Polybrominated diphenyl ethers
Polybrominated diphenyl ethers or PBDEs, are organobromine compounds that are used as flame retardant. Like other brominated flame retardants, PBDEs have been used in a wide array of products, including building materials, electronics, furnishings, motor vehicles, airplanes, plastics, polyurethane foams, and textiles. They are structurally akin to the PCBs and other polyhalogenated compounds, consisting of two halogenated aromatic rings. PBDEs are classified according to the average number of bromine atoms in the molecule. The health hazards of these chemicals have attracted increasing scrutiny, and they have been shown to reduce fertility in humans at levels found in households. Their chlorine analogs are polychlorinated diphenyl ethers (PCDEs). Because of their toxicity and persistence, the industrial production of some PBDEs is restricted under the Stockholm Convention, a treaty to control and phase out major persistent organic pollutants (POPs).
Classes of PBDEs
The family of PBDEs consists of 209 possible substances, which are called congeners (PBDE = C12H(10−x)BrxO (x = 1, 2, ..., 10 = m + n)). The number of isomers for mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, and decabromodiphenyl ethers are 3, 12, 24, 42, 46, 42, 24, 12, 3 and 1, respectively. In the United States, PBDEs are marketed with trade names: DE-60F, DE-61, DE-62, and DE-71 applied to pentaBDE mixtures; DE-79 applied to octaBDE mixtures; DE 83R and Saytex 102E applied to decaBDE mixtures. The available commercial PBDE products are not single compounds or even single congeners but rather a mixture of congeners.
Lower brominated PBDEs
These species average 1-5 bromine atoms per molecule and are regarded as more dangerous because they more efficiently bioaccumulate. Lower-brominated PBDEs have been known to affect hormone levels in the thyroid gland. Studies have linked them to reproductive and neurological risks at certain concentrations or higher.
Higher brominated PBDEs
These species average more than 5 bromine atoms per molecule.
The commercial mixture, named pentabromodiphenyl ether, contains the pentabromo derivative predominantly (50-62%), however the mixture also contains tetrabromides (24-38%) and hexabromides (4-8%), as well as traces of the tribromides (0-1%). In similar manner, commercial octabromodiphenyl ether is a mixture of homologs: hexa-, hepta-, octa-, nona-, and decabromides.
Health and environmental concerns
Since the 1990s, environmental concerns were raised because of the high hydrophobicity of PBDEs and their high resistance to degradation processes. While biodegradation is not considered the main pathway for PBDEs, the photolysis and pyrolysis can be of interest in studies of transformation of PBDEs. People are exposed to low levels of PBDEs through ingestion of food and by inhalation. PBDEs bioaccumulate in blood, breast milk, and fat tissues. Personnel associated with the manufacture of PBDE-containing products are exposed to highest levels of PBDEs. Bioaccumulation is of particular concern in such instances, especially for personnel in recycling and repair plants of PBDE-containing products. People are also exposed to these chemicals in their domestic environment because of their prevalence in common household items. Studies in Canada have found significant concentrations of PBDEs in common foods such as salmon, ground beef, butter, and cheese. PBDEs have also been found at high levels in indoor dust, sewage sludge, and effluents from wastewater treatment plants. Increasing PBDE levels have been detected in the blood of marine mammals such as harbor seals.
A non-peer-reviewed study of 20 mother-child pairs in the United States conducted by the Environmental Working Group found that the median blood levels of PBDEs in children (62 parts per billion) were 3.2 times higher than in their mothers. PBDEs have also been shown to have hormone-disrupting effects, in particular, on estrogen and thyroid hormones. A 2009 animal study conducted by the US Environmental Protection Agency (EPA) demonstrates that deiodination, active transport, sulfation, and glucuronidation may be involved in disruption of thyroid homeostasis after perinatal exposure to PBDEs during critical developmental time points in utero and shortly after birth. The adverse effects on hepatic mechanism of thyroid hormone disruption during development have been shown to persist into adulthood. The EPA noted that PBDEs are particularly toxic to the developing brains of animals. Peer-reviewed studies have shown that even a single dose administered to mice during development of the brain can cause permanent changes in behavior, including hyperactivity.
Swedish scientists first reported substances related to PentaBDE were accumulating in human breast milk. Studies by the Swedish Society for Nature Conservation found for the first time very high levels of higher brominated PBDEs (BDE-209) in eggs of Peregrine falcons. Two forms of PBDEs, Penta and Octa, are no longer manufactured in the United States because of health and safety concerns. Based on a comprehensive risk assessment under the Existing Substances Regulation 793/93/EEC, the EU has completely banned the use of Penta and Octa BDE since 2004. However, both chemicals are still found in furniture and foam items made before the phase-out was completed. The most-common PBDEs that are used in electronics are in a form known as Deca. Deca is banned in Europe for this use and in some U.S. states. For PBDE, EPA has set reference dose of 7 micrograms per kilogram of body weight, which is "believed to be without appreciable effects". However, Linda Birnbaum, PhD, a senior toxicologist formerly with the EPA (now NIEHS) notes concern: "What I see is another piece of evidence that supports the fact that levels of these chemicals in children appear to be higher than the levels in their parents; I think this study raises a red flag." Previous study by EWG in 2003 published test results showing that the average level of fire-retardants in breast milk from 20 American mothers was 75 times higher than the average levels measured in Europe.
It has been postulated that increasing levels of PBDEs in the environment could be correlated with the increasing incidence of feline hyperthyroidism. However, a study in 2007 found that no association could be detected between hyperthyroid cats and serum PBDE levels.
An experiment conducted at Woods Hole Oceanographic Institution in 2005 showed that the isotopic signature of methoxy-PBDEs found in whale blubber contained carbon-14, the naturally occurring radioactive isotope of carbon. Methoxy-PBDEs are produced by some marine species. If the methoxy-PBDEs in the whale had come from artificial (human-made) sources, they would have contained only carbon-12 and no carbon-14 due to the fact that virtually all PBDEs that are produced artificially use petroleum as the source of carbon; all carbon-14 would have long since completely decayed from that source. The isotopic signatures of the PBDEs themselves were not evaluated. The carbon-14 may instead be in methoxy groups enzymatically added to man-made PBDEs.
A 2010 study found that children with higher concentrations of PBDE congeners 47, 99 and 100 in their umbilical cord blood at birth scored lower on tests of mental and physical development between the ages of one and six. Developmental effects were particularly evident at four years of age, when verbal and full IQ scores were reduced 5.5 to 8.0 points for those with the highest prenatal exposures.
Regulations of PBDEs
As of June 1, 2006 the State of California began prohibiting the manufacture, distribution, and processing of flame-retardant products containing pentabrominated diphenyl ether (pentaBDE) and octabrominated diphenyl (octaBDE). PBDEs are ubiquitous in the environment, and, according to the EPA, exposure may pose health risks. According to U.S. EPA's Integrated Risk Information System, evidence indicates that PBDEs may possess liver toxicity, thyroid toxicity, and neurodevelopmental toxicity. In June 2008, the U.S. EPA set a safe daily exposure level ranging from 0.1 to 7 ug per kg body weight per day for 4 most common PBDEs. In April 2007, the legislature of the state of Washington passed a bill banning the use of PBDEs. The State of Maine Department of Environmental Protection found that all PBDEs should be banned. In August, 2003, the State of California outlawed the sale of penta- and octa- PBDE and products containing them, effective January 1, 2008. In May 2007, the legislature of the state of Maine passed a bill phasing out the use of DecaBDE.
The European Union decided to ban the use of two classes of flame retardants, in particular, polybrominated diphenyl ethers (PBDEs) and polybrominated biphenyls (PBBs) in electric and electronic devices. This ban was formalised in the RoHS Directive, and an upper limit of 1 g/kg for the sum of PBBs and PBDEs was set. In February 2009, the Institute for Reference Materials and Measurements (IRMM) released two certified reference materials (CRMs) to help analytical laboratories better detect these two classes of flame retardants. The reference materials were custom-made to contain all relevant PBDEs and PBBs at levels close to the legal limit.
At an international level, in May 2009 the Parties of the Stockholm Convention for Persistent Organic Pollutants (POPs) took the decision to list commercial penta-BDE and commercial octa-BDE as POP substances. This listing is due to the properties of hexa-BDE (hexabromodiphenyl ether) and hepta-BDE (heptabromodiphenyl ether) which are the main components of commercial octa-BDE, and due to the properties of tetra-BDE (tetrabromodiphenyl ether) and penta-BDE (pentabromodiphenyl ether) which are the main components of commercial penta-BDE.
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