Polychlorinated biphenyls (PCBs) are a class of synthetic organic compounds. Each molecule contains two benzene rings.
A benzene ring consist of six carbon atoms to which hydrogen atoms are connected. In PCBs molecules, chlorine atoms interchange some of these hydrogen atoms. Total, 209 different kinds of PCBs can be developed. These different arrangements are called congeners, each containing a particular number of chlorine atoms placed at particular locations. For example figure show a PCB molecule having chlorine atoms at the 2 position on one ring, and the 2’ position on the other.
Physical properties of PCBs:
- Can persevere in the environment
- Extremely unreactive and unaffected by acids and bases breakdown
- Insoluble in water, but freely soluble in fats
- can accumulate in animal fat and along the food chain
- Their level of solubility is mainly reliant on the quantity and location of chlorine atoms in molecule
- cannot simply evaporate
PCBs can be evaluated in:
- Organic samples: human blood, milk, and fatty tissue, animal tissues and in some foods and foodstuff such as, fish and dairy products.
- Environmental samples: air, drinking water, soil, sediment, and solid waste.
Mechanism of transportation of PCB
PCBs are found in significant amounts in sediments. They can persist in sediments for a longer period, be gradually distributed into the water, and evaporate into air. Releases from water into air continue particularly when there is a hot weather. This in air can build up on land by wet deposition when it rains or snows, or by dry deposition when they connected to the dust or soot that deposit on the land. They could be transferred by pests, which have been exposed, to them.
How do PCBs build up in living organisms?
PCBs can accumulate in living organisms from the atmosphere (bioaccumulation) and along the food chain (bio magnification). they persist in fatty tissues than any other body parts. The amount of its bioaccumulation influenced by how quickly they are accumulated, break down and removed by the individual. Many animal species can transform some PCB congeners, such as insects, crabs and vertebrates, as well as some birds, fish, and mammals. The degree of breakdown depend upon the animal species and type of its molecule. The intensity is higher in sediments than in water, therefore the concentrations of PCB are frequently higher in bottom-feeding species. They generally biomagnify along the food chain, which leads to greater concentrations in organisms that are higher up in the food chain.
In the aquatic environment, concentrations will thus be greater in shellfish than in the plankton on which they feed, and even greater in animals at the top of the food chain such as large predatory fish or mammals (seals, dolphins, and whales).
On land, the bio magnification occurs, for instance, through the accumulation from soil or plant leaves to worms or insects and finally to birds and mammals. They can also be found in the eggs of contaminated birds.
Humans can also accumulate this from food. On average, in humans, the concentration in fatty tissues is over a hundred times greater than in the food they eat.
Fate of PCBs in the body
PCBs can enter human cells and tissues with contaminated air, food, or through contact with the skin. Tests on laboratory animals had showed that they are easily absorbed through the digestive tract when ingested, and to a lesser amount through the skin. The main elimination routes are feces, urine, and breast milk. In the gastrointestinal tract, ingested diffuse across cell membranes and enter blood vessels and the lymphatic system. They are particularly those that contain a large number of chlorine atoms, are freely soluble in fats and can store in fat-rich tissues such as the liver, brain and skin. In mothers, They have also been found in placenta, umbilical cord blood, and breast milk. They can go through different transformations in the body and then either be stored in specific tissues or excreted.
Transformations that lead to accumulation:PCBs can easily be converted into persistent metabolites that are not freely excreted and that can store in particular tissues and body fluids. The half-life of these persistent metabolites depends on the structure of the PCB molecule.
Transformations that lead to excretion: PCBs can leave the blood and enter tissues very quickly and converted into water-soluble substances. These can combine with glutathione and glucuronic acid, that are naturally present in the body, forming a substance that is excreted into urine and feces. The degree of transformation depends on the number and position of chlorine atoms of PCB molecule. It is slower when there are more chlorine atoms on both phenyl rings and faster if there are two carbon atoms without any attached chlorine atoms next to each other.
Effects of PCBs on reproduction
Harmful effects on fertility and reproductive organs have been tested in laboratory animals like rats, mice, and monkeys exposed to PCBs through their food.
Rats that were fed comparatively high doses of PCBs showed:
- an increase in weight of testes at 3.5 mg/kgbody weight per day,
- a decreased fertility at 12.5 mg/kgbody weight per day,
- a reduced sperm quantity at 25 mg/kgbody weight per day, and
- a decreased litter size at 35.4 mg/kgbody weight per day.
Harmful reproductive effects have been detected in the progeny of female rats exposed to a mixture of comparatively highly chlorinated PCBs molecule during lactation. This indicates that PCBs transmitted during breast feeding can have a long-term effect.
Monkeys that were fed PCBs indicated:
- a delayed menstruation at 0.1 mg/kgbody weight per day, and
- a decreased fertility at 0.2 mg/kgbody weight per day.
- No harmful effects on fertility were detected below 0.005 mg/kg body weight per day [NOAEL].
Influences on female hormonal activity, have been broadly examined in-vivo and in-vitro in rats and mice. The studies show that PCBs may increase or simulate the estrogen activity or, on the other hand, it prevent the action of estrogens like in case of dioxin-like PCBs.
Developmental effects of swallowed PCBs have been confirmed on many animal species. Contact of PCBs to females during gestation and lactation may restrict with the growth of the fetus.
In monkeys, several behavior tests were performed on the offspring of exposed females. Newly born monkeys were exposed to a mixture similar to PCB composition in human milk, displayed behavioral change with a LOAEL of 0.0075 mg/kg body weight per day. In another study, changes in the growth of nervous system were detected after exposure to high concentration of non-planar PCBs, but not with coplanar (dioxin-like) PCBs.
Effects of PCBs on Immune system
The influences of PCBs on immune system of young rats was evaluated. Fifteen weeks old, rats exposed to high concentration of PCBs had a reduced thymus gland and a decreased number of immune cells indicating that PCBs had impaired the functions of the immune system. The lowest dose at which no harmful effects on the immune system of rats were observed was 1 mg/kg body weight per day. Highly chlorinated PCBs have more adverse effects on immune system of mice than those with low chlorine content.
Biological effects of PCBs
Two main effects have been detected; the decreased production of antibodies in response to an immunological test and alterations in some of the organs of lymphatic system such as spleen and thymus. PCBs can cause reduced production of antibodies, an increased vulnerability to disease, and a decreased thymus weight. PCBs had less adverse effects on adult animals than fetus. These adverse effects occur in all animal species observed at high concentration of PCB, and only in some, such as the rhesus monkey, at low doses. Specific PCB mixtures affect the transmission of nerve impulses in different areas of the brain and spinal cord in rats and monkeys.
PCBs can induce reproductive and immune system malfunctions. Through the “one health’ concept, herbal plants can be harnessed to provide phytoremediation, and immunostimulation for sustainable improvement in environmental, animal and public health.