Re: "other" dioxins

[Melissa TwiningDavis <mdavis@emerald.tufts.edu>]

> 2,3,7,8-tetrachloro-dibenzodioxin "2,3,7,8-TCDD"  is what is generally
> called "dioxin".  

Be careful.  The term dioxin can have two separate meanings. 1) It can 
refer to only 2,3,7,8-tetrachloro-dibenzo-p-dioxin, or 2) It can be used 
to refer to all of the different chemical congeners within the dioxins 
that have the same or similar toxicity levels as 2,3,7,8-TCDD (also known 
as toxic equivalency factors/quotients-TEFs or TEQs).  This use of the 
term dioxin includes the dibenzofurans and PCBs--some of which are 
nearly as toxic as 2,3,7,8-TCDD.  Each article written on dioxin should 
clarify its use of the term.

> I am seeking information of RELATIVE toxicities of the various dioxins.

There is a great deal of controversy over the potential toxicity of 
"dioxins"-definition #2.  According to Alastair Hay (1982), the toxicity 
of chlorinated dibenzodioxins and dibenzofurans is dependent on both the 
number of chlorine atoms and the placement of those atoms on dioxin's two 
benzene rings.  The positioning of the four chlorine atoms at locations 
2,3,7 and 8 is believed to be what gives this congener its powerful 
toxicity.  

If one chlorine atom is removed from 2,3,7,8-TCDD, the toxicity of the 
new congener decreases by a factor of 15,000.  If a chlorine atom is 
added to 2,3,7,8-TCDD, the toxicity of the new congener only decreases by 
a factor of .5.


As for 

> 1,3,6,8-TCDD
> 2,3,7-trichloro-dibenzodioxin
> 2,7-dichloro-dibenzodioxin
> dibenzo-dioxin   (with no chlorines)
> 
> I assume that these chemicals with fewer chlorines are less persistent in
> the environment and are therefore considered less of a "health" concern, but
> the assumption that they are less toxic???
> 
I would come to a different assumption.  I believe that these congeners 
are less toxic, but I do not know that they are less persistent. 
Ultimately, I think that the persistence of the dioxins and its congeners 
may be far more deadly than its immediate toxicity.

Most information is written on TEQs, referring to 2,3,7,8-TCDD, and the 
other exceptionally toxic forms such as the furans.  I don't know where to 
find information on these particular congeners.  Have you come across 
this list in an article(s)?