Re: What is the molecular process of dioxin syntheses ?

Unfortunately I don't know the specific chemical reactions involved in 
dioxin synthesis, although I have found a useful document regarding the 
sources of dioxins that are present.

The United States Environmental Protection 
Agency (EPA) is in the process of preparing a report on sources of 
dioxin and related compounds in the US.  Fortunately they have a draft 
available for review, although I must stress (since the EPA did) that this 
is not yet an official document, so it is subject to change.  

At any rate, in the US in 1995 there were >50 potential categories of 
dioxin sources listed.  They range from the mundane, such as municipal 
waste incineration, to the technical, such as pentachlorophenol production 
by chemical manufacturing processes.  By far the greatest source of dioxin 
was the production of pentachlorophenols, accounting for about 90% of the 
estimated total.  Dioxin is a byproduct in this process.

However, this source of dioxin has long been known, and very little of the 
dioxin produced in pentachlorophenol manufacture ever enters the 
environment in a form where it could be dangerous.  The EPA was kind enough 
to list sources in four categories: air, water, land, and products.  Dioxin 
from pentachlorophenol manufacture belongs to the last category.

Aside from dioxin in other products, the largest amount produced enters the 
atmosphere through incineration.  Municipal and hospital waste incineration 
produce about 1600 g/year of dioxin equivalents, while burning of fossil 
fuels and wood, brush and straw add another 400 or so grams per year.

Inputs of dioxin to water and land are much less well known.  The largest 
known source of dioxins directly to water is through bleaching of wood pulp 
and paper with chlorine.

Again I should point out that these are estimates and only from the US for 
a specific year.  The amounts and sources have changed over time, even over 
the last decade, although earlier estimates are in some cases even more 
error prone, since the sources are still being discovered.

I hope this helps answer your question!