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pcdd/f: environmental levels & env. fate
>Bill Carroll, Chlorine Chemistry Council sez:
>Nope. TCDD. Greater than 30 ppt, some mortality in sac fry. Over 100,
>total mortality. Current Lake Ontario--surprisingly, perhaps--5 ppt (10
>TEQ). The trout are coming back. Compare to levels ca. 100 thirty years
right, and 60 ppt tcdd is the LC50 for lake trout (the more sensitive
species), according to the recent piece by janet ralloff in _science news_
(17 may). when lake ontario concentrations used to be 2.5 times the level
needed for 100% mortality (!), and when many other p.o.p.'s and p.i.p's add
to the burden of the fry of such a critical species to lake ecosystems but
are not considered, and when trout populations are still suffering other
pcdd/f/etc induced stresses -- where does that leave the importance of a
reduction in some e.d.'s in the environment?
>The decay data is recent, and most has so far been done on PCBs. Hites also
>did some a few years ago. There was a paper at Dioxin '96 whose author's
>name escapes me right now. Lots more difficult experiments to do, but it's
i have czuczwa & hites (_es&t_ '86, 20:2:195-200) which corroborates what
you say about lower Cl congeners possibly being more susceptable to
cleavage (more C-H bonds, more water soluble). kwok et al. (_es&t_ '95,
29:6:1591-99) investigate Rx rates w/ the ubiquitous atmospheric radical,
-OH, and also confirming your statements.
however, what are the rules after deposition from gas phase to soil or
water, or on surfaces generally? only ~10% of the key question--actual
exposure--occurs from inhalation, so what if the planar more Ah toxic ones
are more available for exposure, possibly even transformed *from* higher
halogenated ones (i've read one _es&t_ piece that shows at least some
transformation into 2,3,7,8 congeners)? and what about the *pattern* of
substitution? i know that planar pcdd/f/pcb are much more readily absorbed
through the gut than the non-planar less Ah toxic ones.
i don't know about the effect of substitution pattern on environmental
transport, fate and transfer, however. in fact, i don't even know the
trends in their physical characteristics, eg o/w coefficients, according to
substitution degree & pattern (perhaps the higher the halogenation the less
water soluble? but what about as to the pattern of substitution?).
can anyone offer some general guidance?