Dioxins enter the air from thousands of sources including incinerators that burn medical, municipal, and hazardous waste, chemical processing facilities that make pesticides and PVC plastic, and metal refining and smelting operations. TCDD, the most commonly studied dioxin and the one included in the timeline, is a known carcinogen. Airborne dioxins settle on the ground and in water and move up the food chain, entering humans primarily through ingested fish, meat and dairy products. They are persistent contaminants that accumulate in fatty tissue.
All dioxin studies included in the timeline were performed at doses of 0.005 parts per million or less (5 μg/kg bw).