Research Communications Evidence for a Decline in Atmospheric Emissions of PCDD/Fs in the U.K. L . - O . K J E L L E R , † K . C . J O N E S , * ,‡ A . E . J O H N S T O N , § A N D C . R A P P E * ,† Institute of Environmental Chemistry, Umea˚ University, S-901 87 Umea˚, Sweden, Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K., and IACR, Rothamsted Experimental Station, Harpenden, Hertfordshire, AL5 2JQ, U.K.
Introduction Atmospheric deposition is the most important source of PCDD/Fs to agroecosystems nationally in the U.K. and other industrialized countries. Plant-based foods, meat, and dairy products (all derived from agroecosystems) account for a substantial proportion of the human dietary intake of PCDD/Fs (1, 2), so atmospheric sources and inputs have important implications for human exposure (3). Here, we report trends in PCDD/Fs in vegetation samples from the control plot of the Park Grass experiment started in 1856 at Rothamsted Experimental Station, which is a semi-rural location in southeast England. The Park Grass permanent pasture experiment was established in 1856 (on a field previously undisturbed for 200 years) to identify the effects of nutrients on crop yield. It is the oldest experiment on grassland in the world. The PCDD/Fs are two classes of persistent, toxic halogenated aromatics; there are 75 possible PCDDs and 135 possible PCDFs. Attention has focused on the 17 2,3,7,8tetra- through octa-Cl-substituted compounds, which have caused carcinogenic effects on laboratory animals and have been implicated in immunological and reproductive effects on birds and mammals in the wild (3-5). The various 2,3,7,8-substituted molecules are believed to operate collectively through a common site of action (6, 7). This has given rise to the development of the ‘toxicity equivalent factor’ (TEF) scheme that can be used to derive a single toxicity equivalent (TEQ) value in tissues exposed to mixtures of PCDD/Fs (6, 7). The 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (2,3,7,8-TCDD) is the most toxic PCDD/F and is assigned a TEF value of 1. All PCDD/Fs are lipophilic; however, as the number of Cl atoms increases through the homologue groups [e.g., from TCDD/Fs through the penta (Pe)-, hexa (Hx)-, hepta (Hp)-, to octa (O)-CDD/Fs], the PCDD/Fs become more insoluble in water and strongly partitioned onto soil particulates. TCDD/Fs are ‘semivolatile’ organic compounds (SVOCs), with a major proportion of their atmospheric burden present in the vapor phase. * Authors for correspondence. † Umea ˚ University. ‡ Lancaster University. § IACR.
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In contrast, OCDD/Fs are almost exclusively associated with aerosols under ambient conditions (2, 8). Various source apportionment studies have identified the combustion-related formation of PCDD/Fs (particularly the burning of solid wastes, wood and biofuels, leaded petrol, etc.) and the incidental releases of PCDD/Fs formed during the manufacture, use, and destruction of chloroaromatic compounds [CAs, notably pentachlorophenol (PCP) and hexachlorobenzene (HCBz) but also polychlorinated biphenyls (PCBs) and herbicides (i.e, 2,4-D and 2,4,5-T)] as the major sources to the environment, although there is uncertainty as to their relative contributions. These combined anthropogenic activities are estimated to have released a few tens of kilograms of ∑PCDD/F to the U.K. atmosphere annually in recent years (9); contemporary primary emissions in the U.K. are estimated between 560 and 1100 g ∑TEQ/year (10). Despite some controversy as to the possible natural production of PCDD/Fs (11, 12), it is likely that trace quantities of PCDD/Fs were released in the past from combustion, but that releases became much greater than in pre-industrial times during this century (12, 13). Importantly, different sources release rather different (and often characteristic) mixtures of PCDD/Fs; ‘fingerprints’ of mixtures (including the non-2,3,7,8-substituted constituents) in environmental samples may provide clues as to sources for apportionment purposes. Combustion processes are generally a more significant source of tetraand penta-CDD/Fs, while CA production (notably of PCP and HCBz) constitutes a greater source of Hp- and OCDD/ Fs (14, 15). Given the established link between atmospheric deposition and human exposure via foliage f livestock ingestion f dairy products f dietary intake (1-3), legislation in Europe and North America has targeted waste incinerator emissions in an attempt to reduce primary emissions of PCDD/Fs (e.g., ref 10). In this paper, we present evidence from the analysis of archived samples that contemporary rural U.K. herbage ∑PCDD/F concentrations have declined 8-fold since a peak in 1961-1965. By inference, air concentrations will also have reduced over this time. The most recent sample (1991-1993) contained ∑PCDD/F concentrations only slightly elevated above the pre-1900 samples. We discuss the implications of these findings in terms of possible sources by reference to the mixtures of PCDD/Fs present in the samples.
Materials and Methods The Park Grass control plot has never received direct applications of fertilizers, soil amendments, or pesticides. Annually harvested (unwashed) herbage samples have been stored at Rothamsted in sealed containers after being ovendried (16). Twenty composite samples were prepared at Lancaster in 1993 to cover one 3-year interval (1991-1993), 5-year intervals (1861-1965 and between 1931 and 1990), and 10-year intervals (between 1871 and 1930) by bulking the first harvest (made in May/June of each year) in proportion to yield. Annual yield averages about 1 t dry
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a
1740 10600 54 2270 463 285 4818 182 2680 600 183 116 30 1501 124 143 135 2272 892 96 130 116 1234 1300 1410 2710 210 413 18363 10345 28708 0.56 200 443 643
1810
9390 87 1530 370 273
3753 146 1910 525
209 201 45 1561 52 176 184
1366 653 99 287 65 1104 861 993 1850 237 952 16045 7608 23653 0.47 212 441 653
1890 1080 40 142 117 1379 914 1060 1970 832 4170 13057 12330 25387 0.94 261 358 620
164 185 73 1591 124 146 160
2695 153 2470 558
6560 127 1830 209 226
1240
150 142 81 1454 93 138 202
3331 144 2240 498
6720 33 1450 263 298
1330
b
1262 733 24 220 57 1034 589 644 1230 1220 3600 12360 8912 21272 0.72 141 326 467
101 96 41 963 44 79 79
2613 83 1480 288
6530 69 1340 209 244
1320
132 149 45 1250 63 158 170
3980 149 2490 427
9020 62 1440 324 446
2030
1088 1571 719 1050 55 ND (119) 128 142 110 86 1012 1278 906 1260 770 1430 1680 2700 909 979 6540 8350 12657 16507 12168 16551 24825 33058 0.96 1.00 187 198 337 530 524 728
141 104 37 1094 86 68 58
2972 149 1500 366
6670 77 1360 223 239
1320
1511 837 61 193 56 1147 853 823 1680 857 4310 13374 10751 24125 0.80 191 361 552
141 113 35 1233 73 90 118
3307 119 1680 480
6830 91 1570 192 265
1320
1252 980 115 406 181 1682 979 897 1880 860 8430 15358 14052 29410 0.91 196 407 603
199 146 66 1499 78 136 125
4377 170 1360 380
6940 60 1130 281 356
1230
2098 1590 39 1440 80 3149 3530 5160 8690 991 21400 13832 34180 48012 2.47 184 342 526
121 74 57 1463 81 207 60
2929 79 1180 316
5300 37 812 200 209
1530
1649 1500 28 1090 59 2677 2630 3410 6040 1210 17300 11841 27617 39458 2.33 179 269 448
75 108 20 1172 79 104 116
2392 122 1870 274
4390 37 758 98 214
924
3328 3320 30 2570 83 6003 7350 6850 14200 2230 39400 15967 63658 79625 3.99 264 302 566
99 77 62 1256 77 296 165
2418 134 2630 244
4060 35 4100 216 212
1010
17741 11000 ND (79) 11900 288 23188 36200 42000 78200 14200 150000 60658 252171 312829 4.16 1182 992 2174
460 350 50 9040 417 1650 974
6120 363 3480 1310
8110 126 2750 369 701
2790
132 178 37 2489 171 1880 710
4091 208 4700 472
6090 37 1420 372 399
2010
5408 19561 3530 2390 251 ND (130) 3230 1040 87 OAd 7098 3430 4950 9230 8250 7970 13200 17200 3480 1990 37700 44100 25753 18090 61858 86981 87611 105071 2.40 4.81 326 541 446 527 772 1068
163 118 30 3832 86 453 349
4593 128 3360 411
6750 53 2190 436 357
1340
ND, not detected; detection limits given in parentheses. cRatio of 1991-1993/1961-1965.
2363 854 78 220 87 1239 943 1150 2100 161 586 12905 8739 21644 0.68 160 392 552
The toxicity equivalent values are also calculated.
1,2,4,9-/2,3,7,8-/1,2,7,9-/ 1540 2,3,4,6-/2,3,4,7-/2,3,4,8TCDF sum of TCDF 4210 2,3,7,8-TCDD NDb (105) sum of TCDD ND 1,2,3,7,8-PnCDF 292 2,3,4,7,8-/1,2,3,6,9391 PnCDF sum of PnCDF 2833 1,2,3,7,8-PnCDD 378 sum of PnCDD 1730 1,2,3,4,7,8-/1,2,3,4,6,7651 HxCDF 1,2,3,6,7,8-HxCDF 96 2,3,4,6,7,8-HxCDF 160 1,2,3,7,8,9-HxCDF 254 sum of HxCDF 2451 1,2,3,4,7,8-HxCDD 181 1,2,3,6,7,8-HxCDD 292 1,2,3,7,8,9-/1,2,3,4,6,7228 HxCDD sum of HxCDD 4131 1,2,3,4,6,7,8-HpCDF 2100 1,2,3,4,6,7,9-HpCDF 95 1,2,3,4,6,8,9-HpCDF 954 1,2,3,4,7,8,9-HpCDF 144 sum of HpCDF 3293 1,2,3,4,6,7,9-HpCDD 1200 1,2,3,4,6,7,8-HpCDD 1450 sum of HpCDD 2650 OCDF 1370 OCDD 7170 PCDF 14157 PCDD 15681 PCDD/F 29838 ratio (PCDD/PCDF) 1.11 I-TEF/89 PCDD 281 I-TEF/89 PCDF 504 I-TEF/89 785 d
44895 3980 233 780 281 5274 5810 4010 9820 638 30500 16671 101785 118456 6.11 826 480 1306
96 209 188 1995 395 2940 1260
3744 279 14000 282
5020 156 2570 265 319
1870
88 94 17 1477 104 165 84
2819 67 2070 208
4000 121 1330 293 276
1280
3793 2093 1380 1020 67 ND (129) 641 622 81 125 2169 1767 4280 3080 3330 2800 7610 5880 1610 975 32300 15600 10300 11038 49013 26973 59313 38011 4.76 2.44 192 233 312 334 504 567
84 56 66 993 71 370 302
2428 77 4160 210
3100 14 1150 195 283
1030
OA, organic artifact.
26074 3670 77 2470 36 6253 19200 15900 35100 2520 78500 33115 150654 183769 4.55 933 957 1890
245 258 66 3701 144 1980 1650
6241 384 8580 612
14400 126 2400 389 452
5540
239
1
313 73 197 35
0.12 0.10 ND 0.05 0.43 0.08 0.08 0.07 0.07 0.07 0.10 0.18 0.11 0.12
98 63 51 337 78 100 76
323 98 231 106
92 ND (32) ND (46) 143 ND (67) ND (48) ND (58)
224 447 68 ND (57) 68 ND 56 ND (36)
228 368 ND ND ND (49) 123 82 ND (29) ND (58) ND (42) ND (41) ND (34) ND (58) ND (42) ND (41) ND (34) ND (69) 90 32 ND (42) ND 213 113 ND 233 291 131 57 226 285 131 113 459 576 262 170 176 51 49 67 2808 629 1017 523 1163 1443 1003 2232 3752 1974 1409 693 4914 3418 2412 2925 3.23 1.37 1.41 0.31 46 78 99 1.7 60 130 70 122 106 208 169 124
0.19 18 0.27 13 0.34 14 0.16 149 0.25 15 0.10 27 0.09 ND (82)
0.46 0.18 0.59 0.16
519
4
445 1575 63 ND (71) 63 ND 54 41 61 118
263
3
34 ND (40) ND (62) 171 ND (73) ND (52) ND (64)
blank
272
2
0.49 525 521 0.96 ND (53) ND (66) 0.48 60 170 0.79 48 92 039 52 130
0.46
1861- 1871- 1881- 1891- 1901- 1911- 1921- 1931- 1936- 1941- 1946- 1951- 1956- 1961- 1966- 1971- 1976- 1981- 1986- 19911865 1880 1890 1990 1910 1920 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1993 ratioc
Individual and Total PCDD/Fs in Archived Herbage (fg/g) and Blank Samplesa
TABLE 1
FIGURE 1. Trends of ∑PCDD/Fs in the Park Grass herbage at Rothamsted and soil from a nearby arable experiment with winter wheat (see ref 13).
mass/h, typical of unmanured grassland in lowland regions of the U.K. Samples (2-6 g) were Soxhlet extracted using toluene, cleaned up on silica, modified on silica alumina and charcoal as described previously (13, 17), and analyzed by HRGC (DB-5 column)-HRMS (resolution 10 000) in Umea˚. 13C-Labeled standards were used for quality assurance purposes, and method blank samples were also run. The full data set, presented in Table 1, includes these blanks.
Results and Discussion A wide array of PCDD/Fs were detected in all the samples well above the method blanks. Concentrations ranged between 21 and 310 ng ∑PCDD/F per kg of dw, although some individual compounds were sometimes below the detection limits. The most recent sample contained 38 ng/kg, a value typical for contemporary U.K. vegetation (18), with the OCDD, HpCDD, TCDF, and PeCDF homologues dominating. The consistent presence of the full range of tetra- through octa-CDD/Fs in all the samples from the mid-1800s provides evidence in support of the ‘pre-industrial’ formation of these compoundssmost likely from the combustion of coal and wood, metal smelting, etc. (see below)salthough the possibility of postcollection contamination of the samples cannot be ignored (19). The presence of PCDD/Fs in pasture is due almost exclusively to deposition of airborne PCDD/Fs, with negligible uptake from the soil and subsequent translocation. Indeed, Welsch-Pausch et al. (8) have elegantly demonstrated that, at least for the tetra- to hexa-CDD/Fs in Welsh ray grass (Lolium multiflorum), foliar content is predominantly via dry gaseous deposition. The Hp- and OCDD/Fs detected in herbage samples may be due to (more limited) dry gaseous deposition and/or to the retention of fine aerosol-bound PCDD/Fs on the leaf surfaces (2, 3, 8). Changes in herbage PCDD/F concentrations and composition can therefore be used to make inferences about changes in air concentrations and hence changing atmospheric source loadings and types. This needs to be done cautiously, however, because the mixture of PCDD/Fs released from a given source is subject to changes during aerial transport, deposition, and residence in the environment. Concentrations of PCDD/Fs in the herbage remained essentially constant between 1861 and 1945, then rose to a peak in 1961-1965, declined, and then reached a second, lower peak in 1976-1980 (Figure 1). Since then, they have declined and the ∑PCDD/F concentration in the 19911993 sample was similar to that of the pre-1946 samples. Most individual compounds and homologues peaked in the 1961-1965 sample. Importantly, there are general differences between the mixture of PCDD/Fs in the preand post-1946 samples. The Hx-, Hp-, and OCDDs were responsible for the main part of the increase in ∑PCDD/Fs
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through the 1950-1980s. The PCDD/PCDF ratio increased markedly from ∼0.8 (range 0.5-1.1) between 1861 and 1945 to ∼3.8 (range 2.3-6.1) between 1946 and 1993. It is informative to compare the 1991-1993:1961-1965 sample concentration ratios, since this highlights interesting differences between compounds. The ratio for the tetra- and penta-CDD/F homologue groups is ∼0.5, that for the HxCDFs is ∼0.2 and that for the HxCDDs, HpCDD/Fs, and OCDD/Fs is ∼0.1 or less. In other words, the latter group has declined much more sharply since the 1960s peak. 2,3,7,8-TCDD, the most toxic compound, is the only constituent that has not declined significantly over the last 30 years. Importantly, PCDD/F concentration trends in herbage and soil at Rothamsted are different (Figure 1). Herbage broadly reflects air concentrations (8), while soils broadly reflect cumulative deposition (13, 20). The soil is therefore the key storage reservoir for PCDD/Fs in the terrestrial environment (13, 20). Soil concentrations appear to have continued to increase at Rothamsted, presumably because atmospheric deposition fluxes have continued to exceed net losses after the peak 1960-1970s input. Principal component analysis (PCA) was used to examine similarities and differences in the multivariate mixtures of PCDD/Fs in the samples. The data were normalized to the ∑PCDD/F concentrations. Two PCs explained 71% of the data variation; these are plotted against each other in Figure 2a. This identified four clusters of samples (A-D). The samples from 1861 to 1945 (‘A-type’ samples with the PCDFs dominant) all appear on the right of Figure 2a. Slight distinctions can be made between the 1861-1910 (A1-type) and 1911-1945 (A2-type) herbage, principally in the proportions of the OCDD/Fs (Figure 2b). However, both are indicative of mixtures of PCDD/Fs derived from the combustion of coal and wood (15, 21, 22). Changes in the quantity and quality of coal burnt, the introduction of oil, and the introduction of larger scale combustion processes for electricity generation are factors that may contribute to the distinctions between A1 and A2 samples. Post-1946 samples are dominated by the PCDDs, with 1,2,3,4,6,7,9-HpCDD, 1,2,3,4,6,7,8-HpCDD, and OCDD all major contributors. PCDFs have the same congener profile as in the pre-1946 samples, except that OCDF, HpCDF, and especially 1,2,3,4,6,8,9-HpCDF show a slight increase; in other words, the Hp- and OCDD increase is broadly ‘superimposed’ on the A-type pattern. The post-1946 samples can be divided into three separate types: B (19461970), C (1971-1980; 1986-1990), and D (1981-1985) (Figure 2b). B-type samples have important contributions from many of the PCDD/F constituents in PCP and its salt, namely, 1,2,4,6,8,9-HxCDF, 1,2,3,6,7,8-HxCDD, 1,2,3,6,7,9/1,2,3,6,8,9-HxCDD, 1,2,3,4,6,7,8-HpCDF, 1,2,3,4,6,7,8HpCDD, 1,2,3,4,6,7,9-HpCDD, OCDF, and OCDD (14). The C-type pattern is similar to the B-type, but with an increase in the proportion of Hx- and PeCDD. The reason for the shift from B to C is not clear, but could be due to the following: (a) a change in the mix of PCDD/Fs in CA source materials because of changes in the manufacturing processes; (b) an increasing proportion of ‘old’, partially weathered (dechlorinated) congeners from the earlier decades recirculating in the environment; (c) an influence from other sources; (d) other factors. The most recent (1991-1993) sample has a smaller B-type ‘signal’ (Figure 2). The D-type pattern was an unusual one and only occurred in one sample. It contained a high proportion of Pe- and HxCDDs, with specific contributions from 1,2,3,4,7PeCDD and 1,2,3,4,6,8-HxCDD. It is speculated that this
FIGURE 2. (a) Principal components plot showing the grouping of samples of similar composition. (b) Typical mixtures of PCDD/Fs for the sample groupings identified in panel a. (Solid line denotes standard deviation.)
may be due to a specific unknown contamination incident. It is pertinent to consider two questions: (a) What were the source(s) of PCDD/Fs in the oldest samples? (b) Which sources were responsible for the more recent temporal trends of PCDD/Fs? The U.K. has a long history of industrial activity, dating back to the smelting of metals in Roman times and the widespread combustion of timber and coal that started many centuries ago. These processes may account for the low levels of PCDD/Fs observed in the samples from the last century. Other workers have observed PCDD/Fs in ‘old’ samples, such as deep-dated sediment cores (23, 24), while others report ‘nondetect’ levels in deep sediment cores and detectable levels in the more recent deposits (e.g., ref 12). The issue of postcollection/deposition contamination is an important one here that needs to be resolved, and uncertainties remain over the relative importance of pre- and postindustrialized environmental levels of PCDD/Fs. The 1940s saw the introduction of other sources of release of PCDD/Fssthe commencement of manufacture of CAs and their subsequent widespread use in industry and agriculture, which would have superimposed on the earlier combustion sources. PCDD/F releases associated with direct PCP production and use or combusted-PCP treated materials may be an important source of the CA-related emissions (9,25-27). Manufacture of PCP commenced in the 1930s, with the largest volumes used worldwide in the 1960-1980s. It has been estimated that ∼1250 t was used
in the U.K. in 1974, declining to 300-400 t/year by the mid-1980s; the decline has continued to the present day (25, 26). In addition, of course, imported PCP-treated wood and textiles will have entered the U.K. Wood treatment accounted for an estimated 80% of total U.K. consumption, with textile treatment and agricultural use responsible for most of the remainder. This broad usage pattern is consistent with the overall trends measured in our herbage, whereas trends in the total volumes of municipal solid waste (MSW) subject to incineration in the U.K., for example, are not. The amounts of incinerated MSW have increased from the 1960s to the present, with improvements in combustion technology designed to reduce contaminant emissions introduced subsequently (10). PCP-derived PCDD/Fs could enter the atmosphere (and hence reach herbage) directly by volatilization from the source material or during its use in timber treatment, and/ or during the combustion of treated products and various wastes (22, 27). In each case, however, the mixture of PCDD/Fs could be ‘weathered’ as they move from the treated source materials through the environment. In addition, the secondary formation of PCP-derived PCDD/ Fs could occur by photocoupling (28) or by enzymatically mediated reactions (29). Hagenmaier et al. (30) have suggested that the amount of ∑PCDD/F released into the German environment through the use of PCP far exceeds that emitted from MSW incineration. Superficially, study (from just one U.K. semirural/urban site) provides some
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support for their assertion that PCP is an important source of PCDD/Fs found ubiquitously in the environment. However, closer inspection of the data reveals that the story is not clear cut. For example, the ratio of 1,2,3,6,7,8-HxCDD: 1,2,3,4,7,8-HxCDD is a useful marker for technical mixtures of PCP used in Germany, being typically g100 (14). This ratio has also been found to be a useful indicator of PCP influence in the environment. Hence, this ratio should increase dramatically if the use of PCP is solely responsible for the increase of PCDD/F levels in the foliage. It does increase, from a value of about 1-2 in the early samples to 4-14 between 1961 and 1990. During this time, the levels of 1,2,3,4,7,8-HxCDD increased by a factor of 4, even though it is hardly present in most technical PCP products. This suggests that ‘pure’ PCP is unlikely to be the dominant source of the PCDD/Fs in the modern foliage samples. However, the timing and change in mixture of PCDD/Fs post-1946 is indicative of significant fresh inputs of PCDD/ Fs to the environment, which seems likely to be broadly associated with the extensive increase in the production, use, and subsequent remobilization of CAs, likely including PCP. Encouragingly, this study of herbage concentrations provides evidence of a decline in atmospheric PCDD/F concentrations. It is pertinent to note that declines in PCDD/F fluxes to the environment and concentrations in biota (including humans) have been reported from several studies in industrialized countries. Specifically, data relate to air concentrations in Germany (31), sediment cores from various countries (12, 23, 30, 32-34), bird’s eggs from the Baltic Sea (35), German cow’s milk (36) and human tissue from Germany, Sweden and the Netherlands (37-41).
Concluding Remarks This study of archived herbage collected between 1861 and 1993 from a well-characterized rural site in southeast England shows that the concentrations and mixture of PCDD/Fs have changed substantially over time. ∑PCDD/F concentrations peaked in the 1960s and have declined ∼8fold since (∑TEQ concentrations, in contrast, show a decline of only ∼4-fold). The most recent sample (1991-1993) contained ∑PCDD/F concentrations only slightly elevated above the pre-1900 samples. We infer that the changes in concentrations and mixtures of PCDD/Fs reflect changes in atmospheric deposition and, if interpreted cautiously, broad changes in PCDD/F sources to the atmosphere. Combustion-derived sources are believed to have provided a ‘baseline’ input to the air throughout the study period (i.e., since 1860), an input which will have presumably changed in intensity and composition over time as combustion sources, types, and strengths have altered. Inputs from CA production and use appear to have supplemented the combustion-derived inputs post-1946. Source inventory estimates have highlighted various anthropogenic combustion activities (notably waste incineration and metals processing) as key primary sources to the contemporary environment. In the U.K., measures are currently in place which should see reductions in these primary emissions over the next decade (10). However, this study (from just one U.K. site) suggests that secondary inputs (i.e., resulting from CA production and use) may have been substantial in the past and may be responsible for an important proportion of the contemporary U.K. environmental burden (9, 13). Future PCDD/F trends and environmental burdens will clearly be influenced by the
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relative importance of primary (combustion-derived) and secondary sources and by the persistence of the compounds. Further studies are required to assess the inputs, mobility, transformations, and significance of CA-derived PCDD/Fs in the environment and to provide context to the current regulatory efforts that are targeting combustion sources.
Acknowledgments We are grateful to Dr. M. McLachlan (University of Bayreuth) for helpful discussions and insight.
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Received for review September 21, 1995. Revised manuscript received December 29, 1995. Accepted January 10, 1996.
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