What on earth is PBDE?

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While dioxin levels have been declining in the environment and foodstuffs, some new chemicals have recently aroused attention. One of these recent problems is the polybrominated flame retardants. These hit the headlines a while ago when an environmental organisation announced that there are fifty times higher levels of polybrominated diphenylaethers (PBDEs) in Baltic herring than found in their Atlantic cousins. There was an error in reading the actual data; the difference was in fact 4 to 5-fold, and this was based on findings from only one study, findings that could not be confirmed by other groups.

In spite of this error, it is reasonable to ask whether or not these chemicals would be a problem to fish consumers. PBDE compounds are used mainly as flame retardants in plastic-ware and synthetic textiles which are both highly flammable hydrocarbon materials. Their manufacturers attempt to decrease this dangerous property by adding bromine to the material; bromine concentrations of a few percent in synthetic materials prevent combustion. PBDEs are widely used in electrical equipment such as television sets, but the largest volumes are their use in plastic foam materials e.g. in furniture and mattresses.

Thus the question is, as usual, a choice between two evils. Materials that can catch fire are an obvious danger, but on the other hand, flame retardants are slowly eliminated from the environment similarly to the older PCB compounds. As fat soluble compounds, they bioaccumulate in living organisms in the environment, and are biomagnified in the food chain.

PBDE compounds are released to the environment from waste disposal, perhaps most easily when plastics are burned. The concentration of commonly measured PBDE compounds in Baltic fish is about one fortieth of the PCB levels, so the levels are still not very high. PBDEs are secreted to breast milk as fat soluble compounds just like the PCBs and dioxins, and the levels in milk are already declining in the Nordic countries. There is a peculiar difference between Europe and Northern America. In the USA, the levels in humans are tenfold higher than those in Europe and are still increasing. The cause of the difference is not well established, but it is worth noting that synthetic carpeting is very common in the U.S, and in Europe hardwood flooring or washable rugs are more common. Exposure from airborne dust via respiration may be even more important than intake from food.

Adverse effects of PBDE compounds

How toxic are these compounds? There is much less information available on PBDEs than there is on PCBs and dioxins. Their immediate toxicity is very low. The problem is that they are excreted very slowly and therefore accumulate with time into living organisms. The less brominated tetra- and pentabromo-diphenylaethers are absorbed fairly well, and their use in Europe has been discontinued, but they still exist in the environment. Decabrominated diphenylaether which is presently used is almost non-absorbable and excreted more rapidly. Therefore it is not believed to pose a risk to human beings. It is another matter that some studies claim that decabrominated species would lose some of their bromine in nature and as a result, the levels of lower brominated compounds would increase. Clearly more information is needed on this issue.

Endocrine disrupters?

It has been suspected that some PBDE compounds might disturb hormonal functions and be so called endocrine disrupters.[1] Large doses have been shown to cause behavioural changes and to slow down the development of the nervous system in mouse and rat pups, when PBDEs have been given to the dam during gestation. The same doses cause decreases in thyroid hormone levels.

One likely reason is that these compounds accelerate the breakdown of thyroid hormones, and this causes a relative hormone deficiency. The lack of thyroid hormone is a well known cause of slow development of the nervous system. However, the doses used in these experiments led to about thousand-fold higher levels in rat adipose tissue compared to the levels encountered in the fat of human mothers. In view of the putative mechanism of action, it seems that developmental effects in humans are not very likely. As an analogy, very few medicines today have a thousand-fold safety margin.

Another issue is fears that PBDEs may disturb male sexual development. Animal experiments do not help much here either, because the doses used have been very high. Human data are very difficult to interpret, and as yet the evidence associating PBDEs to male sexual developmental disturbances is rather unconvincing.

The safety margin for dioxins[2] is much less. Even with dioxins, the conclusion after very detailed studies is that at the present levels the possibility of harm is much less important than the health benefit of eating fish, their major source. Nonetheless, both authorities and manufacturing industries must monitor the environment to see if there is the accumulation of these persistent and permanent compounds, and attempt to reduce their release as much as possible. Environmental organisations should perhaps pay more attention to the accuracy of their information. They have very important watchdog functions in our society, and cannot afford to lose their credibility.

Brominated flame retardants are useful chemicals, and they increase the safety of flammable synthetic materials. Some of them are biomagnified in the environment, and they have been replaced by safer compounds.

Notes and references

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