Polychlorinated biphenyl

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Polychlorinated biphenyls (PCB-compounds): a group of oily stable chemicals, which are mixtures of many congeners. They are very poorly water soluble and lipophilic (see #physicochemical properties), and therefore accumulate in lipids (fats) of living organisms (see PCB – environmental persistence), and bioaccumulate in trophic levels (see PCB – biomagnification). They contain small amounts (1 to 40 mg/kg) of PCDFs as impurities (see PCB – contaminants). (For detailed information, see International Programme on Chemical Safety, Environmental Health Criteria 140, WHO, Geneva, 1993,; Safe, Crit. Rev. Toxicol. 1994:24:87-149,).

Acute toxicity

toxicity occurring after a single dose within a few weeks. It is generally low, but it depends on the mixture of congeners, because dioxin-like (non-ortho, see ortho-PCBs) PCBs are much more toxic than other congeners. Their toxicity resembles that of dioxins (see PCDD/F - acute toxicity).

Analysis

measurement of concentration of a compound in a sample. PCBs can be analysed by gas chromatography by using electron capture detector. This is a fairly widely available method, but if absolute accuracy and congener-specific analysis is needed, gas chromatography-mass spectrometry (see this) may be needed. This is a very expensive method not available in many laboratories in Europe.

Biomagnification

property of PCB compounds to concentrate from one trophic level to the next (see also biomagnification). Many PCBs are extremely persistent in the environment. Increase in chlorination (see PCB - physicochemical properties) increases both stability and lipophilicity. Therefore they concentrate along the food chain, and species at the "top" of the food chain (such as seals or eagles) are in special danger.

Carcinogenicity

capacity of PCBs to cause cancer. A number of long-term carcinogenicity studies have been carried out in mice and rats. Interpretation is complicated by the lack of information of minor impurities, especially PCDFs. Some tested mixtures were free of PCDFs. In many of these studies hepatocellular adenomas and/or carcinomas (tumours of the liver) were found although the increase was not always significant. PCB mixtures are considered non-genotoxic, PCBs do not cause mutations or chromosomal damage. Therefore rodent tumourigenicity is considered to be of epigenetic nature (promoting rather than initiating effect, see mutagenicity, promoters). IARC classifies PCBs as probable human carcinogens on the basis of animal data. Remarkable caution is needed in extrapolating the available animal data to humans. None of the available epidemiological studies provide conclusive evidence of an association between PCB exposure and increased cancer mortality. (For more information, see International Programme on Chemical Safety, Environmental Health Criteria 140, WHO, Geneva, 1993).

Chemical structure

see chemical structures.

Contaminants

by-products found unintentionally in PCB products. Technical PCBs contain a number of various chlorinated byproducts, e.g. about 40% chlorobenzenes, a few percent chloronaftalenes, and also small amounts of PCDDs and PCDFs (93% PCDFs and 7% PCDDs of the total PCDD/F content in the PCB product which caused the Yusho Rice Oil accident, pentaPCBs, tetraPCBs and hexaPCBs dominating). PCDFs have been detected up to 40 mg/kg (ΣPCDF) in PCBs. Because commercial products were not sold according to composition but according to their physical properties, there may be large variations both between preparations and between lots of a preparation.

Disposal of

PCB oils cannot be burned in usual conditions, because they burn poorly and evaporate to the environment along with their PCDD/F impurities. PCDFs may also be formed during PCB burning. Therefore PCBs are considered problem waste, which must be incinerated by a well-controlled process in a high-quality waste incinerator at the temperature of 1000 - 1200 °C, and with an effective fly-ash filtering system (see also incineration).

Elimination

process of discharging PCB out of the body. Elimination of chemicals out of the body is usually based on two mechanisms, excretion (such as in urine or faeces) or metabolism (chemical breakdown, often in the liver). Only water soluble materials can be excreted in the kidneys to urine, and PCBs as lipid soluble and poorly water soluble chemicals cannot be excreted practically at all as such. Metabolism tries to make them more water soluble, but especially higher chlorinated PCBs (see PCB - physicochemical properties) are metabolised very poorly, and therefore cannot be effectively excreted even with the help of metabolism. Therefore they accumulate in body fats, and their half-life (see this) may be even several years.

Environmental persistence

ability of PCBs to continue existence in the environment. The stability of PCBs is a technical advantage, but it also means that they are extremely persistent in the environment. Increase in chlorination (see PCB - physicochemical properties) increases both stability and lipophilicity. Neither soil microbes nor animals are able to break down effectively the highly chlorinated PCBs (see also ortho-PCBs). This causes very slow elimination (see PCB - elimination). Because some PCBs are more persistent than others are, the spectrum of congeners in the environment, animals and humans is never quite identical to that in the original commercial product. In water, PCBs are adsorbed on sediments and organic matter. This decreases the rate of volatilisation, but also slows down the degradation.

Half-life

time needed to decrease the amount of PCBs to one-half. There is no systematic information on the half-life of all PCB congeners in humans. The half-lives of the most toxic non-ortho PCBs have been estimated to vary from 0.1 to 13 years. Somewhat fragmentary information suggests that the half-lives of PCBs are on the average around one year (with much variation to each direction). See also half-life.

Physicochemical properties

All PCBs are lipophilic (soluble in fats and oils) and practically insoluble in water, but lipophilicity increases by increasing rate of chlorination (see PCB - chemical structure). Technical mixtures are mobile to viscous oils depending on the rate of chlorination, and their boiling point varies from 300 to 400 °C. They resist high temperatures and oxidising conditions without breaking down. Their electrical conductivity is very low which made them suitable cooling liquids for electrical equipment.

Sources

Emissions. PCBs were manufactured from 1930 to 1970s or 1980s (varying in different countries), and the total production was in excess of a million tonnes. PCBs are still manufactured e.g. in Russia. They have spread to the environment in accidents (such as transformer fires or leaks), from volatilisation of waste landfills and incineration of mixed municipal waste (e.g. plastic materials). The virtually universal distribution of PCBs suggests transport in air. Human exposure. Food is the major source for human exposure to PCBs and dioxins, especially fatty foods: dairy products (butter, cheese, fatty milk), meat, egg, and fish. Some subgroups within the society (e.g., nursing babies and people consuming plenty of fish) may be highly exposed to these compounds and are thus at greater risk. Daily intake of PCBs is a few µg per person. PCB concentrations have been screened in two WHO international studies, and in Central Europe the concentrations have decreased in breast milk from 400-800 µg/kg (sum of six marker PCBs [see this] in milk fat) to 200-400 µg/kg from 1987 to 1993. The decrease in environmental concentrations is partly due to prohibition of the use of PCBs in Europe, partly due to improved incineration technology (see also PCDD/F - sources).

Toxicity in humans

This is difficult to evaluate, because the exposure has usually been to a mixture of different congeners and also impurities such as PCDFs. Occupational exposure may be to different congeners than exposure of general public through food, because some congeners are more easily degraded in the environment than others. In occupational conditions skin rashes, itching, irritation of the conjunctivae, pigmentation of fingers and nails, chloracne, liver problems and neurological and unspecific psychological symptoms have been seen. In Yusho and Yu-Cheng incidents (see these) also various skin and nail problems were seen, as well as liver enlargement and immunological problems. In children of Yusho and Yu-Cheng patients skin problems, oedematous eyes, dentition at birth and lowered birth weight were seen, among others. Total exposures in these cases have been estimated at 600 to 1,800 mg per person (ΣPCB). The daily intake of PCBs in general population in most industrialised countries is of the order of some micrograms per person (ΣPCB). Such levels have not been associated with disease. (For a review, see Safe, Crit. Rev. Toxicol. 1994:24:87-149; for detailed evaluation, see International Programme on Chemical Safety, Environmental Health Criteria 140, WHO, Geneva, 1993).

Trade names

Many companies in several countries have manufactured PCBs. The trade names include Apirolio, Aroclor, Clophen, Fenchlor, Kanechlor, Phenoclor, Pyralene, Pyranol, Pyroclor, Santotherm FR, and Sovol. Sometimes the trade name indicates the degree of chlorination, e.g. Aroclor 1254 contains 54 % of chlorine, 12 indicates the number of carbon atoms.

Use

PCBs have been used since 1930 because of their stability and low flammability (see PCB - physicochemical properties) as insulating materials in electrical equipment (electrical capacitors, transformers), as plasticizers (softening materials) in plastic products, and for a variety of other industrial purposes (in gas-transmission turbines, vacuum pumps, hydraulic fluids, adhesives, fire retardants, wax extenders, lubricants, cutting oils, oils in heat exchangers etc.). The total production was in excess of a million tonnes. Common trademarks included Aroclor, Clophen, and Kanechlor (see PCB - trade names).