Polybrominated biphenyls (PBBs), also called brominated biphenyls or polybromobiphenyls, are a group of manufactured chemicals, of the polyhalogenated compounds. Their chlorine analogs are the PCBs. While once widely used commercially, PBBs are now controlled substances under the Restriction of Hazardous Substances Directive, which limits their use in electrical and electronic products sold in the EU.
PBBs usually exist as colorless to off-white solids. PBBs soften at 72 degrees Celsius and decompose above 300 degrees Celsius. They have low vapor pressure, are very soluble in benzene and toluene, and insoluble in water. They are degraded by UV light.
PBBs are used as flame retardants of the brominated flame retardant group. They are added to plastics used in products such as home electrical appliances, textiles, plastic foams, laptop cabinets, etc. to make them difficult to burn.
Exposure to the coplanar stereoisomer 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). The mechanism of toxicity is cellular oxidative stress by aryl hydrocarbon receptor activation.
Early studies on the effects of PBBs on human beings concerned the people in Michigan, USA who consumed PBB-contaminated animal products (see history of PBBs below). A study of 327 girls aged 5–24 years in Michigan found those who were exposed in utero to PBBs at or above a level of 7 ppb found had an earlier age at menarche compared to a case-control group. 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. Some residents complained of nausea, abdominal pain, loss of appetite, joint pain and lethargy, 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 suffered hypothyroidism, 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.
There is stronger evidence that PBBs may have caused skin problems, such as acne, 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 cause weight loss, skin disorders, nervous and immune systems effects, as well as effects on the liver, kidneys, and thyroid gland.
Possibility of carcinogenicity
It is not known for certain if PBBs could cause cancer in human beings, but it has been observed that they can lead to cancer in lab mice exposed to very high concentrations of PBBs. Based on such animal tests, the United States Department of Health and Human Services has determined that PBBs may reasonably be anticipated to be carcinogens. The International Agency for Research on Cancer also suggests that PBBs are possibly carcinogenic to humans.
Michigan PBB contamination incident
Before the 1970s, PBBs were widely used commercially as a flame retardant. Michigan Chemical Corporation (owned by Velsicol Chemical Corporation) in St. Louis, Michigan 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). FireMaster FF-1 (a white powder) is FireMaster BP-6 with the addition of 2% calcium silicate as an anti-caking agent. 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.
In 1973, several thousand pounds of FireMaster BP-6 were accidentally mixed with livestock feed that was distributed to farms in Michigan, USA. Some 1.5 million chickens, 30,000 cattle, 5,900 pigs, and 1,470 sheep then consumed this feed, became contaminated with PBBs and the carcasses were disposed of in landfill sites throughout the state. These events were portrayed in the 1981 film Bitter Harvest and in the book "The Poisoning of Michigan". A 1978 episode of Lou Grant ("Slaughter") portrays a similar, but fictionalized account. One year elapsed before the animals were culled. A study was undertaken on 4,545 people to determine the effects of PBBs on human 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.
All were queried concerning 17 symptoms and conditions possibly related to PBBs. Venous blood was drawn and analyzed for PBB by gas chromatography. Mean serum PBB levels were found to be 26.9 ppb 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.
To evaluate peripheral lymphocyte function, T and B cell quantitation and in vitro responses to three nonspecific mitogens were studied in 34 persons with highest PBB levels (mean: 787 ppb), and in 56 with low levels (mean: 2.8 ppb). Again, no statistically significant differences in lymphocyte function were noted.
Bans and restrictions
Noting the possible hazards on the environment, however, PBBs were listed as one of six controlled substances under the Restriction of Hazardous Substances Directive (RoHS), which was enacted into European 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.
Testing for PBB in plastics
Until recently testing for PBB has been cumbersome. Cycle time, inaccuracy, cost and level of expertise required for the test engineer has precluded the implementation of any screening of plastic component in a manufacturing or in a product qualification/validation environment.
Recently, with the introduction of a new analytical instrument IA-Mass, screening of plastic material alongside manufacturing line is possible. A 5 min. detection cycle and a 20 min. quantification cycle is available to test and to qualify plastic parts as they reach the assembly line.
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