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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?
  tony tweedale