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~2 Sources of Dioxins and Dioxin-like Compounds in the Environment Dioxin and dioxin-like compounds (referred to collectively as DLCs) are ubiquitous in the environment (ATSDR, 1998; Travis and Hattemer-Frey,1989~. People may be exposed to background levels (i.e., low concentrations) of DLCs by breathing air, by consuming food or beverages, or by their skin coming into contact with DLC-contaminated materials (ATSDR, 1998~. However, the major route of human exposure to DLCs is through the food supply; the first step in that pathway is the introduction of DLCs into the environment. This chapter provides a general overview of known sources of DLCs and how these compounds are transported from the environment into the pathways that lead to human foods. Five major sources of DLCs: combustion, metals smelt- ing and refining, chemical manufacturing, biological and photochemical pro- cesses, and environmental reservoirs, are described. Quantitative information on DLCs released from each source is briefly presented and the environmental fate of DLCs, including how these compounds are transported within or to various geographic regions, is discussed. Finally, information is presented on how DLCs enter the food chain. The information presented in this chapter is not an exhaustive summary of environmental sources of DLCs and their environmental fate and transport; such a summary is beyond the scope of this report. For more detailed information, the reader is referred to the Agency for Toxic Substances and Disease Registry's (ATSDR) toxicological profiles for chlorinated dibenzo-p-dioxins, chlorinated dibenzofurans, and polychlorinated biphenyls (PCBs) (ATSDR 1994, 1998, 2000~; the U.S. Environmental Protection Agency's (EPA) draft Exposure and Human Health Reassessment of 2,3, 7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) 53
54 DIOXINS AND DIOXIN-LIKE COMPOUNDS IN THE FOOD SUPPLY and Related Compounds (EPA, 2000~; and the original articles referenced in this chapter. MAJOR SOURCES OF DLCs IN THE ENVIRONMENT With the exception of the dioxin-like PCBs, DLCs are unintended byproducts of combustion. Combustion sources can be of anthropogenic (e.g., waste incin- eration) or natural origin (e.g., forest fires). Industrial (e.g., paper and chemical manufacturing) and biological processes also contribute to DLC production, al- though in smaller quantities. These DLCs are formed in trace quantities and released into the environment. Until 1977, when regulatory action prohibited further manufacture, PCBs were commercially produced in large quantities in the United States. The potential for human exposure to PCBs still exists, however, because they persist in air, soil, and water sediments for many years. Addition- ally, they are also found in older transformers, capacitors, fluorescent lighting fixtures, electrical devices, and appliances that are still in use. Combustion The primary environmental source of dioxins and furans is combustion (Zook and Rappe, 1994, as cited in ATSDR, 1998~. Combustion processes include waste incineration (e.g., municipal solid waste, sewage sludge, medical waste, and hazardous waste), burning of various fuels (e.g., coal, wood, and petroleum products), other high-temperature sources (e.g., cement kilns), and poor or un- controlled combustion sources (e.g., forest fires, volcanic eruptions, building fires, and residential wood burning) (Clement et al., 1985; EPA, 2000; Thoma, 1988; Zook and Rappe, 1994, as cited in ATSDR, 1998~. At present, the major quantifiable source of DLC formation in the United States is from backyard burn barrels (Personal communication, D. Winters, EPA, April 2, 2002) (see Appen- dix Table A-28. Most of the direct releases of DLCs from combustion processes are into the air (Czuczwa and Hites, 1984, 1986a, 1986b, as cited in ATSDR, 1998~. Estimates of polychlorinated dibenzo-p-dioxin and polychlorinated di- benzofuran sources made in EPA's draft inventory (EPA, 1998) indicate that domestic waste burning contributes more than 1,000 g toxicity equivalents (TEQ)/ y, and likely contribute significantly to current DLC releases (Gullett et al.,2001; Wunderli et al., 2000~. DLCs are formed in chemical reactions that occur during the combustion of organic compounds in the presence of chlorinated materials (ATSDR, 1998~. EPA (2000) outlines three mechanisms by which this occurs. First, stack emis- sions of DLCs result from the incomplete destruction of DLC contaminants present in materials delivered to the combustion chamber. Not all of the DLC components are destroyed by the combustion system, thus allowing trace amounts of DLCs to be emitted from the stack. Second, the formation of DLCs from
SOURCES OF DIOXINS AND DIOXIN-~IKE COMPOUNDS IN THE E~IRONMENT 55 aromatic precursor compounds occurs in the presence of a chlorine donor. The general reaction in this formation pathway is an interaction between an aromatic precursor compound and chlorine promoted by a transition metal catalyst on a reactive fly ash surface (Dickson and Karasek, 1987; Liberti and Brocco, 1982, as cited in EPA, 2000~. Last, de nova synthesis promotes formation of DLCs in combustion processes from the oxidation of carbon particulate catalyzed by a transition metal in the presence of chlorine. Intermediate compounds, which are precursors to DLC formation, are produced during de nova synthesis. The forma- tion of DLCs via either the precursor or de nova synthesis pathways requires the availability of gaseous chlorine (Addink et al., 1995, as cited in EPA, 2000; Luijk et al., 1994~. The source of the chlorine is the materials (fuels or feed) in the combustion system. Metals Smelting and Refining There are several types of primary and secondary metal smelting and refin- ing operations, including iron ore sintering, steel production, and scrap metal recovery. Such operations use both ferrous and nonferrous metals. Few data are available on the potential for the formation and environmental release of DLCs from primary nonferrous metal manufacturing operations. The contribution of DLCs produced from primary copper and aluminum smelters is thought to be minimal (Environmental Risk Sciences, 1995; Lexen et al., 1993, as cited in EPA, 2000~. Titanium smelting and refining may be a source of DLCs (Bramley, 1998, as cited in EPA, 2000~. Current information is insufficient to determine if primary magnesium smelting and refining releases DLCs into the environment. Secondary smelting and refining of nonferrous metals such as aluminum, copper, lead, and zinc may result in formation of DLCs, due to combustion of organic impurities (e.g., plastic, paints, and solvents) in the metals and chlorine-contain- ing chemicals (e.g., sodium chloride and potassium chloride) used in the smelting process (Aittola et al., 1992; EPA, 1987, 1997, as cited in EPA, 2000~. Two types of operations used in primary ferrous metal smelting iron sinter production and coke production are potential sources of DLCs (Lahl, 1993, 1994; Lexen et al., 1993; Rappe, 1992a, as cited in EPA, 2000~. Recycled dust and scraps from other processes in the sintering plant introduces traces of chlo- rine and organic compounds that generate DLCs (Lahl, 1993, 1994, as cited in EPA, 2000~. Electric arc furnaces, used in secondary ferrous metal smelting and refining, have also been implicated as a source of DLCs. Chemical Manufacturing Three types of chemical manufacturing processes bleaching of wood pulp in paper manufacturing, chlorine and chlorine-derivative manufacturing, and ha- logenated organic chemical manufacturing lead to the production of DLCs.
56 DIOXINS AND DIOXIN-LIKE COMPOUNDS IN THE FOOD SUPPLY DLCs, primarily the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCCD) and 2,3,7,8- tetrachlorodibenzofuran (TCDF) congeners, are present in effluent and sludge from pulp and paper mills that employ the bleached kraft process (Clement et al., 1989; EPA,1991; Swanson et al., 1988, as cited in ATSDR,1998~. From 1988 to 1992, there was a 90 percent reduction in TEQs generated by pulp and paper mills for 2,3,7,8-TCCD and 2,3,7,8-TCDF (NCASI, 1993, as cited in EPA, 2000~. To help reduce DLCs in effluents from pulp and paper mills, EPA promulgated effluent limitations guidelines and standards for certain segments of the pulp, paper, and paperboard industries (EPA, 1998, as cited in EPA, 2000~. These industries are responsible for more than 90 percent of the bleached-chemical pulp production in the United States. Chlorine gas is manufactured by electrolysis of brine electrolytic cells. The use of mercury cells that contain graphite electrodes has been shown to generate high levels of chlorodibenzofurans (Rappe, 1993; Rappe et al., 1990, 1991; Strandell et al., 1994, as cited in EPA, 2000~. During the 1980s, graphite elec- trodes were replaced with titanium metal anodes at chlorine gas manufacturing facilities in the United States. Hutzinger and Fiedler (1991, as cited in EPA, 2000) reported low concentrations of DLCs in chlorine-derivative products. DLCs can be formed during the manufacture and disposal of halogenated organic chemicals such as chlorophenols, chlorobenzenes, and PCBs (Ree et al., 1988; Versar, 1985, as cited in EPA, 2000~. Chlorophenols, which have been used for various pesticide applications, may be released into the environment from industrial use of these compounds and from disposal of wastes from the manufacturing facilities that produce them. Production of chlorophenols has been limited to 2,4-dichlorophenol and pentachlorophenols (PCPs) since the late 1980s, and disposal of wastes generated from their manufacture is strictly regulated. The production of chlorobenzenes (used as raw materials in the production of phenol, aniline, and various pesticides) by certain processes can inadvertently produce DLCs (Ree et al., 1988, as cited in EPA, 2000), but they are not believed to be generated in the production of mono-, di-, and trichlorobenzene, which are the only forms of chlorobenzene currently produced in the United States. The release of DLCs from the production of chlorobenzenes was estimated to be negligible in 1995 (EPA, 2000~. Coplanar PCBs, which have dioxin-like activity, are entirely anthropogenic in origin and were produced in the United States from 1929 to 1977 (NRC, 2001~. Most countries have banned the production of PCBs, but several potential sources for environmental release remain. These include continued use and disposal of PCB-containing products such as transformers, capacitors, and other electrical equipment that were manufactured before 1977; combustion of PCB-containing materials; recycling of PCB-contaminated prod- ucts, such as carbonless copy paper; and releases of PCBs from waste storage and disposal (NRC, 2001~. PCBs may still be manufactured in Russia and North Korea and therefore, PCBs may be entering the environment both in those coun- tries and in other countries that buy their PCB-containing products (Carpenter,
SOURCES OF DIOXINS AND DIOXIN-~IKE COMPOUNDS IN THE E~IRONMENT 57 1998; NRC,2001~. Other halogenated organic chemicals that, through their manu- facture or disposal, have been associated with the release of DLCs into the envi- ronment include polyvinyl chloride, aliphatic chlorine compounds, and various dyes, pigments, and printing inks. Biological and Photochemical Processes Evidence suggests that DLCs can be formed under certain environmental conditions. DLCs have been found in various types of composts, possibly from atmospheric deposition onto plants that were subsequently composted or from uptake of DLCs from air by the compost (Krause et al., 1994; Lahl et al., 1991, as cited in EPA, 2000~. DLCs are also found in sewage sludge; specific sources include microbial biotransformation of chlorinated phenolic compounds, runoff to sewers from contaminated lands or urban surfaces, household or industrial wastewater, and chlorination operations within wastewater treatment facilities (Cramer et al., 1995; Horstmann and McLachlan, 1995; Horstmann et al., 1992; Rappe, 1992b; Rappe et al., 1994; Sewart et al., 1995, as cited in EPA, 2000~. Evidence also suggests that DLCs can be generated by photolysis of PCPs, but this reaction has only been demonstrated under laboratory conditions (Lamparski et al., 1980; Vollmuth et al., 1994; Waddell et al., 1995, as cited in EPA, 2000), and it is not known if it occurs in the environment. Reservoirs for DLCs Reservoir sources also contribute to global DLC exposure. These reservoirs, representing a recirculation of past DLC generation, may be classified as long-, intermediate-, and short-term, or as static and dynamic sources. DLCs may be released from reservoirs by volatilization, particle and vapor deposition, suspen- sion or resuspension into air and water sediments, and direct consumption by land and aquatic animals and humans. The impact of various reservoir sources on human DLC burdens is dependent on their direct or indirect contact with humans or with human food supplies. DLC contamination of geological formations represents a long-term, static source. Ball clay deposits (Hayward et al., 1999) that are resurrected and put into circulation by mining and by feeding these deposits to land and aquatic animals is one example of reservoir activation. Other examples may be uncovered as vari- ous mineral or other geological deposits are mined and brought to the surface for further processing or use. Intermediate reservoirs may be characterized as soils and sediments in wa- terways. Newly generated DLCs precipitate into these areas, dependent on the atmospheric conditions and point source generation capacities. These DLC de- posits persist for many years without substantial degradation. Limited volatiliza- tion and sunlight-generated degradation may reduce DLC levels within these
58 DIOXINS AND DIOXIN-LIKE COMPOUNDS IN THE FOOD SUPPLY reservoirs, but the levels are relatively static or are cumulative in their concentra- tions. Soil and waterway exposures by grazing animals, direct soil contamination of fresh vegetables and fruits, and aquatic exposure to contaminated waterway sediments are examples of direct input of these reservoir sources into the human food supply. Aerosol or environmental suspension represents a short-term reservoir as DLCs that are adsorbed onto particulate matter or that are in a volatilized state are transported until they precipitate from the atmosphere as rainfall or by particle aggregation and deposition. These means of transport may vary by geographical and temporal duration. This process is one method by which DLCs can be spread over large geographical areas, including intercontinental transport, from the point of generation to the point of entry into intermediate- and long-term reservoirs. Fat deposits from animals and fish exposed to contaminated environmental and feed sources are an additional short-term reservoir and may be more dynamic than other sources. The generational periods of some species (e.g., bovines) may be long enough to characterize those species as intermediate reservoirs. DLC levels in adipose tissue are a function of the exposure experiences and species metabolic capacities for specific animal or aquatic populations. These fat depos- its may be directly introduced to the human population through foods of animal or aquatic origins or by the use of their fats in processed-food preparation, or indirectly through the recycling of animal- and aquatic-origin fats in feed. GENERAL INFORMATION ABOUT THE QUANTITATIVE INVENTORY OF RELEASES OF DLCs The most recent analysis of emission estimates of DLCs for the United States have been developed for two reference years, 1987 and 1995, by EPA (2000~. EPA chose to use data from 1987 because: (1) prior to 1987 there was little empirical data on source-specific emission estimates, (2) soon after 1987, there was widespread installation of DLC-specific emission controls at a number of facilities and, therefore, 1987 is the latest time representative of the emissions before those controls were installed, and (3) around 1987 there were significant advances in emissions measurement techniques and in techniques to measure low concentrations of DLCs in environmental samples. Data from 1995 were used because it is the latest time period that could be practically addressed by EPA in the time frame that its report was scheduled for completion. Since 1995, EPA has promulgated regulations limiting DLC emissions for several source categories that contribute to the DLC inventory (e.g., municipal waste combustors, waste incinerators, and pulp and paper facilities using chlorine bleach processes). There- fore, the 1995 data may not be indicative of current DLC releases. Because not every facility in the United States for each of the source categories has been tested for DLC releases and emissions, EPA developed an extrapolation method to estimate national DLC emissions from tested facilities. For a detailed descrip-
SOURCES OF DIOXINS AND DIOXIN-~IKE COMPOUNDS IN THE E~IRONMENT 59 tion of that methodology, the reader is referred to EPA (2000~. Some investiga- tors have suggested that EPA's methodology may have led to the underestimation of releases (Bruzy and Hites, 1995; Harrad and Jones, 1992; Rappe, 1991, as cited in EPA, 2000~. It is likely that unknown sources of DLCs still exist, also leading to the underestimation of releases. EPA (2000) made several general conclusions about the quantitative inven- tory of sources for DLCs. These include: . . The best estimates of releases of DLCs to air, water, and land from reasonably quantifiable sources were approximately 3,300 g TEQDF WHO98 (3,000 g I-TEQ) in 1995 and 14,000 g TEQDF whops (12,800 g I- TEQ) in 1987. A wide variety of sources are responsible for the environmental releases of DLCs in the United States; however, combustion sources are respon- sible for the majority of releases of TCDDs and TCDFs (approximately 85 percent of these compounds from quantifiable sources in 1995~. Met- als smelting and manufacturing and chemical manufacturing contributed approximately 10 percent and 5 percent, respectively, of environmental releases of DLCs from quantifiable sources in 1995. The primary environmental releases of dioxin-like PCBs are electrical equipment. There was an estimated decrease in releases of DLCs between 1987 and 1995 of approximately 76 percent. This decrease was primarily due to reductions in air emissions from municipal and medical waste incinera- tors. Insufficient data are available to comprehensively estimate point source releases of DLCs into water. The inventory includes only a limited set of activities that results in direct environmental releases to land. Direct releases to land include land appli- cation of sewage sludge, commercial sludge products, and pulp and paper mill wastewater sludge, use of 2,4-D pesticides, and manufacturing wastes from the production of ethylene dichloride and vinyl chloride. EPA regulates biosolids (sewage sludge) under 40 C.F.R. part 503, §405(d). A recent report by the National Research Council (NRC, 2002) concluded that "there is no documented scientific evidence that the Part 503 rule has failed to protect public health." The report does, however, recommend a need to update the scientific basis of the rule to assure the public and protect public health. Significant amounts of DLCs that are produced annually are not consid- ered environmental releases (e.g., DLCs generated internal to a process but destroyed before release and waste streams that are disposed of in approved landfills) and, therefore, are not included in EPA's national inventory.
60 DIOXINS AND DIOXIN-LIKE COMPOUNDS IN THE FOOD SUPPLY ENVIRONMENTAL FATE AND TRANSPORT OF DLCs Transport in Air Atmospheric transport is a major dispersal mechanism for DLCs in the envi- ronment. The presence of DLCs in remote, nonindustrial locations suggests that long-range atmospheric transport of the compounds occurs. The atmospheric distribution and transformation profile for a specific congener depends on its vapor pressure, the atmospheric temperature, and the particle concentration in the air. For a given congener, the less chlorinated it is (i.e., the lower its vapor pressure), the warmer the atmospheric temperature, and the fewer particles that are present, the greater the fraction that will be found in the vapor phase. The more chlorinated DLCs (i.e., pentachlorinated and greater) are associated almost exclusively with the particle-bound phase (ATSDR, 1998), whereas TCDD, which has an intermediate vapor pressure, is associated with both the vapor and particle- bound phases. A percentage of DLCs in the particle-bound phase appear to reach equilibrium between the particle-bound and vapor phases, while some portion remains irreversibly bound to the particles. Dioxin-like PCBs have greater vola- tility than other DLCs with similar chlorination and are associated more closely with the vapor phase. Like DLCs, they appear to travel great distances in the atmosphere. DLCs are removed from the atmosphere by either dry or wet deposition. Dry deposition occurs for both the particle- and vapor-associated compounds. The more highly chlorinated DLCs adsorbed to atmospheric particles are removed primarily by gravity to the surface of soil, vegetation, or water. The less chlori- nated congeners in the particle or vapor phase are removed by atmospheric turbu- lence and diffusion. Vapor-phase DLCs are generally deposited directly onto vegetation or soil covering (leaves and detritus). The greater volatility of PCBs means that they may volatilize from soil and water surfaces, thus acting as sources of atmospheric inputs, as demonstrated in the Great Lakes (EPA, 2000~. DLCs may be deposited onto soil, water, or vegetation via wet deposition, either suspended in the water or associated with precipitation particles. DLCs have been measured in precipitation in most locales (EPA, 2000~. Wet deposition is the most efficient removal process for particle-bound DLCs from the atmo- sphere (ATSDR, 1998~. Transport in Soil DLCs have low water solubility and are highly lipophilic and, as such, tend to partition from the atmosphere to soil and vegetation surfaces. As some DLCs are considered semivolatile, particularly the less chlorinated congeners, some small portion of deposited DLCs may reenter the atmosphere as a result of vola- tilization from these surfaces or bound to airborne soil particles (EPA, 2000~.
SOURCES OF DIOXINS AND DIOXIN-~IKE COMPOUNDS IN THE ENVIRONMENT 61 Once below the soil surface, soil-bound DLCs do not appear to move up or down via volatilization without a carrier; this is particularly true for the terra- and higher chlorinated DLCs. The presence of a solvent such as oil may facilitate the diffusive movement of DLCs through soil. As most DLCs that attach to soil particles have little potential for leaching through or volatilizing from soil, they are removed from soil surfaces primarily by soil erosion (wind or water) and runoff to water bodies. For DLCs that are not eroded, burial is the major fate process. Without the presence of a carrier in the soil, as may occur at a hazardous waste site or spill, DLC contamination of the underlying groundwater from soil is unlikely; however, some leaching through soil may occur for the dioxin-like PCBs, particularly if the soil has a low organic content and is less likely to bind the compounds (ATSDR, 2000~. Suzuki and colleagues (1998) and Sinkkonen and Paasivirta (2000) modeled the transformation and fate of DLCs for various environmental media. These models estimate steady-state and nonsteady-state concentrations of DLCs for water, soil, and sediment, and they predict environmental fluxes. Although the models lack quantitative and qualitative precision, they do indicate that soil is an important reservoir source for DLCs, and that degradation rates in soil and sedi- ment may be significant determinants of the environmental transformation and fate processes of these compounds. Transport in Water As DLCs enter the water column through soil runoff and erosion or atmo- spheric wet or dry deposition, they adhere to particles in the water column and are ultimately removed by sedimentation. A slight amount of volatilization from the water column back into the atmosphere is possible, especially for the lighter DLC congeners. However, volatilization from water is the predominant removal mech- anism for the dioxin-like PCBs in the water column. The low water solubility of DLCs, including the dioxin-like PCBs, means that only a small portion of these compounds will dissolve in water. Once adsorbed to particulate matter in the water column, the DLC-contain- ing particles will settle out to the sediments and eventually be buried, although resuspension into the water column may occur as a result of sediment agitation due to floods, biological activity, or other phenomenon. Resuspended sediments may travel downstream for great distances before settling back out of the water column, accounting for the presence of DLCs in sediments at considerable dis- tances from a source. The more volatile PCBs may also move from the sediments back into the water column and ultimately into the atmosphere, although this process is unlikely for other DLCs. This process is greater in the summer months and with shallow waters (ATSDR, 1998~.
62 DIOXINS AND DIOXIN-LIKE COMPOUNDS IN THE FOOD SUPPLY Transformation Processes and Persistence DLCs are stable compounds and are highly resistant to most environmental degradation processes and to hydrolysis (EPA, 2000~. Transformation processes for DLCs appear to be primarily through photooxidation in the atmosphere, pro- ducing hydroxyl radicals and some photolysis in air and on soil and water sur- faces. Only a few studies are available on the transformation of DLCs under natural environmental conditions; they will not be discussed in this report. In air, photooxidation via hydroxyl radical reaction appears to be a major degradation process for both particle-phase and vapor-phase DLCs. Tropospheric lifetimes are variable: 0.5 days for monochlorodibenzo-p-dioxins, approximately 2 days for 2,3,7,8-TCDD,10 days for octachlorodibenzo-p-dioxins (OCDD), and 39 days for octachlorodibenzofurans (Atkinson, 1991, as cited in EPA, 2000~. The reaction of vapor-phase PCBs with hydroxyl radicals results in a half-life that varies from 11 days for tetrachlorinated congeners to more than 94 days for heptachlorinated congeners (Atkinson, 1987, as cited in EPA, 2000~. Other stud- ies have indicated that the atmospheric lifetimes for vapor-phase reactions of DLCs with nitrate and ozone were approximately 5 days and greater than 330 days, respectively (Kwok et al., 1994~. Atmospheric photooxidation rates in- crease in the summer months and decrease with increasing DLC chlorination. Vapor-phase DLCs are subject to some photolysis, with rates decreasing as the DLC chlorination increases (EPA, 2000~. The atmospheric half-life of 2,3,7,8- TCDD in summer sunlight was estimated to be 1 hour (Podoll et al., 1986~. Photolysis of particle-bound congeners is much slower (8 percent of 2,3,7,8- TCDD adsorbed to fly ash was lost after 40 hours of irradiation) (Mill et al., 1987~. In both cases, the transformation is photodechlorination from more to less chlorinated congeners. As the majority of these studies have been conducted under laboratory conditions, photolysis rates under actual environmental condi- tions may be different (EPA, 2000~. For dioxin-like PCBs, as the chlorination increases, so does the photolysis rate. Unlike other DLCs, the less chlorinated PCBs are more resistant to photolysis and may be formed as products of photoly- sis of the more chlorinated congeners (EPA, 2000~. Photolysis is the major fate process for DLCs in water, but it is relatively slow. Dioxin congeners substituted in the 2,3,7, and 8 positions, rather than the 1,4,6, and 9 positions, are more readily degraded (ATSDR, 2000~. In sunlight, 2,3,7,8-TCDD in natural waters is expected to have a half-life ranging from approximately 3 days in summer to 16 days in winter (EPA, 2000~. The presence of organic molecules may increase or decrease the photolysis rates for the DLCs. Photolytic degradation of PCBs increases with chlorination of the compound; however, the OCDD congener is resistant to this process (ATSDR, 1998~. Addi- tionally, PCBs suspended in fresh surface water react with hydroxyl radicals with half-lives of 4 to 11 days (Sedlak and Andren, 1991, as cited in EPA, 2000~. In soils, DLCs bind strongly to organic matter with the result that degrada- tion below the soil surface is virtually nonexistent. DLCs at the soil surface are
SOURCES OF DIOXINS AND DIOXIN-~IKE COMPOUNDS IN THE E~IRONMENT 63 subject to some photolysis, although at a slower rate than when in water (ATSDR, 1998~. Degradation of the di- and trichlorinated DLCs in sandy loam soil has been reported at 15 months (Orazio et al., 1992, as reported in EPA, 2000), although no significant degradation was seen for the terra- to octachlorinated DLCs. Photolytic degradation of DLCs on soil surfaces has been determined for various soil types. DLCs appear to persist longer on soils with higher organic content, although the photolytic degradation rates for most soil types are equal after about 5 days (Kieatiwong et al., 1990, as cited in EPA, 2000~. Paustenbach and colleagues (1992, as cited in EPA, 2000) estimated that 2,3,7,8-TCDD has a half-life of 25 to 100 years in subsurface soil and 9 to 15 years in the top 0.1 cm of soil, indicating that photolysis occurs only to the depth of penetration of ultraviolet light (EPA, 2000~. DLCs applied to agricultural fields in sewage sludge showed no degradation after 43 days under natural sunlight in the late summer (Schwarz and McLachlan, 1993, as cited in EPA, 2000~. Although microbial degradation has been demonstrated for some DLC con- geners, this does not appear to be an important transformation process. Hexa- chlorinated congeners were degraded by 70 to 75 percent by the white rot fungus Phanerochaete sordida, with 2,3,7,8-TCDD and the pentachlorinated congeners being more resistant (Takada et al., 1994, 1996, as cited in EPA, 2000~. DLCs appear to be subject to the greatest reductive degradation under anaerobic condi- tions, although some aerobic degradation has been reported. Microorganisms from Passaic River sediments were able to degrade 2,3,7,8-TCDD by 30 percent within 7 months, with a resulting increase in the less chlorinated forms; similar results were seen with the incubation of OCDD, which was converted to less chlorinated forms (Barkovskii and Adriaens, 1995, 1996, as cited in EPA, 2000~. Unlike most DLCs, PCBs have been shown to be aerobically and anaerobi- cally dechlorinated by several microbial species (ATSDR, 2000~. Aerobic bio- degradation decreases with increasing chlorination (EPA, 2000), but can still result in destruction of the PCB molecule. The half-life of tetra-PCB congeners in surface waters and soils exposed to ultraviolet light is estimated to be 7 to 60 days and 12 to 30 days, respectively; the half-lives of penta- and greater chlorinated PCBs in surface waters and soils exposed to ultraviolet light are estimated to be greater than 1 year (EPA, 2000~. PCBs are associated strongly with anaerobic sediments and so anaerobic biodegradation of PCBs does occur (ATSDR, 2000~. Although this process dechlorinates PCBs, it does not destroy the molecules (Liu et al., 1996; Sokol et al., 1994~. ENTRY OF DLCs INTO THE FOOD CHAIN As discussed above, pathways of DLC entry into the food chain include atmospheric transport of emissions and their subsequent deposition on plants, soils, and water (Fries, 1995a) and industrial discharges directly into bodies of
64 DIOXINS AND DIOXIN-LIKE COMPOUNDS IN THE FOOD SUPPLY water or on land (Bobovnikova et al., 2000; Connolly et al., 2000~. This section summarizes evidence suggesting that DLCs enter the food chain by these path- ways. Aerial transport of DLC-containing emissions is considered the primary path- way of entry of DLCs into the food chain (Fries, 1995a, 1995b) as a result of their deposition onto plants and soils. Another source of residues in the food chain is DLCs that are released from sediment and soil that were sinks for past introduc- tions into the environment (Bushart et al., 1998~. Minor introductions may also occur as a result of the application of sewage and paper-mill sludge to land (Rappe and Buser, 1989; Weerasinghe et al., 1986) and the use of wood treated with pentachlorophenol (Firestone et al., 1972; Ryan et al., 1985; Shull et al., 1981~. More than 95 percent of contaminants deposited by air in terrestrial envi- ronments reach soil (Fries and Paustenbach, 1990~. The transfer of semivolatile lipophilic compounds such as DLCs is predicted to occur through volatilization end particulate deposition (Fries, 1995a, 1995b), although McCrady and colleagues (1990) and Bacci and colleagues (1992) found that no TCDD was detected in plants when suitable vapor barriers were provided. Uptake and translocation of TCDD from soils to plants is not believed to be a major route for DLCs. Isensee and Jones (1971) showed a lack of absorption and translocation after foliar application of TCDD, and several studies report that little or no TCDD was measured in seeds and aerial parts of crops grown in contaminated soil (Isensee and Jones, 1971; Jensen et al., 1983; Wipf et al., 1982~. According to reviews by Esposito and colleagues (1980) and Norris (1981), approximately 0.1 percent of TCDD applied to soil was found in above-ground portions of oats and soybeans, although some absorption did occur from nutrient solutions. While volatilization is clearly a major factor in the worldwide distribu- tion of DLCs, there was no evidence presented to the committee to confirm that transfer to forage crops was a major pathway of concern. While there may be some contamination of forage crops through particulate-bound DLCs, the lack of lipids in most forage crops makes it unlikely that vapor-phase DLCs will accu- mulate on plants. In 1977, one year after the unintended release in Seveso, Italy, no traces of TCDD were found in the flesh of apples, pears, and peaches or in corncobs and kernels grown near the factory. However, approximately 100 pg/g of DLCs were detected in the peel of fruits (Wipf et al., 1982~. Tree bark has been shown to be a useful environmental monitor of vapor-phase PCBs (Hermanson and Hites, 1990~. Little vapor transfer of the highly chlorinated congeners from soil is ex- pected because the vapor pressure of a homologous series of compounds, such as DLCs, decreases with increasing chlorination (Fries, 1995a). Therefore, a pre- dictable transport mechanism for the highly chlorinated congeners is by transfer of soil particles as dust or by splashing during precipitation. This mechanism does not appear, however, to be particularly important for animal forage crops
SOURCES OF DIOXINS AND DIOXIN-~IKE COMPOUNDS IN THE ENVIRONMENT 65 (Fees, 1995b). Concentrations of DLCs (pnmanly hepta and octa) in hay were determined to be unrelated to soil concentrations by Hulster and Marschner (1993), and residues of polybrom~nated biphenyls (which are similar in chemical composition to DLCs) were not detected in harvested forages grown in soil with concentrations as high as 300 ppb (Fees and Jacobs, 1986~. The predicted concentrations of TCDD on plants are highest in pastures due to average pasture yields that are lower than the average yields of other forages (Fees and Paustenbach, 1990~. Also, the use of pasture as a roughage source for animals is an important factor because soil ingestion is added to direct plant contamination as a source of exposure (Fnes,1995b). In general, soil ingestion is related inversely to the availability of forage when pasture is the sole animal-feed source. For example, the amount of soil ingested by food animals is as little as 1 to 2 percent of dry-matter intake during periods of lush plant growth, but it rises to as much as 18 percent when forage is sparse (Mayland et al., 1975; Thorton and Abrahams, 1981~. This chapter provides a general overview of known sources of DLCs and how these compounds are transported from the environment into pathways lead- ing to human foods. The data presented constitute the bulk of the general back- ground information that the committee used to fam~lianze itself with issues re- lated to environmental sources of DLCs and their entry into the food supply. Environmental releases of DLCs have been decreasing in recent decades. The sources contributing the majority of DLC releases are from combustion, in par- ticular from nonregulated sources. REFERENCES Addink R. Bakker WCM, Olie K. 1995. Influence of HC1 and C1 on the formation of 2 polychlori- nated dibenzo-p-dioxins/dibenzofurans in a carbon/fly ash mixture. Environ Sci Technol 29:2055-2058. Aittola J. Paasivirta J. Vattulainen A. 1992. Measurements of organochloro compounds at a metal reclamation plant. Organohalogen Compd 9:9-12. Atkinson R. 1987. Estimation of OH radical reaction rate constants and atmospheric lifetimes for polychlorobiphenyls, dibenzo-p-dioxins, and dibenzofurans. Environ Sci Technol 21:305-307. Atkinson R. 1991. Atmospheric lifetimes of dibenzo-p-dioxins and dibenzofurans. Sci Total Environ 104: 17-33. ATSDR (Agency for Toxic Substances and Disease Registry). 1994. Toxicological Profile for Chlo- rinated Dibenzofurans. Atlanta, GA: ATSDR. ATSDR. 1998. Toxicological Profile for Chlorinated Dibenzo-p-dioxins. Atlanta, GA: ATSDR. ATSDR. 2000. Toxicological Profile for Polychlorinated Biphenyls (PCBs). Atlanta, GA: ATSDR. Bacci E, Cerejeira MJ, Gaggi C, Chemello G. Calamari D, Vighi M. 1992. Chlorinated dioxins: Volatilization from soils and bioconcentration in plant leaves. Bull Environ Contam Toxicol 48:401-408. Barkovskii AL, Adriaens P. 1995. Reductive dechlorination of tetrachloro-dibenzo-p-dioxin parti- tioned from Passaic River sediments in an autochthonous microbial community. Organohalogen Compd 24:17-21.
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