Dioxin Controvery

["Susan K. Snow" <sksnow@1stnet.com>]

  The following was posted recently to the "WASTENOT Organic Waste
  Collection, Processing, Composting," online discussion list.  The author
  is Gary A. Breitenbeck of Louisiana State University.  
  I share this to you for the purpose of discussion.
  Susan Snow
  
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  Recent discussions on this list, the net and in the popular press
  regarding the concentrations, toxicology and especially fate of dioxins,
  furans and similar organic compounds in composts, soils, wastes and food
  raised a number of interesting questions.  I was vaguely aware of the
  Times Beach disaster in the early '80's and subsequent attempts to link
  health problems of Viet Nam vets with dioxin contaminants in Agent
  Orange (phenoxy herbicides),  but was under the impression that
  "dioxin"  had receded into secondary importance as an environmental
  concern.
  
  Because of my involvement with the beneficial use of paper mill sludges,
  MSW compost, biosolids and other materials that may contain dioxins, I
  was compelled to look into the matter.  The following comments are the
  result of a  review of the literature that was by no means exhaustive.
  They pertain to TCDD (2,3,7,8-tetra chlorodibenzo-P-dioxin), the
  compound  that most seem to refer to when using the generic term
  "dioxin".
  
  The current controversy seems to be fueled by the release of a draft of
  the EPA's reassessment of dioxin.  This document apparently has not yet
  undergone peer review, and a quick reading suggests that many
  conclusions are yet equivocal.  Nevertheless,  it seems to have given
  birth to another whirlwind environmental debate driven by an emotional
  commitment to preconceived ideas rather than a balanced review of the
  evidence.   A number of activist Web pages have been established (search
  "dioxin").  An interesting counterpoint:
  http://www.heartland.org/dioxin.html.
  
  The most recent "peer-reviewed" document reviewing  the environmental
  implications of  TCDD appears to be that of Arthur and  Frea (1989, J.
  Environ. Qual. 18:1-11).   In addition to fate and transport, they
  briefly review the toxicological effects  (acute toxicity, chronic
  toxicity, teratogenicity, mutagenicity and  carcinogenicity) known at
  that time.   A great deal of the controversy regarding the toxic effects
  of TCDD appears to arise from the fact that the effects of this compound
  are highly species-specific as well as dependent on the sex of the test
  animal and the route of exposure.  Findings from studies using human
  subjects exposed to high (in some instances, extremely high)
  concentrations of TCDD are inconsistent, but most indicate that risks to
  the general population are very low.   A search of  recent dissertation
  abstracts indicates that the toxicology of TCDD remains an active area
  of research.
  
  TCDD and its precursors appear in low concentrations as contaminates
  during the production of various chlorinated aromatics.   TCDD is also
  produced during combustion of many materials such as polyvinyl chloride
  in the presence of naturally occurring phenols (e.g, lignin in wood).
  Waste combustion followed by long-range airborne transport has been
  suggested as a major source of environmental TCDD.   The very high
  temperatures that occur in well-managed incinerators (oxymoron?) destroy
  TCDD.  It is not known to be produced biologically.  Well managed
  compost piles do not achieve the temperatures needed to chemically form
  TCDD (180-400 °C; 350-750°F).  A pile on fire does.
  
   The principal mechanism of TCDD degradation appears to be photolysis
  (i.e., it rapidly decomposes in the presence of light), but only in the
  presence of a proton donor (e.g. on a leaf but not in a glass of clean
  water).  Once in the environment, the strong affinity of  TCDD for
  soils, sediments and organic matter greatly reduces it exposure to
  light.  Several studies indicate that TCDD is recalcitrant to both
  abiotic and biological degradation  in soils.  Most of these studies
  were performed in the laboratory, and none involved addition of  a
  readily decomposable substrate.  I could not locate reports specifically
  documenting the fate of TCDD during composting.
  
  Although TCDD is highly recalcitrant to microbial degradation,  a number
  of specific organisms have been identified that can degrade TCDD
  aerobically.   TCDD also undergoes biologically mediated reductive
  dehalogenation when incubated under anaerobic conditions, suggesting the
  possibility of a two-phase degradation pathway (anaerobic/aerobic)
  similar to that of DDT.
  
  Plant roots do not appear to readily assimilate TCDD, though low
  concentrations have been found in tubers of some species.  Uptake, like
  toxicity,  appears to be species-specific.  Limited data suggest that
  TCDD is not translocated through the vascular tissue of plants.
  Studies to assess the uptake of volatile TCDD using transport models
  conclude that this mechanism is not significant except where very high
  concentrations of free-phase TCDD exist due to its relatively low
  volatility.
  
  My general impression is that the limited hazards posed by TCDD should
  not deter efforts to develop alternative methods of waste disposal that
  sustain the productivity of agricultural land resources.  Additional
  research is need to determine if  the management practices used in
  compost operations and wastewater treatment facilities significantly
  influence the concentrations of TCDD and related compounds in finished
  composts and biosolids.   The health effects of TCDD merit further
  study, but it is highly improbable that trace levels of  TCDD in the
  environment are largely responsible for society's woes, as some seem to
  suggest.   On the other hand, I doubt you will ever again see me
  standing downwind of someone's backyard trash fire.