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Is Dioxin as Toxic as the Press Makes it out to Be? |
I. What
is Dioxin?
Dioxin is a class of chemicals properly named poly-chlorinated
dibenzo-p-dioxins. There are about 75 congeners in this group, of
which the most potent is 2,3,7,8 - tetrachloro p - dibenzodioxin. (TCDD)
Dioxin's biological effect is to bind to the Ah receptor
in a cell, and behave as an extremely persistant synthetic hormone.
Dioxin disturbs key physiological signaling systems. It's planar
shape facilitates binding. Dioxin has an extremely high affinity
for the receptor site.
Dioxin's toxicity is enhanced by the fact that it is stored
in the fat, and bio-accumulates with little excretion, and a long half-life.
Dioxin is found in nature from a number of sources both natural
and artificial. These include, but are not limited to minor impurities
in herbicides and paper processing, in combustion processes such as forest
fires, car exhaust, and incinerators, improper handling and dumping of
chemical and medical waste, and as a by-product of Al, Mg, Ni, Fe, and
steel smelting.
II. Human Exposure
There are a number of ways for humans to be exposed to dioxin.
The average amount of dioxin in human tissue in the United States and Canada
is from 5 to 10 parts per trillion.
As dioxins are released into the atmosphere, they precipitate
back to the earth, where they land upon grassy fields and the like.
Cows and other livestock consume these grasses, so therefore, it is natural
to expect a very small concentration of dioxin in both beef and milk (products
of the fatty tissue of cows) with a slight emphasis on meat. This
is indeed the case as most studies show.
Exposure also occurs with chemical plant workers, and constitutes
a minor occupational hazard in almost all cases, except of course direct
experience with pure dioxin and performing chemistry with it.
In addition to the amount of dioxins that precipitate out
of the atmosphere, industry also produces atmospheric amounts of dioxin,
again in very small concentrations.
One of the few actual dangerous sources of dioxins occurs
with chemical plant accidents, where parts of the plant may explode and
spread not only dioxin, but numerous other dangerous chemicals as well.
This was the case with the plant in Seveso.
The other major exposure of humans to dioxin, and arguably
the greatest case against large exposures to dioxin is the Agent Orange
issue in Southeast Asia during the Vietnam War. Many thousands of
gallons of a compound code-named Agent Orange (1:1 n-butyl ester mixture
of 2,4,5-trichlorophenol and 2,4 dichlorophenol) were used to defoliate
the Vietnamese jungles. As a point of fact, these two materials are
the precursors to the most likely mechanism of TCDD synthesis. It
is not surprising that TCDD shows up as a trace impurity in Agent Orange.
III. Health
Effects
One of the most well known health effects of dioxin exposure
found in the laboratory as well as in the major accidents is a severely
disfiguring acne appropriately dubbed "chloracne".
It is important to point out that results so far have shown
that the overall risk of cancer from dioxin exposure is not increased,
but only the frequency of certain types of cancer over others; i.e. soft
tissue sarcomas and lymphomas. This may indicate not a carcinogenicity,
but merely an aiding factor for other carcinogens. For
more information look at slide #7.
IV. Animal
Results
Dioxin
has been fed to animals, as well as painted on their skin, and injected
into avian eggs, with a remarkable plethora of toxicological yet indeterminate
effects. Conditions were neither consistant nor reproducible. For
more information look at slide #8.
V. EPA Classification
The EPA considers TCDD as well as several other related compounds
and isomers as "probable human carcinogens."
The EPA considers animal evidence of carcinogenicity sufficient to conclude
that dioxin is an animal carcinogen in most of the animals that were studied.
Because of certain test factors, however, the EPA has declared
human evidence to be inadequate.
VI. Dioxin
& the Environment
As mentioned previously, dioxin is deposited from the air
onto plants and soil, and from there it enters animals and humans in the
food chain.
Regulations have been put in effect to limit the amount of
particulate matter released from incinerators. Special scrutiny has
been payed to the fact that dioxin is found in highest concentration after
the gasses have cooled down in the scrubbers, so additional scrubbing is
needed as well as higher temperature scrubbers, (> 800 Degrees Celsius).
Various thermal and solvent desorption methods have been
put into use around accident sights, one of the most successful being the
solvation of dioxin in an immobile polymer, and it's subsequent destruction
by UV light from the sun. In 9 days, it was reported that 90% of
the original amount of dioxin was destroyed by this method.
VII. Conclusions
Because of the regulations in place, and an increased knowledge
of avoidance technique, as well as an increased awareness of dioxin's presence
in the environment, dioxin doesn't pose a serious threat in the U.S.
Because dioxin bio-accumulates, it should be watched carefully
nonetheless to make sure that sudden increases or decreases in environmental
concentrations are kept track of through assessment of human concentrations.
Due to the results of animal and human studies, dioxin should
still remain on the list of possible human carcinogens, until more conclusive
results are available.
Dioxin is not as much of a human health risk as the media
has lead us to believe.
Slides
