PCB’s, PBB's and Dioxins

 PCB’s

• Polychlorinated biphenyls
• Mixtures of biphenyls with 1-10 chlorine atoms per molecule

• Oily fluids with high boiling points, great chemical resistance, low electrical conductivity and high refractive index
• Used as plasticizers in polyvinyl polymers, as insulators, coolants in transformers and heat exchange fluids, Inks, paints, sealants, insulators, flame retardants and microscope immersion oils
• Chemically inert and biologically stable- Correlated with their degree of chlorination
• Structure similar to DDT and so is their persistance and potential for biomagnification

• PCB residues have been detected in large numbers in environmental samples and they accumulate in higher trophic level animals
• Conc. of PCB over 1 ppm detected in people with no known occupational exposure
• Predatory fish (trout, salmon etc)- accumulation of 20-30 ppm
• Predatory and fish eating birds- several 100’s of ppm

• PCB contaminated cooking oil-leaky heat exchanger- 1000’s poisoned in Japan-liver damage and skin conditions like chloracne (consumptions of several grams)
• Low intakes from general environmental condition not known to harm but it is implicated in egg shell thinning and reproductive failure of predatory-fish eating birds
• PCB’s and other chlorinated hydrocarbons (DDT) suspected as potential human carcinogens

• Various degrees of chlorination & High number of isomers- are the difficulties in identification and quantitative measurement of PCB’s
• Photochemical conversion of DDT’s to PCB’s is another reason for widespread occurrence

• Biphenyls-used by some microorganisms as sources of C and energy- Mono, di, tri and tetra chloro biphenyls are subject to biodegradation
• Extensively chlorinated biphenyls are persistent, environmental pollutants- substituents prevent ring hydroxylation

• Government regulations strictly ban PCB’s from food processing equipments and foodpackaging materials-phasing out the use as plasticizers

 PCB’s- Relatively resistant to biodegradation

• Aerobic degradation by white rot fungi (Phanerochaete), Acinetobacter and Alkaligenes

• Anaerobic, reductive dehalogenation- Degradation usually by co-metabolism, enhanced by the addition of less chlorinated analogues like dichlorobiphenyl-Seen commonly in soils, sediments, aquatic environment- More extensively chlorinated PCB’s more likely to be dechlorinated than less chlorinated ones- Meta and para positions dechlorinated in preference to ortho position-Little knowledge about anaerobic degradation

PBB’s

• Polybrominated Biphenyl’s- used as flame retardants
• Polychlorinated terphenyls –PCT’s – with 3 attached phenyl rings-used as plasticizers

PCT's

• Pollution potentials similar to PCB’s
• Manufactured on a small scale than PCB’s
• Accidental mixing of PBB flame retardants and cattle feed additives in a chemical plant-fed to dairy cows in Michigan-led to destruction of many dairy herds-economic loss
• No human casualities- but large numbers of people consumes PBB’s through dairy products before the cause was traced and eliminated
• This incident reminds of the long residence time and biomagnification potential of PBB’s as environmental pollutants

Dioxins

• 2,3,7,8-tetrachlorodibenzodioxin (TCDD), chlorodibenzodioxins, chlorodibenzofurans
• Frequent contaminants of PCB’s and the herbicide 2.4,5 –T
• Some of the dioxins arise during manufacture, others are formed through thermal degradation of chlorophenols and chlorobiphenyls

• Characterised by high recalcitrance
• High degree of acute and chronic toxicity
• Contamination as low as ppb range hazardous to human health
• Short-term exposure of humans to high levels of dioxins may result in skin lesions, such as chloracne and patchy darkening of the skin, and altered liver function.
• Long-term exposure is linked to impairment of the immune system, developing nervous system, the endocrine system and reproductive functions.
• Chronic exposure of animals to dioxins has resulted in several types of cancer

• Due to the omnipresence of dioxins, all people have background exposure and a certain level of dioxins in the body, leading to the so-called body burden.
• Current normal background exposure is not expected to affect human health on average. However, due to the high toxic potential of this class of compounds, efforts need to be undertaken to reduce current background exposure.

• 2,3,7,8-tetrachlorodibenzodioxin (TCDD) and related compound contamination in human settlements were reported by contaminated waste oil (United States) and by explosion ofchemical reactor(Italy) -Evacuated and closed indefinitely

• Biodegradability doubtful
• Oxidation of TCDD and other chlorinated hydrocarbon pollutants by the lignin degrading Basidiomycete Phanerochaete chrysosporium reported

Reference

Microbial Ecology: Fundamentals and Applications- Ronald M. Atlas, Richard Bartha