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Polybrominated diphenyl ethers

Chemical structure of PBDEs
Identifiers
CAS Number	90193-67-2
ECHA InfoCard	100.082.305
CompTox Dashboard (EPA)	DTXSID70107595
Properties
Chemical formula	C12H10−xBrxO
Molar mass	Variable
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references

Polybrominated diphenyl ethers or PBDEs, are a class of organobromine compounds that are used as flame retardants. 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 polychlorinated diphenyl ethers (PCDEs), polychlorinated biphenyls (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 life-saving benefits of fire retardants led to their popularization. Standards for mass transit vehicles continues to increase as of 2021.

Because of their toxicity and persistence, all commercially relevant PBDEs have been marked for elimination 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)Br_xO (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.

Lower-brominated PBDEs with 1–4 bromine atoms per molecule are regarded as more dangerous because they more efficiently bioaccumulate. They have been known to affect thyroid hormone levels, and studies have linked them to reproductive and neurological risks at certain concentrations or higher. Higher-brominated PBDEs are less acutely dangerous but biotically and photochemically debrominate to lower-brominated congeners.

Production

PBDEs were produced commercially via the bromination of diphenyl ether, with three technical-grade mixtures being sold, varying by degree of bromination.

In the United States, PBDEs were marketed with the trade names DE-60F, DE-61, DE-62, and DE-71 applied to pentaBDE mixtures, DE-79 applied to octaBDE mixtures, and DE 83R and Saytex 102E applied to decaBDE mixtures. The available commercial PBDE products were not single compounds or even single congeners but rather mixtures of congeners.

Technical pentaBDE predominantly contained pentabromo derivatives (50–62%); however, the mixture also contained tetrabromides (24–38%) and hexabromides (4–8%), as well as traces of the tribromides (0–1%). Technical octaBDE was a mixture of homologs: hexa-, hepta-, octa-, nona-, and decabromides. Technical decaBDE was 97% decabromide, with small amounts of octa- and nonabromides.

Commercial production of PBDEs began in the 1970s, and continued until the early 2010s. Cumulative global production is estimated to have been 175 kt for pentaBDE, 130 kt for octaBDE, and 1600 kt for decaBDE.

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. In 1995 for example, it was found that Buffalo River fish (common carp) were contaminated with this compound and the tissue concentrations were ranged from 13.1–22.8 ng/g wet wt. 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 working on 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. A 2024 study found most forms of black plastic household displayed high PBDE concentrations of up to 3% by weight due to the use of recycled E-waste in the production process. Black plastic spatulas and other cooking implements exposed to high temperatures would be especially likely to leach the material into food. 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.

There is also growing concern that PBDEs share the environmental long life and bioaccumulation properties of polychlorinated dibenzodioxins.

"With the exception of one small case-control study reporting possible associations between adipose PBDE concentrations and risk of pancreatic cancer, there is no evidence for carcinogenicity of PBDEs in human studies."

Case studies

PBDEs have been shown to have hormone-disrupting effects, in particular, on estrogen and thyroid hormones and can interfere with both metabolic and oncogenic pathways. 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 more highly brominated PBDEs (BDE-209) in eggs of peregrine falcons. Two forms of PBDEs, penta- and octaBDE, 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 European Union has completely banned the use of penta- and octaBDE since 2004. However, both chemicals are still found in furniture and foam items made before the phase-out was completed. The most common PBDEs used in electronics are decaBDE. DecaBDE 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 at 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." A previous study by the Environmental Working Group 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.

Increasing levels of PBDEs in the environment may be responsible for the increasing incidence of feline hyperthyroidism. A study in 2007 found PBDE levels in cats 20- to 100-fold greater than median levels in U.S. adults, although it was not adequately powered to establish an association between hyperthyroid cats and serum PBDE levels. Subsequent studies have indeed found such an association.

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 stable isotopes of carbon because 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 after correcting for sex, ethnicity, tobacco smoke exposure, and mother's IQ.

Sediment contamination

Increasing environmental concentrations and changing distributions of PBDEs in sediments of the Clyde River Estuary in Scotland, UK have been assessed. Analysis of six sediment cores each of 1 m depth from Glasgow city to Greenock revealed that total concentrations increased toward the river bed surface (0–10 cm). Amounts of PBDE ranged from 1 to 2,645 µg/kg (dry wt. sediment) with a mean of 287 µg/kg (dry wt. sediment). Down-core PBDE congener profiles showed that higher concentrations were due to elevated levels of BDE-209. The majority of the sediment records clearly showed a change from mainly lower molecular weight BDEs 47,99, 183, 153 at lower depths to BDE-209 near the surface, a change in congener and homologue group patterns that corresponds to the restrictions of penta- and octaBDE commercial mixtures under EU law in 2004–2006.

While biodegradation is not considered the main pathway for PBDEs, photolysis and pyrolysis can be of interest in studies of transformation of PBDEs.

Regulations of PBDEs

In August, 2003, the State of California outlawed the sale of penta- and octaBDE and products containing them, effective January 1, 2008. 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 the four most common PBDE congeners. 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 May 2007, the legislature of the state of Maine passed a bill phasing out the use of decaPBDE. The U.S. importers and manufacturers of PBDEs have now withdrawn them from sale, by the end of 2004 for pentaBDE and octaBDE, and by the end of 2013 for decaBDE.

The European Union decided to ban the use of two classes of flame retardants, in particular, 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 released two certified reference materials 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 on Persistent Organic Pollutants (POPs) decided to list commercial pentaBDE and commercial octaBDE as POP substances. This listing is due to the properties of hexaBDE and heptaBDE, which are components of commercial octaBDE, and to the properties of tetraBDE and pentaBDE, which are the main components of commercial pentaBDE. In 2017, it was decided to also list decaBDE.

Alternatives

Major decaBDE producers have switched to manufacturing decabromodiphenyl ethane, a structurally related compound that has also come under suspicion as an environmental pollutant. Non-halogenated alternatives also exist.

References

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External links

• U.S. EPA. An Exposure Assessment of Polybrominated Diphenyl Ethers. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-08/086F, 2010
• Sandy, Martha. Polybrominated Diphenyl Ethers: Recommendations to Reduce Exposure in California. Office of Environmental Health Hazard Assessment. 25 Apr. 2008
• Polybrominated diphenylethers (PBDEs) | U.S. Environmental Protection Agency

• Flame retardants
• Endocrine disruptors
• Bromoarenes
• Diphenyl ethers