What are Polychlorinated Biphenyls (PCBs)?

Polychlorinated Biphenyls (PCBs) are persistence organic pollutants having one or more chlorine atoms attached to the joined benzene rings. 

What are Polychlorinated Biphenyls (PCBs)?Polychlorinated Biphenyls (PCBs)

There are more than 209 types of PCBs having different number and location of chlorine atoms attached to the benzene ring. PCBs are distributed into coplanar, mono-ortho-substituted PCBs, and non-dioxin-like PCBs. Coplanar and some mono-ortho-substituted PCBs have dioxin-like properties.

Properties

The toxicity level of PCBs depends upon the arrangement of chlorine atoms in the molecule. Many type of PCB have dioxin-like properties. These PCBs have one chlorine atom at the ortho-position in benzene ring. The benzene rings of such molecules can interchange and adopt a coplanar arrangement, which have the similar toxicity as the polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs).

However, many other types of PCB have more than two chlorine atoms at the ortho-position in the benzene ring.In such molecules, the two benzene rings are not in the similar plane, and slow non-dioxin-like properties.

Industrial PCB are a combination of various types PCB and comprises of small quantity of PCDFs or PCDDs. The contamination level of such molecules is significantly higher as compared to PCBs [ATSDR 2000].

Uses

PCBs are insulated and non-flammable therefore they were commonly used in making different things for approximately 50 years (1929 and 1977).

interesting reading:  Polychlorinated Biphenyls and Its Potential Effects on GI Health

They were used in producing

  • Diffusion pump oils
  • Pesticide extenders
  • Heat conversation and dielectric liquids in electric appliances like transformers and capacitors
  • Hydraulic and greasing liquids
  • As constituents in sealing compounds, paint industry, glues, flame-retardants, and plasticizers.

In 1977, the United States banned the production of PCBs because of their potential carcinogenicity.

Trade Names

The following trade names are used for commercial PCB mixtures:

  • Aroclor
  • Asbestol
  • Askarel
  • Clorphen
  • Diaclor
  • Dykanol
  • EEC-18
  • Eucarel
  • Nepolin
  • Pyranol

Source of PCBs

PCBs can be emitted in the environment by or from

  • Removal of PCB-containing products in public landfills
  • Prohibited or inappropriate depositing of waste material that contained PCBs, for example, transformer liquids
  • Leakages from electrical transformers and capacitors having PCBs
  • Contaminated waste places

PCBs have been categorized in at least 500 of the 1,598 harmful waste places on the EPA’s National Priorities List, and low concentrations of PCBs can be observed all over the world [ATSDR 2000].

After emitted into the environment, PCBs

  • Accumulates in animal and human tissues as they shift up the food chain,
  • Gradually metabolize
  • Driven and transferred within the environment [ATSDR 2000; Safe 2007].

After emitted into the environment, PCBs adsorb effectively to water, air, soil, and sediment. Consequently, these mixtures have a tendency to persist in the environment, with half-lives from months to years. PCBs can be transferred to the plants via soil inconsequentially.

Cycling of PCBs includes volatilization from soil and water surfaces into the atmosphere, with the following elimination from the atmosphere by wet or dry deposition, then revolatilization.

interesting reading:  Polychlorinated Biphenyls and Its Potential Effects on GI Health

Exposure Routes

The major sources of PCBs contamination are the atmosphere and the factories. As they degrade slowly, highly chlorinated PCB complexes can persevere in the environment for years.

  • The principal route of exposure to PCBs is the ingestion of infected foodstuffs, especially beef, mutton, chicken meat, and fish.
  • Industrial contact with PCBs occurs generally via the inhalation and dermal routes.
    • Industrial contact with PCBs may also result in waste site cleaning or dumping activities.

Who will be affected by PCBs?

  • People who eat large quantities of infected fish.
  • Children whose mothers consume large amounts of contaminated fish during pregnancy or nursing
  • Farmhouse people who consume contaminated food; and
  • Persons who are living near burners, PCB dumping facilities, or NPL harmful waste places where they have been identified.
  • People with abnormal liver function may degrade PCBs inefficiently than healthy people.
  • Labors can be contaminated by PCBs through:
    • repair of equipment manufactured before 1977
    • accidents or leaks relating PCB,
    • waste site cleaning or dumping activities

Metabolism PCBs

The liver is the main site of a breakdown of PCBs, which involves hydroxylation and conjugation with glucuronic acid and sulfates. They are broken down by the microsomal monooxygenase method catalyzed by cytochrome P-450 to phenols, which can be conjugated or further hydroxylated to form a catechol [Safe SH 2007]. Glucuronide and sulfate conjugates are removed mostly in the urine, while hydroxylated metabolites are removed mostly in the bile.

The rate of specific degradation of PCB influenced by the number and location of chlorine atoms. Steele et al. (1986) expected the half-life in humans for less chlorinated PCB compounds (Aroclor 1242) as 6-7 months and high chlorinated PCB compounds as 33-34 months. Phillips et al. (1989) calculated half-lives of 2.6 and 4.6 years for the less (Aroclor 1242) and high (Aroclor 1254) chlorinated PCB compounds respectively in capacitor workers. Generally, lower chlorinated biphenyls are easily break down than highly chlorinated biphenyl. Consequently, highly chlorinated biphenyls have a tendency to persist in the body for a longer time than lower-chlorinated biphenyls.

interesting reading:  Polychlorinated Biphenyls and Its Potential Effects on GI Health

Excretion

PCBs are mainly removed from the body when they conjugated and changed into more polar substances. The main routes of elimination of PCBs are urine and feces.

Summary

PCBs are organic compounds having carbon, hydrogen and differing number of chlorine atoms. Their production has been banned in many countries since the 1970s as they have harmful effects on human and animal health.

PCBs have been used in many products like plastics, paints, and adhesives. They are unaffected by acids, bases, and heat, so they have also been used in electric equipment like transformer. Removal of waste that comprises can cause environmental pollution. They can stay in the atmosphere and accumulate in human and animal tissue by the food-chain. Humans may be contaminated with them by ingesting infected food and water or inhaling polluted air. The rate of metabolism and excretion of it in the body depend on the type of PCB.

Aftab Qamar

Aftab Qamar

Dr Aftab Qamar DVM from PMAS Arid Agriculture University Rawalpindi Currently doing M.Phil. in animal nutrition from Guangdong , China

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