Comparing Source Emissions to Deposition Estimates

Studies comparing emissions from known sources of PCDD/Fs to aerial deposition rates in Sweden56 and the United Kingdom57,58 found that annual depositions were 8-20 times higher than annual emissions, suggesting that the bulk of dioxin present in the atmosphere is due to unidentified sources. Estimates of atmospheric deposition rates for PCDD/Fs vary from 1-2 ng TEQ m 2 yr 1 for limited US data to 2-30 ng TEQnU2 yU1 for more extensive European data. Based on an assumed combined wet and dry particulate and gaseous deposition 

54 US Environmental Protection Agency, Dioxin Levels in Fish Near Pulp and Paper Mills, Office of Water Regulation and Standards, Monitoring and Data Support Group, Washington, 1988. 

in Biological Basis for Risk Assessment of Dioxins and Related Compounds. Banbury Report 35, ed. M. A. Gallo, R. J. Scheuplein and K. van der Heijden, Cold Spring Harbor Laboratory Press, Plainview, NY, 1991, p. 121. 

Harrad, A.P. Stewart and K. C. Jones, presented at Dioxin 92,12th International Symposium on Chlorinated Dioxins and Related Compounds, Tampere, Finland, August 1992. 

The above estimates suggest that there may be significant unidentified sources of PCDD/Fs. This possibility is highlighted by Rigo,13 who assigns 82% of total annual emissions in the US to other, unidentified and natural sources, and by Travis and Hattemer-Frey,34 who have estimated that as much as 90% of total TCDD input into the US environment is unaccounted for by specific sources. This view is questioned by Thomas and Spiro,59 who believe that careful analysis of dioxin emissions provides no evidence for significant missing dioxin sources. 

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