Furans are similar chemicals and are often found with dioxins.
Although there are several hundred dioxins and furans, most are only slightly toxic and some are nontoxic. A single dose of 6 millionths of a gram of the most toxic dioxin or furan, called 2,3,7,8-TCDD, is lethal for a rat. The toxicity of a given dioxin or furan depends on the animal that is exposed. Other animals are somewhere in between. How toxic dioxins and furans are for humans is still uncertain.