It looks as if Canada is poised to ban flame retardants known as PBDEs, which are have been linked with learning deficits and behavioral abnormalities in lab animals, and are found at high levels in some people.

That’s the good news. The bad news is that some tests are finding alarmingly high levels of the compounds in kids. You win some, you lose some.

On a positive note, it seems that in the latest round of reporting on PBDEs, reporters are picking up on one of the issue’s complexities: that the harm caused by PBDEs may be more closely linked to timing than to dose. That is, an incredibly small dose at just the wrong stage of infant development can have more of an effect than a far larger dose later on in life.

From the Toronto Globe and Mail story:

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The traditional mantra of toxicologists has been that the dose makes the poison, or that exposures have to be large to have an effect, with larger exposures packing more punch than smaller ones.

In experiments with rodents, effects have been noted on the offspring of rats given only one exposure of 60 parts per billion, an amount that a few decades ago scientists would have dismissed as too low to have an impact … When researchers upped the dose to 300 ppb, there was hardly any increase in activity …

Young male mice given traces of the chemicals four and 10 days after birth exhibited behavioural abnormalities, but the same dose given to 19-day-olds caused no changes at all, compared with control animals.

To sum up: a five-fold increase in the dosage of PBDEs did little extra damage; but delaying exposure by just nine days made the effects disappear. Which is hard to square with the traditional understanding of the old toxicologists’ saw, “the dose makes the poison.”

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