Polybrominated biphenyls (PBBs), also calledbrominated biphenyls orpolybromobiphenyls, are a group of manufactured chemicals that consist ofpolyhalogenated derivatives of abiphenyl core. Their chlorine analogs are thePCBs. While once widely used commercially, PBBs are now controlled substances under theRestriction of Hazardous Substances Directive, which limits their use in electrical and electronic products sold in the EU.

The family of PBBs consists of 209 possible substances, which are calledcongeners (PBB = C₁₂H_(10−x)Br_x (x = 1, 2, ..., 10 =m +n)). The number ofisomers for mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, and decabromobiphenyl are 3, 12, 24, 42, 46, 42, 24, 12, 3 and 1, respectively.^[1]

PBBs usually exist as colorless to off-white solids. PBBs soften at 72degrees Celsius and decompose above 300 degrees Celsius. They have lowvapor pressure, are very soluble inbenzene andtoluene, and insoluble inwater. They are degraded byUV light.

PBBs are used asflame retardants of thebrominated flame retardant group. They are added to plastics used in products such as homeelectrical appliances,textiles,plastic foams,laptop cabinets, etc. to make them difficult to burn.

A number of substituted PBBs found application in medicine, namelybromophene andbromofenofos.

Exposure to the coplanarstereoisomer 3,3',4,4',5,5'-hexabromobiphenyl (but not the non-coplanar stereoisomer) in genetically susceptible mice is known to cause immunotoxicity and disorders related to the central nervous system, and even at doses as low as 2.5 mg/kg, excess neonatal fatalities are observed (LD₅₀ is from 5–10 mg/kg).^[2] The mechanism of toxicity is thought to be related toaryl hydrocarbon receptor activation.^[2]

There is evidence that pre- and post-natal exposure to PBB can cause earlymenarche.^[3]

Early studies on the effects of PBBs onhuman beings concerned the people inMichigan, United States who consumed PBB-contaminated animal products (seehistory of PBBs below).^[4] A study of 327 girls aged 5–24 years in Michigan found those who were exposedin utero to PBBs at or above a level of 7 ppb found had an earlier age at menarche compared to acase-control group.^[5] Michigan dairy farmers exposed to PBBs had significant immune system abnormalities including reduced numbers of circulating blood lymphocytes, increases in lymphocytes with no detectable surface markers, and reduced functional response to specific test antigens.^[6] Some residents complained ofnausea,abdominal pain,loss of appetite,joint pain andlethargy, though it could not be clearly established that PBBs were the cause of these health problems.Workers who were exposed to PBB during PBB production sufferedhypothyroidism,^[7] although no deterioration in memory performance was found in PBB-exposed workers in tests conducted several years after final PBB exposure, and there was also no correlation of performance with PBB concentration.^[8]

There is stronger evidence that PBBs may have caused skin problems, such asacne, in consumers of the contaminated food. Some workers exposed to PBBs by breathing and skin contact for days to months also developed acne.

Studies in animals exposed to large amounts of PBBs for a short period or to smaller amounts over a longer period show that PBBs can causeweight loss,skin disorders,nervous andimmune systems effects, as well as effects on theliver,kidneys, andthyroid gland.

It is not known for certain if PBBs could causecancer in human beings, but it has been observed that they can lead to cancer inlab mice exposed to very high concentrations of PBBs. Based on suchanimal tests, theUnited States Department of Health and Human Services has determined that PBBs may reasonably be anticipated to becarcinogens. TheInternational Agency for Research on Cancer also suggests that PBBs are possibly carcinogenic to humans.

Before the 1970s, PBBs were widely used commercially as aflame retardant. Michigan Chemical Corporation (MCC) inSt. Louis, Michigan, which was then owned byVelsicol Chemical Corporation, was a major producer of the FireMaster range of PBB-based flame retardants. FireMaster BP-6 (a yellow-brown powder) is a mixture of many different PBB congeners with 2,2',4,4',5,5'-hexabromobiphenyl and 2,2',3,4,4',5,5'-heptabromobiphenyl being significant constituents by mass (60-80% and 12-25%, respectively).^[9][10] FireMaster FF-1 (a white powder) is FireMaster BP-6 with the addition of 2% calcium silicate as an anti-caking agent.^[10] Mixed bromochlorobiphenyls and polybrominated naphthalenes, as well as lower brominated compounds formed by incomplete bromination, have also been found as minor constituents of FireMaster products.^[11]

In summer 1973, several thousandpounds of FireMaster BP-6 were accidentally mixed withlivestock feed that was distributed to farms inMichigan because the MCC plant also produced a feed precursor ingredient,magnesium oxide, which was sold to the feed manufacturer.^[12] Some 1.5 millionchickens, 30,000cattle, 5,900pigs, and 1,470sheep then consumed this feed, became contaminated with PBBs and the carcasses were disposed of in landfill sites throughout the state.^[13] In 1976, the Michigan Department of Community Health established a PBB registry to gather and analyze data on exposed residents. It now resides at theRollins School of Public Health atEmory University,^[14][15] and is maintained by epidemiologist Michele Marcus.^[16]

Michigan Farmer magazine staff members Richard Lehnert and Bonnie Pollard broke the news of the contamination. The magazine continued coverage of the issue until the eventual bankruptcy proceedings of the farm cooperative responsible for the accidental contamination and subsequent distribution of the feed.^[17] These events were also portrayed in the 1981 documentaryCattlegate by Jeff Jackson, the true-fiction filmBitter Harvest starringRon Howard, and in the bookThe Poisoning of Michigan by Joyce Egginton. A 1978 episode ofLou Grant ("Slaughter") portrays a similar, but fictionalized account. One year elapsed before the animals were culled.^[12]

This incident is cited amongst a handful of other noxious substances as the driver for PresidentGerald Ford's reluctant approval in 1976 of theToxic Substances Control Act, which "remains one of the most controversial regulatory bills ever passed".^{[18][better source needed]}

A study was undertaken on 4,545 people to determine the effects of PBBs onhuman beings. These include three exposure groups – all people who lived on the quarantined farms, people who received food from these farms and workers (and their families) engaged in PBB manufacture – as well as 725 people with low-level PBB exposure.^[19]

All were queried concerning 17 symptoms and conditions possibly related to PBBs.Venous blood was drawn and analyzed for PBB bygas chromatography. Meanserum PBB levels were found to be 26.9ppb by weight (26.9 μg/kg) in farm residents, 17.1 in recipients, 43.0 ppb in workers, and 3.4 ppb in the low exposure group. No associations could be established between serum PBB levels and symptom prevalence rates.^[19]

To evaluate peripherallymphocyte function,T andB cell quantitation andin vitro responses to three nonspecificmitogens were studied in 34 persons with highest PBB levels (mean: 787 ppb), and in 56 with low levels (mean: 2.8 ppb). No statistically significant differences in lymphocyte function were noted.^[19]

However, as of 2015 these studies are still ongoing, and 40 years later adverse reproductive-system effects (as measured by theApgar score of the newborns) continue to be found in the grandchildren of those who consumed tainted farm products.^[20][21]

Noting the possible hazards on the environment, PBBs were listed as one of six controlled substances under theRestriction of Hazardous Substances Directive (RoHS), which was enacted intoEuropean Law in February 2003. RoHS legislation lists PBBs as a "restricted substance" group. Other countries followed suit, resulting most recently in restriction dates instituted in China on March 1, 2007, and South Korea on July 1, 2007.

• Aftermath of Michigan ContaminationArchived 2015-12-28 at theWayback Machine

• Endocrine disruptors
• Flame retardants
• Bromoarenes
• Persistent organic pollutants under the Stockholm Convention
• Persistent organic pollutants under the Convention on Long-Range Transboundary Air Pollution
• Biphenyls
• 1973 disasters in the United States
• 1973 in the environment
• Presidency of Gerald Ford

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