"Dioxin" is a catch-all term.
The are 208 dibenzo-dioxins and dibenzo-furans.
This total of 208 different chemical compounds does not
include the large number of polychlorinated biphenyls
(PCB's) that have a structural similarity to the
(2,3,7,8-tetrachlorodibenzo-p-dioxin) is considered to be
the most toxic dioxin, but also one of the most toxic
chemicals known to modern science. (See Acute
In addition to TCDD, there are
sixteen other dioxins and furans that are considered to be
highly toxic. At the current time, when dioxin and
furan levels are determined, TCDD and the other sixteen
dioxins/furans are normally measured.
Dioxins are identified by the numbering system shown below. TCDD has a chlorine atom in the
2,3,7 and 8 positions.
have a chemical structure very similar to the dibenzo-dioxins.
Dioxins have two oxygen atoms joining the two aromatic
rings, while furans have only one oxygen atom in the
molecule. The number system for
2,3,7,8-tetrachloro-dibenzofuran (2,3,7,8-TCDF) is shown in
the chemical structure below.
Dioxins and furans that have chlorine atoms in the 2,3,7 and
8 positions are especially toxic.
Dioxins and furans are stable molecules that can be created
very easily under a wide range of chemical reaction
conditions. Chemical reactions normally produce a
combination of several different dioxins and furans.
For this reason, dioxins and furans are found in the
environment as mixtures, never just as a single dioxin or
The biological half-lives and the environmental
half-lives of the seventeen most toxic dioxins/furans vary
considerably. The biological half-life is
defined as the time required for a living organism to reduce
the steady-state level of a dioxin or a furan to 50% of its
original level once further dioxin/furan exposure is
discontinued. The environmental half-life has a
has a biological half-life in humans of approximately 6 to
7.6 years and the other highly toxic dioxins/furans have
biological half-lives in humans ranging from 3 years to
The biological half-life of
according to the age of the human. At birth, the
average half-life for all dioxins/furans is approximately 6
months; at one year of age, approximately 12 months; at five
years of age, 48 months (4 years) and at 55 years of age,
more than13 years.
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