"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 dioxins/furans.  TCDD (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 Toxicity).

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.
 

TCDD

Dibenzo-furans 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.

TCDF

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 furan.

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 similar definition.

TCDD 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 almost 20 years. 

The biological half-life of dioxins/furans varies 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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