Dioxins are among the most toxic chemicals. It is a general term used to describe hundreds of chemicals which are highly persistent in the environment.
They are formed as a by-product of industrial processes involving chlorine, such as waste incineration or paper bleaching. But they also occur naturally in soil fungus and in grasshoppers.
The most highly toxic is TCDD (tetrachlorodibenzodioxin). One sign of exposure to the substance is chloracne, an unpleasant skin complaint.
They are so dangerous because they accumulate in the body fat over many years, and don't break down easily. For years they have been linked to cancer and two years ago became particularly associated with breast cancer. They can also cause severe reproductive and developmental problems.
Because of industrial processes, every mouthful of food we eat contains dioxins at a very low level. When humans first used fire to cook their food, they were creating dioxins by burning wood.
It is not often fatal for humans because, despite prolonged exposure, we appear to be able to tolerate large quantities in the body. But some are far more sensitive to them than others.