Where Did All Those Dioxins Come From?

3.2.5 Anthropogenic vs. Natural Sources of Dioxin (Charge Question 10)

The background for this question is the observation in the late 1970s that dioxins are produced by the combustion of many common materials, including municipal solid waste. This led some scientists to suggest that dioxins had been with us since "the advent of fire" and that dioxins could be produced by natural combustion (for example, by forest fires). At that time, there were some suggestions that observed levels of dioxins were primarily the result of coal combustion or perhaps of wood burned in small stoves. This speculation was largely refuted by sediment core studies, both in the United States (primarily in the Great Lakes) and in Europe, which indicated that environmental dioxin levels increased significantly beginning about 1935-40 (see Volume II, pages 3-92 to 3-94).

Since the advent of fire clearly predated this time, it can be concluded that dioxins were largely anthropogenic and associated with events taking place around 1935-40. What were these events? Coal combustion could be ruled out because the consumption of coal in the United States was essentially constant from the turn of the century until about 1970; this record did not agree with the sediment core data.

The explanation is likely to be the introduction of chlorinated organic compounds (polyvinyl chloride and chlorinated pesticides are but two examples) in the 1935-40 time-frame. Other sources such as leaded gasoline (which commonly contained ethylene dichloride and ethylene dibromide), diesel emissions, and PCBs are also possibly significant contributors. Although the details of dioxin formation are not yet quantitatively understood, the introduction of these chlorinated products into wastes that were combusted appears to be the most likely cause of the increased dioxin deposition measured in sediments.