FEATURE
The U.S. Dioxin Inventory: Are There Missing Sources? VALERIE M. THOMAS AND THOMAS G. SPIRO
F
ew environmental issues are as contentious as dioxins. Establishing an inventory of dioxin emissions in the United States is an area of fundamental controversy, an issue acknowledged in EPA's recent draft dioxin reassessment (1,2). One approach that has been tried is to compare emissions estimates from the known sources with deposition estimates to see if they balance. Some researchers have concluded that environmental levels exceed known emissions by a substantial margin, suggesting the existSome attempts to ence of significant unknown sources of dioxin (3-5) or that balance atmospheric natural sources such as forest fires have been underesemissions of dioxins timated (6) But careful analysis sugw i t h deposition have gests that there is no evidence for significant misssuggested "missing ing dioxin sources. There are only four published measources." But the surements of dioxin deposition in the United States, evidence disappears all from the Northeast. Dioxin deposition is expected in the uncertainties to vary strongly with location, and current data proin deposition data. vide little basis for determination of average U.S. dioxin deposition. The issue of natural sources is interesting but historical deposition records strongly implicate human activity as the main dioxin source As part of its risk assessment, EPA has constructed an inventory of dioxin emissions in the United States, and a new study has been published recently (7). Similar inventories have been published for Canada, Britain, Sweden, and other countries (3, 4, 8). The outcomes of these investigations are broadly similar, although there are significant differences in detail. The new ranking of U.S. sources 8 2 A • VOL. 30, NO. 2, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
agrees well with EPA's (Figure 1), although the uncertainties are large (a factor of 10 or more). Modern analytical methods make it possible to detect dioxin at very low levels, but it is not a straightforward matter to find out how much is produced. Dioxin is not one chemical but many. The term refers collectively to the congeners of chlorinated dibenzodioxins and furans. The 2,3,7,8-tetrachloro species is believed to be the most toxic, and toxicity diminishes as the number of CI substituents increases or decreases from four. The closely related chlorinated dibenzofurans, which generally accompany the dioxins, have similar toxicity patterns but are less toxic overall. Dioxin emission levels are often reported as the toxicity-weighted sum of the tetrathrough octa-chlorinated dioxins and furans, and the numbers in this cirticle cire in these toxicity equivalent units (TEQs) (i). The ratio of TEQ to total TT1aSS of dioxins and furans depends on the congener distribution but tvnically is about T60 Constructing dioxin source estimates The dioxin source estimates are constructed by multiplying an emission factor—the amount of dioxin emitted per kilogram of material combusted—by the total combustion in each category. For example, the estimated emissions from municipal waste incineration is the product of the estimated average weighted emission factor (~ 100 ng TEQ/kg of waste combusted) multiplied by the amount of waste burned annually in U.S. municipal waste incinerators (—30 billion kg/year). For most combustion categories, the total amount combusted is fairly well known, well within ±50%. But the emission factor is much less certain because of the difficulty in obtaining a meaningful average when the actual dioxin output is highly variable. The data for municipal waste incinerators (Table 1) are more detailed than for any combustion source and permit categorization of the various kinds of facilities. For this reason, the problem of arriving 0013-936X/96/0929-82A$12.00/0 © 1996 American Chemical Society
at a representative average emission factor stands out most clearly in this case. The 100 ng TEQ/kg average dioxin emission factor cited earlier for municipal waste incineration is derived by weighting the emission factor in each category by the total refuse burned in the category. However, the distribution of the data is highly skewed. Nearly one-half of the tests have been on relatively new incinerators equipped with spray dryers and fabric filters (SD/FF). This is not surprising, because until recently dioxin testing was not required at municipal waste incinerators, and an emissions test for dioxin costs about $30,000. The SD/FF-equipped units have relatively low dioxin emission factors (0.2-0.8 ng TEQ/kg). Almost all of the dioxin from municipal waste incineration is emitted from the older incinerators equipped with only electrostatic precipitators (ESP). Dioxin emissions data are available for only 11 of these incinerators, and their emission factors are widely scattered, ranging from 10 to 700 ng TEQ/kg. The 700 ng TEQ/kg value dominates the entire estimate; incinerators in this category (mass burn refractory wall, widi ESP) are calculated to emit 70% of the dioxin while burning only 10% of the garbage. But this emission factor rests on a single test! Recently the municipal waste incineration industry has estimated an emissions total that is 30% lower than the estimates reported earlier by us or EPA (2 kg/year TEO vs 3 kg/year) (9) This difference is within the estimated uncertainty For medical waste incinerators, the data have been much more fragmentary, leading to a wide divergence between our estimate, 0.7 kg TEQ/year, and EPA's estimate, 5 kg TEQ/year. EPA assumed twice as much medical waste combusted as we did and used an emission factor based on facilities with no pollution control equipment, whereas we averaged the data for facilities with and without such equipment. Thus EPA's value for dioxin from medical incinerators seems to be a worst-case estimate. The American Hospital Association has now drawn up its own inventory (11) of U.S. medical waste incinerators and has arrived at a dioxin total about half as large as ours (Figure 1). Scarce deposition data Missing sources of dioxin have been suggested because the total deposition or environmental loading is claimed to exceed known emissions by a substantial margin (5). But data on environmental levels are still scarce or missing. The loadings used in previous mass balance calculations have been calculated by multiplying the area of the United States by an average deposition rate based on the four deposition measurements that have been published: Indianapolis (1991) and Bloomington (1991), Ind.; Green
FIGURE 1
U.S. annual dioxin emisions from combustion sources The two most recent dioxin source estimates show good agreement between average values for most combustion sources, although uncertainties are still large. The range bars span the combined ranges of EPA and Thomas and Spiro estimates. Estimates from EPA (7, Figure 11-3), Thomas and Spiro (7), medical waste incineration industry!/7), and municipal waste incineration industry (9). Sources
Estimated annual dioxin emissions (g TEQ/year)
Medical waste incineration Municipal waste incineration Cement kilns and boilers Wood burning (industrial) Secondary copper smelting Forest fires Petroleum combustion Wood burning (residential) Hazardous waste incineration Sewage sludge incineration Coal combustion Kraft black liquor boilers Drum reclamation Secondary lead smelting Tire combustion
Lake (near Syracuse), N.Y. (1979-84); and Lake Superior (1982) {12-14). Although the four locations give comparable measurements (1-2 ngTEQ/m2/year), it is unlikely that they are representative of the U.S. land mass, with its vast tracts of mountains, deserts, and prairies. In any event, multiplying this rate by the land area (~ 1013 m2) yields 10-20 kg/year of dioxins, not a great deal larger than the total emissions estimated by EPA (9 kg TEQ/year) or ourselves (6 kg TEQ/year). In choosing a low source estimate and a high deposition estimate, one can find a severalfold discrepancy between them (5), but in view of the uncertainties in both estimates, the discrepancy is of doubtful significance. Moreover, a new analysis US different method involving urban air concentrations and production-to-concentration ratios for of pollutants yields dioxin emission estimates that are in rough agreement with the source totals (7) Thus there is no compelling reason to think
there are large missing sources Nevertheless, in the absence of a more accurate mass balance, candidates for additional dioxin sources continue to surface and need to be evaluated. Dioxins can be formed as impurities in the manufacture of organochlorine chemicals, and indeed dioxin contamination of the herbicide 2,4,5VOL. 30, NO. 2, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 8 3 A
TABLE 1
Dioxin emissions from municipal waste incinerators, early 1990s Emissions data on municipal waste incinerators (70) show that dioxin emissions can be greatly reduced by control of combustion conditions and installation and careful operation of new pollution control equipment. There are more data on municipal waste incineration than any other combustion process, but even this information is patchy; some emission factor estimates rely on only one measurement.
Combustor type
Number of facilities tested
Mass burn refractory wall ESP 1 SD/FF 1 Mass burn waterwall ESP 4 SD/ESP 2 SD/FF 7 Mass burn rotary ESP 0 SD/FF 2 Refuse-derived fuel ESP 2 SD/ESP 2 SD/FF 3 Modular starved air ESP 2 None 1 Modular excess air ESP 2 Total
29
Emission factor, ng/kg/TEQ
MSW combusted, 109 kg/year
TEQ dioxin emissions, g/year
700 0.5
3 0.15
2100 0.08 50 16 5
10 8 0.5
5 2 8
10 0.8
0.5 1
5 0.8
150 0.8 0.2;
5 1.5 0.9
750 1 0.2
33 13
0.5 0.5
16 70
16
0.4
6
100
s
29
3000
TEQ, toxicity equivalent units; MSW, municipal solid waste; ESP, electrostatic precipitator; SO, spray dryer; FF, fabric filter. " Value is weighted average.
trichlorophenol (2,4,5-T) was the basis of the longrunning and still unresolved controversy about the effect on Vietnam veterans and civilians of spraying with Agent Orange. We estimated that the spraying of chlorophenol-based herbicides and pesticides in the United States was responsible for as much as one-half of the environmental emissions of dioxin in 1970 (7). Most uses of chlorophenol pesticides were phased out in the 1970s and 1980s, but some investigators have proposed that the disturbance of old dioxin reservoirs may be a significant contributor to current dioxin deposition {15). The largest volume organochlorine product is polyvinyl chloride (PVC). Some dioxin is thought to form during its production, but there are no data from which to make a quantitative estimate. Dioxin emissions have been estimated for PVC combustion in building fires and for accidental polychlorinated biphenyl (PCB) fires. Once widely used, PCBs are no longer produced but are still present in old electrical transformers and capacitors. These fires may produce amounts of dioxins t h a t 3X6 significant locally, but they are unlikely to be important entries in the national inventory. Dioxins are produced in paper manufacture when wood pulp is bleached with chlorine. Contamina8 4 A • VOL. 30, NO. 2, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
tion of fish in nearby waterways has been a major concern, and the U.S. pulp and paper industry has been implementing measures to reduce dioxin production. EPA estimates that dioxin production from the U.S. pulp and paper industry is about 5% of the total national emissions. Perhaps the most interesting question is whether natural sources of dioxin are important. There are many natural organochlorine products; peroxidase enzymes, capable of incorporating halogen atoms into carbon compounds, are widespread in nature (6). There have been reports of dioxin emission from compost piles, but data are insufficient to make quantitative estimates. Wood burning is known to produce dioxins, and although the rates are low, enough wood is burned for industrial and residential wood combustion to produce significant total amounts of dioxin. Our estimates for these two sources (see Figure 1) differ from EPA's because of different judgments on emission rates. More data have been added since these studies were performed (Table 2), and the estimates will probably be revised toward the middle ground. Excluding the high entries for industrial burning of PCPtreated wood chips or for residential burning of household waste, the average emission factors are about 2 ng TEQ/kg for both industrial and residential wood burning. ^We used the values 0 7 and 7 ng TEQ/kg, respectively, based on fewer data and EPA used 4 and 1 ng/kg, respectively.
The forest fire contribution These numbers are important with respect to the contentious issue of forest fires, which on several occasions have been named as the largest dioxin source. The amount of wood burned in U.S. forest fires is comparable to industrial and residential wood burning. Because there are no dioxin emission data on forest fires per se, both we and EPA used the emission factors for residential wood burning to arrive at the estimated forest fire contribution (0.4 and 0.08 kg, respectively). If a revised emission factor of 2 ng TEQ/kg were used, then the estimate would fall in between, at 0.2 kg. This is a small fraction of the total dioxin inventory. However, it is possible that wood stoves have a dioxin emission factor different from that of forest fires because of differences in combustion conditions (16). Also, the wood used in residential burning may have a lower average chlorine content because it lacks die salt-containing sap that is present in the bark and foliage of burning trees. But the relative importance of chloride ions versus organically bound chlorine in dioxin formation from combustion is not understood. Tree bark may absorb significant quantities of anthropogenic chlorinated organics, which may increase dioxin emissions from forest fires (17). Thus, we have very little information on which to base a quantitative assessment of natural dioxin sources. However, the historical record of dioxin deposition provided by sediment cores strongly implies that anthropogenic sources have become dominant. For example, in Siskwit Lake (located on an island in Lake Superior), the dioxin deposition rate (Figure 2) increased eightfold between 1940 and 1970, the period of great expansion in the industrial use
of chlorine. Since 1970, the rate has declined about 30%, concurrent with decreased production of chlorophenols and improved technology for municipal incinerators. It is difficult to reconcile these trends with predominantly natural sources, especially when one considers that the area of U.S. forests consumed by fire diminished by more than a factor of four between 1940 and 1970 through more effective fire control (7). The inventory of U.S. dioxin emissions is improving. There is good agreement between the two most recent studies and ample evidence that the main current sources of dioxins are incinerators. More important, improvements in the design, pollution control, and operation of these facilities promise significant emissions reductions soon. However, a lack of data and of basic understanding of the environmental behavior of dioxins must be addressed. These data deficiencies can be rectified by obtaining geographically representative measurements of dioxin deposition. A better understanding of the environmental behavior of dioxins calls for research on the importance of vapor-phase versus particulate transport the environmental behavior of different congeners and the significance of processes that reintroduce dioxins into the atmosphere after deposition
FIGURE 2
Historical dioxins flux to remote North American lake Sediment cores from Siskwit Lake, located on an island in northern Lake Superior, provide a historical record of atmospheric dioxin fluxes (14). An eight-fold increase in the deposition rate between 1940 and 1970 correspond: to a great expansion in the industrial use of chlorine. The decrease since 1970 (about 30%) parallels decreased production of chlorophenols and a reduction in municipal incinerator emissions.
References (1) Estimating Exposure to Dioxin-Like Compounds (External Review Draft); U.S. Environmental Protection Agency: Washington, D.C., 1994; Vols. I—III (EPA/600/6-88/ 005Ca,b,c). (2) Health Assessment Document for 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds (External Review Draft); U.S. Environmental Protection Agency: Washington, D.C., 1994; Vols. I-III (EPA/600/BP92/001a,b,c). (3) Rappe, C. Banbury Report 35: Biological Basis for Risk Assessment of Dioxins and Related Compounds; Cold Spring Harbor Press: New York, 1991. (4) Harrad, S. J.; Jones, K. Sci. Total Environ. 1992,126, 89107. (5) Rigo, H. G. Solid Waste Technol. 1995, Jan./Feb., 36-39. (6) Gribble, G. Environ. Sci. Technol. 1994, 28, 310A-318A. (7) Thomas, V M.; Spiro.T. M. Toxicol. Environ. Chem. 1995, 50, 1-37. (8) Sheffield, A. Chemosphere 1985, 14, 811-14. (9) "Comments on EPA's Estimating Dioxin Exposure Document"; Integrated Waste Services Association: Washington, D.C., 1995. (10) Locating and Estimating Air Emission Sources of Dioxins and Eurans. Draft final report. Office of Air Quality Planning and Standards. U.S. Environmental Protection Agency: Research Triangle Park, N.C., 1993. (11) "Comments on the U.S. EPA Dioxin Exposure and Health Document;" American Hospital Association: Chicago, 111., Jan. 13, 1995. (12) Koester, C; Hites, R. Environ. Sci. Technol. 1992,26,137582. (13) Smith, R. M. et al. Chemosphere 1992, 25, 95-98. (14) Czuczwa, J.; Hites, R. Environ. Sci. Technol. 1986,20, 195200. (15) Kjeller, L-O.; Rappe, C. Environ. Sci. Technol. 1995,29, 34655. (16) Reinhardt, T. E.; Ward, D. C. Environ. Sci. Technol. 1995, 29, 825-32. (17) Simonich, S. L.; Hites, R. A. Science 1995, 269, 1851-54. (18) Schatowitz, B. et al. Organohalogen Compd. 1993,11, 30710. (19) Vikelsoe, J.; Madsen, H.; Hansen, K. Organohalogen Compd. 1993, 11, 405-8.
Valerie Thomas is a member of the research staff at Princeton University'' Center for Energy and Environmental Studies. Thomas Spiro is the Eugene Higgins Professor of Chemistry, Princeton nniversity.
TABLE 2
Dioxin emissions from biomass combustion Dioxin emissions data from wood burning (7) can be used to estimate emissions factors for forest fires.
Facility
Dioxin emission factor, TEQ, ng/kg
Industrial w o o d incinerators Wood-fueled incinerator (scrubber, multiclone, ESP) Before pollution control 8 After pollution control 1.6 Wood-fueled incinerator (ESP) 1 Wood-fueled incinerator (multiclone) 0.8 Wood-fueled incinerator (multiclone, ESP) 0.7 W o o d chip furnace 3 Natural w o o d chips 1.8 Chipboard chips 0.6 Chlorophenol-treated w o o d waste chips 120 Industrial average 2 ± 2.6* Nonindustrial biomass c o m b u s t i o n Residential boilers Domestic furnace (straw) Cigarettes Household w o o d stove a Natural beech w o o d Household waste W o o d stove (natural beech wood) W o o d stoves (natural beech, birch, and spruce) 0 Nonindustrial average
0.3 4 4 1 1400 0.32 1.9 2 ± 1.6"
TEQ, toxicity equivalent units; ESP, electrostatic precipitator. 3 Values from Reference 18. * Excluding chlorophenol-treated wood. c Value from Reference 19. rf Exnliirlinn household masts.
VOL. 30, NO. 2, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 8 5 A
