Chemistry

Polychlorinated biphenyls also known as PCBs belong to a group of 209 synthetic organic chemicals composed of between 1 and 10 atoms of chlorine attached to biphenyl rings (1). These individual chlorinated biphenyl rings, congeners, were mainly manufactured as mixtures comprising of 60-90 congeners. The PCBs have a general formula of C12H10-xClx (1). PCBs are sparingly soluble in water but highly soluble in oils, fats and organic solvents. They are stable compounds and not easily degradable although some thermal, biochemical and chemical factors can degrade PCBs through spontaneous (metabolically) or intentional (incineration) processes (1). Degradation of PCBs by heat requires enormous application of heat energy and metabolic degradation proceeds slowly as compared to other compounds. The once highly controversial PCBs were produced as complex isomers with varying numbers of chlorine atoms. They were manufactured and sold with a 4 digit number attached to their trade names where the first two numbers referred to the carbon atoms attached to the biphenyl rings and the other two numbers referred to the chlorine percentage by mass in the PCBs mixture (1). For instance, Aroclor 1260 contained 12 atoms of carbon attached to the biphenyl rings and had 60 percent chlorine in its total mass (1). The commercial applications of PCBs were mainly made possible by their physical and chemical stability such as thermal and chemical resistance which attracted uses such as electrical insulation and as lubricants. Because of these properties and the poor understanding of their toxicity levels, PCBs may make a come-back although they were banned due to environmental contamination.  However, this calls for detailed studies that will disapprove the existing studies of PCBs on the harmful health hazards caused by the compounds.

History, Properties and Uses of PCBs The PCBs are classes of chemical compounds that never existed naturally but were slowly released into the environment beginning 1900s by industrial processes and human activities (1). In 1929, Monsanto became the only American company to first manufacture PCBs which made a breakthrough in industrial chemistry. PCBs were highly celebrated for their properties of preventing explosions and fires and the fire code made PCBs a requirement for fire prevention (1). The PCBs oils have low reactivity, are inflammable, have high resistance to electricity, are excellent insulators and are subtle even when exposed to pressure and heat. These properties made PCBs oils to be used as insulators for capacitors and transformers and dielectric fluids. The oils slowly gained applications in hydraulic fluids, systems for heat transfer, casting wax, adhesives, pigments, plasticizers, compressors, fluorescent light ballasts and much more (1). The PCBs application declined in 1970s and then completely stopped in 1977 due to environmental and health concerns (1).Health Effects of PCBsPCBs as Carcinogens A number of studies have also shown PCBs to be associated with cancer of the biliary tract and that of the liver. From animal studies, rats that feed on PCBs have a higher propensity of developing hepatomas or liver cancers. This may bring into a conclusion that PCBs are likely to be carcinogenic compounds among humans. The Environmental Protection Agency (EPA) and the International Agency for Research on Cancer (IARC) have classified PCBs as potential human carcinogens and some studies under the National Toxicology Program have already concluded that PCBs can cause human cancer (1). The National Institute for Occupational Safety and Health has classified PCBs as possible occupational carcinogenic compounds. Among the studies on exposed workers have shown some changes in urine and blood and this are indicators to liver damage.

The most affected population consists of the mothers and children who are exposed to PCBs. Metabolism of these compounds in the liver is slow and often leads to hydroxyl conjugates which are highly poisonous and have a potential to cause hepatomas (1). A number of melanomas have also been observed apart from hepatomas. PCBs exposure causes skin cancers perhaps due to the interference with melanocytes that produce melanin. Melanin is essential for the protection of humans against ultraviolet rays from the sun. Absence of melanin paves way for destructive effect of UV rays on the deoxyribonucleic acid (DNA) which eventually leads to a number of mutations (1). Consumption of fish contaminated with PCBs lead to the development of biliary tract cancer, cancer of the gall bladder and cancers of the gastrointestinal (GIT) cancers. The main entry of PCBs into the human system is through the GIT where humans consume heavily PCBs contaminated fish. The PCBs get into the system and circulate in the entire body. The PCBs eventually reach their destiny in the fat tissues where they are periodically released to cause detrimental health effects. While there is blood-brain barrier, the PCBs can get into the brain and cause brain cancer (1).

Women exposed to PCBs sometimes develop breast cancer. The mechanism for the development of breast cancer can be thought to be that of activation of protooncogenes to oncogenes. Protooncogenes consist of class of human genes that have the property to cause cancer but upon activation. The protooncogenes are usually activated to oncogenes by a number of factors which include chemical and viral activation factors. PCBs, particularly the metabolites such as hydroxyl PCBs have a potential to activate protooncogenes and oncogenes (1).Non-Carcinogenic Effects of PCBs on Human Health Large amount exposure to PCBs has been observed to cause various skin conditions like rashes and chloacne. Animal experiments have shown that PCBs cause skin conditions presented as acne. PCBs have also been found to cause anemia, thyroid, stomach, and liver damages in animal experiments.  The changes in behavior, reduced immune defense and impaired reproduction observed in animal studies show the potentiality of PCBs of causing devastating effects in humans. While these effects have been observed, PCBs have not shown to cause any birth effects. The compounds are not therefore classified as teratogenic despite the heated controversy surrounding the topic of PCBs (1).

PCBs cause women to give birth to babies with reduced weights. Women that heavily feed on PCBs contaminated fish have babies with reduced weights than the ones who do not feed on PCBs contaminated fish. There are also abnormal infant behavior responses to tests. The behaviors include greatly declined short-term memory and motor skills problems which last for a number of years. The children often have a lowered IQ than those born of mothers not exposed to PCBs (1).

There may be also some problems of developing skills in visual recognition and general learning. Among other effects to babies born of mothers exposed to heavy PCBs concentration include deranged immune system and various effects to the thyroid functions. Perhaps one of the ways through which children get exposed to PCBs toxicity is through breastfeeding. However, placental PCBs transfers are also not unusual as the mother can pass the PCBs to the fetus during early stages of development in the mothers womb. This affects their immune function. The PCBs just like dioxin binds to essential immune receptors and influence the working of the T cells and the lymphocytes. There has been an increasing association of Epstein-Barr virus infection with increased PCBs exposures (1). In addition, since the immune system is weakened, there are higher chances of individuals developing non-Hodgkins lymphoma. Animal experiments show an increased risk of PCBs exposed animals to get infected with Epstein Barr virus. This similar effect is seen among individuals exposed to dioxins and dibenzofurans (1).

PCBs also affect the levels of estrogen thus contributing to the overall problems of reproduction. There are high chances of males being feminized in the womb of mothers. In some cases, the babies turn out to be intersex meaning that they are neither nither males nor females. The babies born of mothers with PCBs contamination also have chances of developing both the female and male reproductive organs. While the two organs may develop, neither the penis nor the vagina will be fully functional there may be some aberrations in the normal functioning of the reproductive system. This observation has mainly been made in whales and polar bears exposed to PCBs. These animals that are exposed to PCBs develop both the female and the male reproductive organs and they do not usually reproduce. This condition is known as the endocrine disruption and therefore PCBs are referred to as endocrine disrupting chemicals (EDCs) (1).

Other non-cancer effects of PCBs such as the ocular and dermal effects have also been documented among monkeys as well as humans subjects. Liver toxicity among rodents has also been documented. There have also been studies to evaluate serum cholesterol, serum triglyceride and blood pressure and linked to the increased levels of serum PCBs among human subjects (1).Environmental Effects of PCBs

PCBs do not degrade easily and therefore remain in the environment for a long time. They continue to be maintained in air, soil and water constantly exposing humans to devastating health risks .The higher the chlorination of biphenyls, the more they are resistant to degradation. Unfortunately, the qualities that make PCBs much valued are the same qualities that make the PCBs to be environmental unfriendly the chemical and thermal stability. The compounds therefore increasingly build up in the environment as more PCBs are introduced. The PCBs increasingly got released into the environment prior to their ban in 1977 through releases and leaks mainly from PCBs containing electrical transformers(1). It is also suggested that PCBs got released through industrial incinerators and the burning of municipal waste. The PCBs enter the human food chain hence leading to harmful health hazards. Organic matter which is contaminated with the PCBs is consumed by worms. The same organic matter provides for nutrients for plant growth. These worms are then eaten by small fishes which are eaten by larger fishes. These small larger fishes are eaten by humans. The eagles also feed on the large fish and since the eagles have a longer lifespan, they continue to feed on the larger fish that fed on small fishes which were heavily contaminated with PCBs from worms. This eventually leads to the concentration of PCBs in the fatty tissues of eagles in a process known as bioaccumulation. Similarly, as humans continue to feed on the contaminated fish, bioaccumulation of PCBs in the fatty tissues also increases and the health consequences are always devastating (1).

A common historical case of PCBs contamination occurred in Hudson River where PCBs were dumped by the General Electric Company between 1946 and 1977(1). The chemicals entered the human food chain through humans consuming fish that fed on contaminated zooplanktons and phytoplanktons. While some of these chemicals may be biotransformed into non-toxic metabolites, most congeners of PCBs take long to be metabolized hence get retained in fat tissues.Conclusion

The controversial PCBs still hold significant debate among health and environmental protection activists since their ban in 1977. Their potential physical and chemical properties that attract an array of uses continue to make chemists to start thinking of reusing the PCBs. However, for these compounds to be reintroduced, efforts have to be made to make them less toxic. Structural chemists and pharmacologists have a challenge to come up with compounds of PCBs that are not toxic to humans. Conjugation of some groups or moieties can help reduce toxicity levels of PCBs. It is hoped that the successful transformation of these compounds will lead to the reintroduction of the PCBs in uses such as the use in capacitors and transformers, voltage regulators, electromagnets, carbonless carbon paper and oil-based paints. Meanwhile intensive public education should be provided to the members of the public and educate them on the dangers of polluting the environment. The health hazards such as cancer, immunotoxicity, neurotoxicity and developmental problems should be communicated in a clear way. Knowledge on environmental protection seems critical in preventing such problems that were caused by deliberate dumping of PCBs by the General Electric Company between the years 1946 to 1977.