PCBs--Polychlorinated biphenyl (a pair of benzene rings joined by a single carbon-to-carbon bone.

PBBs-- Polycbrominated biphenyl

• Each ring on the biphenyl structure can have up to 5 chlorine atoms, so it is theoretically possible to make 209 different kinds (congeners) of PCBs.

• In practice, most PCBs are mixtures of 70 or more of these different congeners.

• First made in 1881, commercialized in 1920s.

• Monsanto Company sold PCBs with the trade name ‘Aroclor’ with a 4-digit ID code. The last two digits in the code indicated the percentage of chlorine (by wt) in the total mix.

Chemical Properties of PCBs

• High boiling point• low water solubility• low conductivity• hard to burn• resistant to acid and bases• mostly inert

Use of PCBs

• Adhesives• Ballasts for fluorescent light fixtures• Carbonless copy paper• Insulation in transformers• High pressure hydraulic fluids• Machine tool-cutting oils• Specialized lubricants and gasket sealers• Plasticizers• Protective coatings for wood, metal, and concrete• Some varnishes and epoxy paints contained PCBs• Braided cotton and asbestos were impregnated

with PCBs to improve their fire resistance.

• In 1966, Sören Jensen, a Swedish chemist from the University of Stockhom, reported evidence that PCBs had entered the food chain as early as 1944.

• Scientists soon found PCBs everywhere, most disturbingly, in human hair and fat.

Why do we worry about PCBs?

1. Cause thin-shelled eggs in birds because they stimulate enzymes that make estrogen more soluble and readily excreted. Low estrogen levels keep calcium reserves low and make little calcium available for eggshell formation

2. Chronic exposure has adverse health effect although acute toxicity of PCBs is low.

3. As PCBs become more chlorinated, they also become more carcinogenic.

4. It is virtually impossible to find environmental samples that do not contain some trace amounts of PCBs and other chlorinated chemicals. Even foods grown in “chemical free” fields can receive trace amounts of PCBs in atmospheric deposits from incinerator exhausts.

Note: Burning PCB-contaminated waste releases PCBs into the atmostphere.

5. One of the more prominent groups of persistent compounds. Sediments contaminated with these toxicants 20 or more years ago still show disturbingly high level.

Incident of contamination

• In 1974, Michigan Chemical Corporation accidentally included 10-20 bags of “Firemaster” (a PBB-containing flame retardant, with a truckload of “Nutrimaster”, that they were delivering to a feed mill.

• As a result, contaminated milk and eggs exposed most of the population in Michigan to PBBs.

• The incident became a multimillion dollar environmental catastrophe.

Good news--

• Microorganisms slowly dechlorinated PCBs under anaerobic conditions.

• PCBs in many aquatic environments will become naturally attenuated with time if left undisturbed.

• Biphenyl addition increases the population of microorganisms that cometabolize PCBs.