While researching some background information for my recent post (Seveso disaster), I came across a few interesting articles about dioxin. I am an organic chemist by training and spent several years in laboratory and so even now I sometimes succumb to the urge to know more about some interesting molecules.
Some molecules are useful, some are beautiful (at least for chemists), and some are villains. From time of my fresher university courses, I always considered dioxin to be a really bad guy, both very dangerous and very persistent.
And a guy who enjoyed really bad company. When US armed forces started using herbicides in Vietnam in 1962, for many years their favorite herbicide was Agent Orange (the herbicides were given names based on colored identification band painted on their 208-litre storage barrels). When herbicide spraying ended in 1971, about three-fifths of recorded herbicide usage represented Agent Orange.
Agent Orange is a chloraromatic compound, and as soon as you start to manufacture such kind of compounds there is always a probability that dioxin TCDD will be produced as well as an unwanted byproduct. This also happened during Agent Orange manufacture and so very significant quantities of TCDD were applied together with the herbicide.
You may read "The extent and patterns of usage of Agent Orange and other herbicides in Vietnam", a Nature article from 2003 for thorough treatement of the subject (if you are not a Nature subscriber, you can find it here).
The second source of extensive data about dioxin's long term effects on human health was brought up by the Seveso accident and so today we know much more about its effects than we did a few decades ago.
Dioxin has been classified as a human carcinogen in 1997 by International Agency for Research on Cancer (IARC)[http://www.iarc.fr/], so legally it is a human carcinogen and any discussion should be settled.
On the other hand I have found a few recent studies and evaluations which challenge this classification. And a few which seem to confirm it.
So what about the title question, is dioxin a human carcinogen or not? I hoped to get at least semi-definitive answer but I could not. Being a practicing chemist, a specialist in scientific informatics and the main programmer of a CLP classifier and Safety Data Sheets creator I do not consider myself to be someone without a clue in this area.
But after spending quite a few hours researching all kind of informations about a very notorious substance I am not convinced that it is really as dangerous as it is claimed to be. However, it may be much more dangerous than my current gut feeling assumes. I simply do not know.
While unable to find a convincing answer, I have decided to share my attempts anyway. I hope it will be useful as a proof of how things start to get difficult and complex if you try to honestly evaluate data about environmental and health risks. Our dream at EcoMole is to really help our environment by bringing as much knowledge as possible to anyone who cares. But as you can see, it is not an easy quest.
A few resources: