31.12 What are Dioxins?
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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
31.12 What are Dioxins?
"Dioxins" are a group of closely-related compounds which are known as
polychlorinated dibenzo-p-dioxins (PCDDs). "dioxins" also commonly includes
polychlorinated dibenzofurans (PCDFs). All organic molecules that contain
chlorine are also members of the "organochlorine" family.
1 9 1 9
2/ \ _o_ / \8 2/ \ ___ / \8
| O |_o_| O | | O |_o_| O |
3\ / \ /7 3\ / \ /7
4 6 4 6
Dibenzo-p-dioxin Dibenzofuran
As dioxins are fat soluble, they will accumulate in fatty tissue. In general,
it is only the biologically active ( molecules containing the 2,3,7,8
substitution ) congeners that accumulate, with levels of the higher
homologues predominating [20]. It is important to remember that of all the
dioxins and furans, only those containing 4 to 8 chlorine atoms, _and_ with
chlorine atoms in the 2,3,7,8 positions are currently considered toxic.
The compounds only containing 0 to 3 chlorine atoms are currently not
considered toxic, however once all four of the 2,3,7,8 positions are filled,
the most toxic congener is created ( 2,3,7,8 TCDD = "dioxin" ). As additional
chlorines are added, the toxicity decreases, except that 2,3,4,7,8
pentachlorodibenzofuran is more toxic than 2,3,7,8 tetrachlorodibenzofuran.
There is evidence that suggests concentrations of dioxins and furans in
human adipose tissue are falling [21]. The analysis for dioxin can reliably
detect ppq ( parts per quadrillion = picograms/kilogram ) levels, but some
evidence suggests dioxins may still have toxic effects at such low levels.
The toxicity of dioxins is currently being carefully assessed by the US EPA
- who are due to present a comprehensive report in the next few months. The
draft of the report, and various reviews, have been available for public
comment and external peer review. A good discussion of current perceptions
is available in a special report published in the January 1995 Environmental
Science and Technology [22], where both sides of the debate are presented.
Dioxins can arise naturally from forest fires, but the major sources are
from incineration and the manufacture and use of organic chemicals. The most
well-known source is perhaps as an impurity in the defoliant Agent Orange
which was widely used in Vietnam. Agent Orange was a 50:50 mixture of the
n-butyl esters of the herbicides 2,4-D ( 2,4-dichlorophenoxyacetic acid ) and
2,4,5-T ( 2,4,5-trichlorophenoxyacetic acid ), and dioxin was present as
an impurity in the 2,4,5-T. Another well known incident was an industrial
accident at a 2,4,5-trichlorophenol manufacturing plant in Seveso, Italy
on 10 July 1976 which resulted in the release of 1-5 kg of dioxin.
Other dioxin sources involve combustion ( leaded gasoline, coal combustion,
metallurgical processes )[20].
As the various congeners have differing toxicity, dioxins are usually
reported using Toxic Equivalents systems. These assign to each congener a
toxicity factor relative to 2,3,7,8-TCDD, and these factors are used to
calculate the 2,3,7,8-TCDD Toxic Equivalent. The International Toxic
Equivalent Factor (I-TEF) system, proposed by the Challenges to Modern
Society Committee of the North Atlantic Treaty Organisation is widely used.
Food is the major source of dioxins for humans, and typical dietary intakes
in the US for a 65kg adult were estimated to be between 18-192 pgTEQ/day [23],
and UK intakes were estimated to be 125 pgTEQ/day [20]. The Regional Office
for Europe of the World Health Organisation suggests 10 pg/kg body weight/day
for 2,3,7,8-TCDD ( 600 pgTEQ/day for 60kg person ), as a Tolerable Daily
Intake, whereas the US-EPA suggests an intake of 0.006 pg/kg/day over a 70
year life will lead to one excess cancer in one million people.
Sources of Dioxins in the UK diet pgTEQ/day Meat, meat products, poultry, and offals 38 Cow's milk 23 Fats and oils 19 Milk products 12 Fish 7.7 Eggs, cereal products, fruit, and vegetables 25.3
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