Dioxin and furan deposition rates began to increase in the 1930s, peaked in the 1950s, and then began declining. The pathways from dioxin emission source to human consumption are illustrated in this figure. Dioxins emitted from combustion and industrial sources, or re-entrained from environmental reservoirs, are transported to distant locations through atmospheric or aquatic pathways. The dioxins are deposited on agricultural crops, taken up in the food supply, and then bioaccumulated and biomagnified through the food chain. This is the predominant pathway for human exposure, excluding isolated exposure episodes resulting from industrial or waste disposal accidents.
Rural environments have substantially lower dioxin and furan levels than urban environments.
Average dioxin TEQ levels in the United States food supply are displayed in this table. The designation ' ND ' in the title section of this slide refers to non-detect, and ' DL ' abbreviates detection limit. Because dioxins are found at very low levels, it can be difficult for a laboratory to determine if a measurement is real or an artifact of the instrumentation. The level at which this uncertainty occurs is called the detection limit. Measured results above the detection limit are considered valid.