A Burning Issue

Flame retardants (FRs) have been used for decades to help prevent death and injury from fires, but it is clear that some of these compounds are entering the environment and food chain. I believe that FRs – legacy and emerging – and dioxins should not be considered in isolation. Instead, a holistic approach is needed.

But before I delve into the what’s, how’s and why’s, please allow me to apologize for the acronym onslaught and address the potentially confusing issue of terminology in this field:

1. Legacy or established FRs (bromine: BFRs; chlorine: CFRs; phosphorous: PFRs) are chemicals that are extensively documented regarding production and use. Furthermore, we have a significant amount of data on the chemistry, fate, exposures, and environment and health issues, which is to say (eco-) toxicity and/or human health effects.
2. Emerging FRs are chemicals that are documented in terms of production and use, and that have also been shown to occur/distribute to the environment and/or wildlife, humans or other biological matrices.
3. Novel FRs are chemicals that have been documented as potential FRs and have been shown to be present in materials or products.
4. Potential FRs are chemicals reported to have applications as FRs, for example, in patents, but where we are unsure about levels of usage (1).

I won’t go too deeply into the history, but suffice to say that PCBs and polybrominated biphenyls (PBBs) are no longer used. As a quick aside, one of the first persistent organic pollutant (POP) incidents occurred in 1973, when a PBB mixture – FireMaster BP-6 – was accidentally (and frighteningly) mixed with livestock feed in Michigan. It led to the quarantining and slaughter of nearly 30,000 cattle and thousands of other animals (1). Polybrominated diphenyl ethers (PBDEs) were widely used in plastics, upholstery, textiles, and foams, and can make up over 15 percent of some products by weight. Tetrabromobisphenol-A (TBBPA) and hexabromocyclododecane (HBCDD) were brought in to replace PBDEs.

The lesson here is that demand arising from the regulation of BFRs will always be met by an increasing number of alternative FRs – after all, industry must still comply with fire safety regulations. Indeed, hundreds of emerging FR compounds have been registered. To put that into perspective, current production of BFRs exceeds 100,000 tonnes per year.

Environmental impact

Why does that matter? Well, as already indicated by the terminology above, emerging FRs have already been found in the environment – and some in foods. As you’d expect, flame retardants must be chemically stable, which is typically embued by halogenated aromaticity and low aqueous solubility. Therefore, it’s almost inevitable that any replacement FR will have similar characteristics, and it’s also likely that they’ll share much in common with other POPs: persistency, bioaccumulation and toxicity.

Many BFRs are being regulated; octa- and nona-BFRs have essentially been phased out in many countries. And the European Commission has asked all member states to gather data on the levels of BFR compounds in foods. The Commission Recommendation (2014/118/EU) recognized that levels of these substances in food of animal origin could be related to their presence in animal feed – expect a further recommendation on monitoring animal feed in 2015.

Why is the European Commission so concerned? It comes off the back of a series of six very comprehensive scientific opinions on the topic from the European Food Safety Authority – an excellent source of background information for those who are interested (www.efsa.europa.eu).

There’s a big challenge here; hundreds of compounds pose potential risk, so we decided to prioritize method development using four factors: environmental persistence (our criterion was over 500 days), bioavailability, toxicity, and occurrence in biota and food. We identified a top ten (see box) – and I’d be very interested in anyone’s views on our selection.

When it comes to risk assessment – or the impact of exposure on human and environmental health effects – a vicious circle exists. It’s difficult for analytical chemists to get funding to develop methodologies to assess exposure to chemicals if there’s no data on toxicology. But toxicologists can’t find funding to research the toxic actions of compounds if there’s no evidence of exposure. How do we break this vicious circle? Students can certainly help. Otherwise, it’s a case of scraping together enough funding to start the ball rolling. I wish we had a more open approach to the problem of emerging compounds...

The dioxin–furan link

Most, if not all, BFRs can form brominated dioxins and furans (PBDD/Fs) when they degrade. They are created by thermal breakdown of brominated organics (burning BFRs in plastics) and are highly toxic and persistent. Moreover, mixed  (and highly toxic) chlorinated- and brominated-dioxins (PXDD, PXDF, PXB) may be formed in the presence of chlorine.

How do we deal with these mixtures? We’ve already done it for chlorinated dioxins, so we’re well ahead of the game. We can use the very elegant WHO-TEQ scheme, a simplified expression of the toxic equivalency (TEQ) of the different PCBs and dioxins as one number:

WHO-TEQ = ∑[PCDDi× TEFi] +∑[PCDFi× TEFi] +∑[PCBi× TEFi].

I was fortunate enough to be involved in a review on the toxicity of these compounds in 2013 (2). It was noted that PBDDs, PBDFs, and some dioxin-like biphenyls (dl-PBBs) may contribute significantly to the total TEQ. But we need more data on exposure. Also, other mixed halogenated PXDD/Fs and PXBs are found in foods, admittedly at lower levels; however, as there are many more congeners (a grand total of more than 5000) and only very few are measured, the potential contribution to total dioxin toxic equivalency could be significant.

The key point is that they are present and a lot more measurements need to be done. If we start looking at compounds that we know have the same type of toxic effect, we will get a much more comprehensive view of the risk.

The analytical challenge

The measurement of mixed halogenated and brominated dioxins is challenging to say the least. Mainly, because there are so many congeners and so few analytical standards – and there are even fewer 13C analogs. For that reason, I’ve only scratched the surface. Clearly there is a lot more to be done and many other compounds also need to be considered, such as polychlorinated naphthalenes (PCNs), which also exhibit dioxin-like toxicity.

To see the whole picture, perhaps we need to revisit our elegant and simple equation from above and make it less simple and elegant:

TEQ = ∑[PCDDi× TEFi] +∑[PCDFi× TEFi] +∑[PCBi× TEFi] +∑[PBDDi× TEFi] +∑[PBDFi× TEFi] +∑[PBBi× TEFi] +∑[PXDDi× TEFi] +∑[PXDFi× TEFi] +∑[PXBi× TEFi] +∑ [PCNi× TEFi] …………… + ∑many more?

The truth is, we don’t know the importance of these compounds – and we don’t know what we’re not looking for. Indeed, we start to echo Donald Rumsfeld (US Secretary of Defense, 2002): “... there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say there are some things we do not know. But there are also unknown unknowns; the ones we don’t know we don’t know.” I’m not sure that’s a place I want to be. The way out? Well, I believe we need next-generation mass spectrometry (MS), we need to combine MS with measurement of biological effect; for example, using cell based or receptor assays. And we need to be aware of the impact of cleanup methods.

The use of the TEQ scheme for dioxins was the first – and very sensible – attempt at regulating chemicals as mixtures. Why? Because of the similar mode of toxic action through the Ah receptor. But if we acknowledge the value of this approach and can prove that we are exposed to other chemicals with the same mode of action, then surely these compounds should also be included in the scheme. Other environmental chemicals, such as some of the BFRs, do not directly share this mode of action– but they can be converted into compounds that do. All of these factors need to be taken into account when undertaking risk assessment to ensure we sufficiently protect human and environmental health.

http://now.eloqua.com/e/er?s=1788&lid=19752

*The opinions and conclusions expressed are solely the views of the author and do not necessarily reflect those of Fera or any other organization.

Top Ten Emerging BFRs

1. Hexabromobenzene (HBB)
2. 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE)
3. 5,6-dibromo-1,10,11,12,13,13-hexachloro-11-tricyclo[8.2.1.02,9]tridecene (DBHCTD)
4. 1,2,3,4,7,7-hexachloro-5-(2,3,4,5-tetrabromophenyl)-bicyclo[2.2.1]hept-2-ene (HCTBPH)
5. Pentabromotoluene (PBT)
6. Pentabromobenzyl acrylate (PBB-Acr)
7. Pentabromoethylbenzene (PBEB)
8. 1,2,4,5-tetrabromo-3,6-dimethylbenzene (TBX)
9. Decabromodiphenyl ethane (DBDPE)
10. Octabromotrimethylphenyl indane (OBTMPI)