to ban their production within the country. This ban took effect on January 1, 1978, as part of the Toxic Substances Control Act. To follow the law, the Environmental Protection Agency (EPA) stopped new PCB manufacturing 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 disposed of. The agency also provided guidelines for safely removing and getting rid of PCBs from existing equipment.
The EPA set the "maximum contaminant level goal" for PCBs in public water systems to zero. However, because of the limits of water treatment technology, the actual allowed level (called the "maximum contaminant level") is 0.5 parts per billion.
Methods of destruction
PCBs were useful in industry because they do not easily react with other substances. This includes their ability to resist burning. However, they can be completely destroyed when burned at very high temperatures, such as 1000 °C. When burned at lower temperatures, some PCBs change into more dangerous harmful substances, like polychlorinated dibenzofurans and dibenzo-p-dioxins. When burned properly, the final 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 uses heat and certain chemicals called alkali metal carbonates.
A method called thermal desorption is very good at removing PCBs from soil.
PCBs are not easily broken down by chemical reactions. They do not react with oxygen or water, which are common in industrial processes. They also have high heat resistance, do not conduct electricity well, are not easily dissolved in water, do not catch fire easily, and are not affected by acids or bases. These properties made them useful in many industrial uses.
Several chemical methods can break down or reduce PCBs. One common method uses mixtures of certain chemicals called glycols, which can remove some or all of the chlorine in PCBs. Other methods use substances like sodium or sodium naphthalene, which can also break down PCBs. Vitamin B12 has also been shown to help in this process.
Using microorganisms to break down PCBs from polluted areas is called bioremediation of polychlorinated biphenyl. Some microorganisms remove chlorine from PCBs by breaking the C-Cl bonds. This process is usually slower than other methods. Enzymes taken from microbes can also help break down PCBs. In 2005, a type of bacteria called Shewanella oneidensis was found to break down a large amount of PCBs in soil samples. A low electrical current can help speed up the breakdown of PCBs by microorganisms.
Research has shown that some types of fungi, called ligninolytic fungi, can also break down PCBs.
Bioremediation
Removing PCBs from sediments in estuaries and coastal rivers is very challenging because the water above the sediments can cause chemicals to become airborne again during cleanup. The most common way to remove PCBs is to dig up the sediments and bury them in landfills. This method is problematic because it can cause PCBs to be released back into the water when sediments are disturbed, and it can harm ecosystems.
A more cost-effective and safer method is bioremediation, which uses living organisms to clean up polluted sediments. Phytoremediation, a type of bioremediation that uses plants, has been shown to remove various pollutants like mercury, PCBs, and PAHs from soil. A study in New Bedford Harbor found that Ulva rigida, a type of seaweed found worldwide, can effectively remove PCBs from sediments. During a bloom, Ulva rigida forms a thick layer over the sediment, allowing it to absorb large amounts of PCBs. Within 24 hours, PCB concentrations in Ulva rigida can reach 1580 micrograms per kilogram. Live seaweed absorbed more PCBs than dead seaweed, but dead seaweed also absorbed significant amounts.
Homologs
For a full list of the 209 types of PCBs, refer to the PCB congener list. Note that biphenyl is not officially considered a PCB congener because it does not have chlorine atoms attached. However, it is often included in scientific writings about PCBs.