Dioxins are not particularly toxic to people but are deadly to Guinea Pigs and some other animals
Note: many of the facts below are from a summary of Dioxin evidence in John Emsley's book The Consumer's Good Chemical Guide.
One of the things about dioxins that creates unwarranted worry is that chemists have devised ways to detect them in incredibly small quantities (in the 1950s they were lucky to reliably detect parts per thousand; by the 1980s fractions of parts per trillion were easily detected). This means they can now be detected almost everywhere. They are also fairly stable (though they are broken down by some bugs and by the UV component of sunlight) and slightly volatile, so they do get around a lot.
It is also worth noting that there are a large range of different dioxins most of which are not at all toxic. The two archetypal nasty members of the class are shown below:
There are nearly two hundred other dioxins which differ from the two shown by having different numbers and locations of the chlorines attached to the rings.
One of the reasons that caused concern about them was they they were significant contaminants in a range of chlorinated weedkillers (2,4,5-T and 2,4-D, for example, the major ingredients of Agent Orange). The wide use of these weedkillers exposed many people to their contaminants (both because of use and manufacture).
One of the reason dioxins came to be regarded as one of the most deadly known toxins is that they are, to guinea pigs. A dose of 1 micro-g per kg will kill guinea pigs. In hamsters, however, the toxic dose is 5,000 micro-g per kg and there good evidence that people are much less sensitive to them than this. As a reference, botulinum toxin (one of the nastiest known human toxins) kills people at a dose of 10 micro-g per kg.
While we can't do laboratory experiments on people to estimate the acute and chronic activity of dioxins, but we can estimate the effects from looking at people who have been accidentally exposed from chemical accidents. There have been several accidents in manufacturing plants and R&D labs, some of which exposed the public to dioxins.
The most obvious human effect of dioxin exposure is chloracne which is nasty but reversible. Emsley quotes Michael Gough, an expert on the topic:
...no human illness, other than the skin disease chloracne, which has occurred only on highly exposed people, has ever been convincingly associated with dioxins.
One big worry, though, is whether exposure might cause cancer in the longer term (dioxins look like the sort of things that might cause cancer). But as Gough summarised more recently:
There are fewer studies of ‘environmentally exposed’ people, primarily because there are few populations that have been exposed to higher than ‘background’ levels. The two most studied populations are those that lived around a chemical plant in Seveso, Italy, which blew up in 1976; and the residents of Times Beach, Missouri, which was exposed to dioxin-contaminated oil. These studies find that there are no increases in overall disease rates or cancers. It is possible to associate dioxin exposures with elevated occurrence of a specific cancer or disease in some studies, but the occurrence of other cancers and diseases is lower than expected, and there is little consistency among studies’ results. This suggests that the varying disease rates result from fluctuations of occurrence observed whenever small populations are studied.
Another fact putting dioxins in perspective has become apparent as analytical techniques have improved: they are natural compounds not just evil byproducts of the chemical industry. In fact we have been exposed to them since we invented fire and the natural world has had some level of dioxins in it since plants first burned, long before people appeared. This happens because wood and plants always contain some chloride (wood is 0.2% chloride) and, when material containing chlorine and carbon burns, dioxins are produced in small quantities. It may even be that people are particularly resistant to dioxins compared to some animals as we have been exposed to higher levels than most animals ever since we started using fire to cook.
While some animals are exquisitely sensitive to dioxins, the best evidence suggests people are not. And, while some dioxins cause nasty human diseases like chloracne, there is little evidence that any cause other conditions even when exposure is chronic. They are not near the top of the list as human toxins.