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Response to Comment on “Global Mass Balance for Polychlorinated Dibenzo-p-dioxins and Dibenzofurans” SIR: In our previous paper, we estimated that the global atmospheric deposition rate of polychlorinated dibenzop-dioxins and dibenzofurans (PCDD/F) was about 13 000 kg/yr and that the PCDD/F emissions rate into the atmosphere was about 3000 kg/yr (1). We stated that this lack of mass balance (about four times more coming out of the atmosphere than going into the atmosphere) was most likely “due to the poor characterization of the known PCDD/F sources”. By “sources”, we meant anthropogenic sources, particularly those in developing countries. Based on a literature review (2) and other writings by Gribble (3, 4), Shanefield suggests that “a significant part” of the missing emissions could be due to natural PCDD/F sources. We disagree. Before we address the available facts, it is helpful to define what Shanefield and Gribble seem to mean by “natural sources”. Both of these authors are referring to the de novo synthesis of PCDD/F by either biological or thermal processes from other naturally occurring precursors. The term “thermal processes” refers to natural combustion (forest or grass fires, for example) of natural materials (raw timber but not pentachlorophenol-treated dimensional lumber, for example). The key idea here is that natural compounds, which are not anthropogenic, can be converted into other compounds, which appear to be anthropogenic, by natural processes. We believe that there is no experimental evidence to support the abundant, natural production of PCDD/F. In Gribble’s review of the literature (2), he bases his assertion that “forest and brush fires are the major source of PCDDs and PCDFs in the environment” (my emphasis) on two papers published in 1982 and 1985 (5, 6). Later, Gribble (3, 4) cites several other papers (7-9) to further support his assertion. While we agree that the combustion of wood can produce small amounts of PCDD/F, none of the studies cited by Gribble indicate that natural combustion can form substantial amounts of PCDD/F. In our paper (1), we have estimated that the amount of PCDD/F produced by the combustion of biomass (which includes wood) is about 350 kg/yr on a global basis; this is about 3% of our total global deposition estimate. [Other authors have estimated PCDD/F emission rates from biomass combustion in the United States alone (10, 11).] Certainly, the available, quantitative data indicate the these so-called natural sources are not very large and could not be “a significant part” of the mass balance difference (which is on the order of 10 000 kg/yr). There is another problem with some of the studies used by Gribble to support his contention. To prove the de novo synthesis of PCDD/F in a forest fire, one would have to make sure that the forest had not accumulated PCDD/F or their precursors by deposition from the atmosphere, a wellknown process (12, 13). In other words, Tashior et al. (7) should have measured the composition of the fuel (the forest) before combustion as well as the emissions from the combustion. If part of a forest had been sprayed by
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silvex [2-(2,4,5-trichlorophenoxy)propionic acid], a pesticide widely used in silviculture, the burning of that forest would certainly produce PCDD/F, but the resulting compounds could not be considered natural. In addition, tree bark accumulates relatively large amounts of organochlorine pesticides (12). If the studies of wood combustion cited above (5-9) had not removed the bark from the wood prior to combustion, then PCDD/F could have been formed indirectly from these anthropogenic sources. It is also important to remember that the air used to support the wood’s combustion could be contaminated with small amounts of PCDD/F or their precursors (14), which could lead to the appearance that PCDD/F had been formed in the combustion process itself. Based on the experimental evidence, it is clear that the combustion of biomass could produce small amounts of PCDD/F, but the amount of PCDD/F produced in this way is just not enough to significantly effect the overall mass balance of these compounds. Furthermore, there are several experimental studies that specifically rule out major natural sources of PCDD/F. These studies have focused on the historical record of PCDD/F inputs into the environment. If natural sources were important, then the concentrations of these compounds as a function of time would not be related to anthropogenic activity. There are now considerable data, derived from lake sediment core studies (15-18), indicating that the environmental abundance of PCDD/F increased by at least a factor of 25 between 1935 and 1970. This finding is a strong indicator that the sources of these compounds are not natural but rather are related to the increase in the production of chlorinated organic chemicals that took place during that time period. One of these sediment core studies (15) was conducted on an lake, the watershed of which had suffered a forest fire in 1937. No elevation whatsoever of PCDD/F concentrations was observed in this lake’s sediment core at this time. These sediment core studies (1518), which have been ignored by Shanefield and Gribble, are widely accepted. In fact, the recent Dioxin Reassessment conducted by the U.S. EPA used these data to conclude that PCDD/F in the environment are primarily the result of anthropogenic activities (19). Natural sources are certainly not “significant” sources of PCDD/F to the environment as Shanefield has suggested. Natural sources are certainly not “the major source” of these compounds to the environment as Gribble has suggested. Incidentally, Gribble’s mistake has led to an interesting exchange of editorial comments in Science (20-22).
Literature Cited (1) Brzuzy, L. P.; Hites, R. A. Environ. Sci. Technol. 1996, 30, 17971804. (2) Gribble, G. W. Environ. Sci. Technol. 1994, 28, 310A-319A. (3) Gribble, G. W. Today’s Chemist at Work 1995, March, 15 ff. (4) Gribble, G. W. Environ. Sci. Technol. 1996, 30, 184A. (5) Nestrick, T. J.; Lamparski, L. L. Anal. Chem. 1982, 54, 22922229. (6) Sheffield, A. Chemosphere 1985, 14, 811-814. (7) Tashiro, C.; Clement, R. E.; Stocks, B. F.; Radke, L.; Cofer, W. R.; Ward, P. Chemosphere 1990, 20, 1533-1536. (8) Clement, R. E.; Tosine, H. M.; Ali, B. Chemosphere 1985, 14, 815-819.
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(9) Eduljee, G. H.; Atkins, D. H. F.; Eggleton, A. E. Chemosphere 1986, 15, 1577-1584. (10) Thomas, V. M.; Spiro, T. G. Environ. Sci. Technol. 1996, 30, 82A-85A. (11) Thomas, V. M.; Spiro, T. G. Toxicol. Environ. Chem. 1995, 50, 1-37. (12) Simonich, S. L.; Hites. R. A. Science 1995, 269, 1851-1854. (13) Simonich, S. L.; Hites. R. A. Environ. Sci. Technol. 1995, 29, 2905-2914. (14) Eitzer, B. D.; Hites, R. A. Environ. Sci. Technol. 1989, 23, 13891395 and 1396-1401. (15) Hites, R. A. Acc. Chem. Res. 1990, 23, 194-201. (16) Smith, R. M.; O’Keefe, P.; Aldous, K.; Briggs, R.; Hilker, D.; Connor, S. Chemosphere 1992, 25, 95-98. (17) Kjeller, L. O.; Rappe, C. Environ. Sci. Technol. 1995, 29, 346355.
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(18) Brzuzy, L. P.; Hites, R. A. Environ. Sci. Technol. 1995, 29, 20902097. (19) Anonymous. Environ. Sci. Technol. 1995, 29, 26A-28A. (20) Ableson, P. H. Science 1994, 265, 1155. (21) Spiro, T. G.; Thomas, V. M. Science 1994, 266, 249. (22) Ableson, P. H. Science 1994, 266, 350-352.
Louis P. Brzuzy and Ronald A. Hites* School of Public and Environmental Affairs and Department of Chemistry Indiana University Bloomington, Indiana 47405 ES9620144
