Dioxin Queries

[ttweed@wildrockies.org (Tony Tweedale)]

>4. Can anyone please check the following information :
>        EPA - safe dose level is 0.006 pg TCDD/kg of body weight/day
---

U.S .EPA and many outside scientists reinvestigated this 'safe' dose for
lifetime cancer (only) risk, and decided a *slightly* higher dose (0.01
pg/kg/d) is closer to the truth.  This was part of US EPA's "Dioxin
Reassessment", the draft of which came out almost 5 years ago.  Personally,
I have taken to using it, disclosing that it still in draft form, but
usually adding that it is the result of the most exhaustive risk assessment
for any chemcial or set of chemicals ever, and that the only reason it is
still in draft form is that the Dioxin Reassessment has become a political
hostage of the US EPA's, more or less (as it indicates the general
population's dose of dioxins is far above this 'safe' dose).  And don't
forget to add that this 'safe' dose does not consider either 1) non-cancer
effects (some serious ones may (there is some evidence) occur at even lower
doses; or 2) the effects of other dioxins that are not accounted for in
this dose (not to mention the effects of thousands of other synthetic
chemicals in our bodies).
--

inhalation risk:  You asked twice.  Aside from specific populations, e.g.
downwind of an incinerator, most (all?) assessments agree that inhalation
represents less than 5% of dioxin risk (c.2-3%).  Think of every person's
spatial zone of inhaled air as the numerator, and the land+water area as
the denominator [simplifying--in reality, both the 3rd & 4th (time)
dimensions are involved in the numerator, but I think this basic idea
explains why so much of dioxin risk is estimated to come from deposition
onto vegetation that's eaten by ruminantsor directly by people (the same
food chain bioaccumulation happens from water surface deposition (plus
direct injection--the final (I think) source is land deposits distributed
to the env., such as dioxins in landfilled incinerator ash))].  The only
plant genus that is known to takle up dioxins by the roots from the soil is
the cucarbita genus, which includes some melons & squash, incl. pumpkins,
but not (I think) cucumbers.  I guess you also have to consider adsorption
from deposition onto skin, and if I recall this pathway is considered very
small part of the risk.

Note that either absorption through the gut or metabolism of dixons
accounts for the fact that most of our body burden is the 2,3,7,8
substituted dioxins & furans that are (unrealistically) regarded as the
only toxic ones.  But tho the non-planar (non 2,3,7,8 substituted D & F)
have other serious toxicities (not as well studied), the fact that they are
either excreted or transformed into 2,3,7,8 substituted D & F makes it a
moot point, I guess.  I don't know about relative PCB (planar and rotated
rings) metabolism and accumulation.  I do know some of the non-planar ones
are quite toxic and relatively well studied, so maybe the body does
accumulate them...  Note the US EPA Dioxin Reassessment considers the
planar PCBs when calculating exposure, body burdens, and the 'safe' dose
(but doesn't consider the non-planar PCDD/F/PCBs...).


Generally (ultraviolet) photodegradation is regarded as the only mechanism
that signifcantly affects dioxins in the environemnt (obvioulsy for only as
long as they are exposed to the sun).  But the literature is thin--unless
new studues exist, the literature of 3-4 years ago said that people don't
even really know what the products of the degradation are--non dioxins,
2,3,7,8-substituted ones, or other ones.  Note that many half-life
calculation include physical removal (e.g. by wind or erosion).  True
half-lives in the environemnt (ie degradation only) rnage from a decade or
two, to several decades and longer).

Tony Tweedale

Causality is a concept not subject to empirical demonstration. -David Hume
(1711-'76)

Temperate but endangered planet.  Enjoys weather, northern lights,
continental drift.  Seeks caring relationship with intelligent life form.
      -Friends of the Earth