Dioxin

The dioxins are a group of similarly structured chemical compounds that are highly toxic to life.  They are never deliberately created, but are commonly created as a by-product of other processes, such as PCBs for electrical equipment, metal refining, bleaching and incineration of waste.  These chemicals bind to the molecules that express genes and can either be directly toxic or can be a carcinogen.  They can be dangerous in even very small doses, but the toxic dose for any particular compound can vary greatly.  
Dioxins are often found in the food chain, generally when foraging animals such as cows eat grass contaminated with industrial waste.  Most governments measure the levels in livestock for human consumption on a regular basis.  The allowable exposure for humans is measured on the scale of less than a millionth of a gram (a few hundred picograms).  
Dioxins can also be harmful to plant life.  Agent Orange, a defoliant used in Vietnam during the Vietnam War, contained dioxins as their primary active ingredient, and many areas where the chemical was used are stlll non-arable.  
Dioxins are also very persistent in the environment.  They cannot be easily incinerated (indeed, they are commonly created by incineration) and resist chemical breakdown.  They can be broken down by exposure to ionizing radiation, such as ultraviolet light.  In addition, because they are not water-soluble, they cannot be removed by the kidneys and tend to bioaccumulate.
Dioxins and dioxin-like compounds at Wikipedia
Category:Chemistry
Category:Toxins