The natural world is, as everyone knows, not the only source of toxic chemicals. In fact, people worry more about synthetic chemicals than natural ones. Organophosphorous insecticides are typical of the fast-acting synthetic chemicals. They affect the nervous system and some members of this chemical family are so toxic that they cannot even be used in insecticides because they would harm the users.
One common chemical of concern to the public is dioxin, a byproduct of industrial processes. This chemical happens to be extremely toxic to female guinea pigs--it takes only a dose of 0.0006 mg/kg b.w. to cause death--while it takes a dose about 5,000 times greater to kill a hamster. Fortunately, humans are more resistant to dioxin.
In addition to dioxin, one notable example of extreme toxicity of manufactured chemicals is