Dioxin is the trace contaminant in Agent Orange, used in the Vietnam War, suspected of causing increased incidence of cancer in veterans exposed to the defoliant. Dioxin is the stuff that is present in trace amounts in effluent from chlorine bleaching of wood pulp containing lignin. Dioxins have been known to be potent carcinogens for some time. Studies, however, have been contradictory. Different animal species react differently to low doses of the poison.
Hamsters are unaffected by a dose of dioxin that is lethal to a guinea pig.
Female rats develop liver cancer upon exposure, but male rats do not. These counterintuitive results appear in studies undertaken under laboratory conditions. TCDD is an extremely potent toxin. However, at low level exposures to humans, dioxin's effects are more difficult to pin down. Just as a hormone, TCDD reversibly binds to an intercellular receptor at the cell membrane.